33. Inter-integrated circuit interface (I2C)

33.1 Introduction

The I2C peripheral handles the interface between the device and the serial I ² C (inter-integrated circuit) bus. It provides multimaster capability, and controls all I ² C-bus-specific sequencing, protocol, arbitration and timing. It supports Standard-mode (Sm), Fast-mode (Fm) and Fast-mode Plus (Fm+).

The I2C peripheral is also SMBus (system management bus) and PMBus ^® (power management bus) compatible.

It can use DMA to reduce the CPU load.

33.2 I2C main features

• • I ² C-bus specification rev03 compatibility:
  • – Slave and master modes
  • – Multimaster capability
  • – Standard-mode (up to 100 kHz)
  • – Fast-mode (up to 400 kHz)
  • – Fast-mode Plus (up to 1 MHz)
  • – 7-bit and 10-bit addressing mode
  • – Multiple 7-bit slave addresses (2 addresses, 1 with configurable mask)
  • – All 7-bit-addresses acknowledge mode
  • – General call
  • – Programmable setup and hold times
  • – Easy-to-use event management
  • – Clock stretching (optional)
• • 1-byte buffer with DMA capability
• • Programmable analog and digital noise filters
• • SMBus specification rev 3.0 compatibility ^(a) :
  • – Hardware PEC (packet error checking) generation and verification with ACK control
  • – Command and data acknowledge control
  • – Address resolution protocol (ARP) support
  • – Host and device support
  • – SMBus alert
  • – Timeouts and idle condition detection
• • PMBus rev 1.3 standard compatibility
• • Independent clock

a. To check the compliance of the GPIOs selected for SMBus with the specified logical levels, refer to the product datasheet.

For information on I2C instantiation, refer to Section 33.3: I2C implementation .

33.3 I2C implementation

This section provides an implementation overview with respect to the I2C instantiation.

Table 208. I2C implementation

1. X = supported.

33.4 I2C functional description

In addition to receiving and transmitting data, the peripheral converts them from serial to parallel format and vice versa. The interrupts are enabled or disabled by software. The peripheral is connected to the I ² C-bus through a data pin (SDA) and a clock pin (SCL). It supports Standard-mode (up to 100 kHz), Fast-mode (up to 400 kHz), and Fast-mode Plus (up to 1 MHz) I ² C-bus.

The peripheral can also be connected to an SMBus, through the data pin (SDA), the clock pin (SCL), and an optional SMBus alert pin (SMBA).

The independent clock function allows the I2C communication speed to be independent of the PCLK1 frequency.

For I2C I/Os supporting 20 mA output current drive for Fast-mode Plus operation, the driving capability is enabled through control bits in the system configuration controller (SYSCFG).

33.4.1 I2C block diagram

Figure 345. Block diagram

33.4.2 I2C pins and internal signals

Table 209. I2C input/output pins

Table 210. I2C internal input/output signals

33.4.3 I2C clock requirements

The I2C kernel is clocked by I2CCLK.

The I2CCLK period \( t_{I2CCLK} \) must respect the following conditions:

where \( t_{LOW} \) is the SCL low time, \( t_{HIGH} \) is the SCL high time, and \( t_{filters} \) is the sum of the analog and digital filter delays (when enabled).

The digital filter delay is \( DNF[3:0] \times t_{I2CCLK} \) .

The PCLK1 clock period \( t_{PCLK} \) must respect the condition \( t_{PCLK} < 4/3 t_{SCL} \) , where \( t_{SCL} \) is the SCL period.

Caution: When the I2C kernel is clocked by PCLK1, this clock must respect the conditions for \( t_{I2CCLK} \) .

33.4.4 I2C mode selection

The peripheral can operate as:

• • slave transmitter
• • slave receiver
• • master transmitter
• • master receiver

By default, the peripheral operates in slave mode. It automatically switches from slave to master mode upon generating START condition, and from master to slave mode upon arbitration loss or upon generating STOP condition. This allows the use of the I2C peripheral in a multimaster I ² C-bus environment.

Communication flow

In master mode, the I2C peripheral initiates a data transfer and generates the clock signal. Serial data transfers always begin with a START condition and end with a STOP condition. Both START and STOP conditions are generated in master mode by software.

In slave mode, the peripheral recognizes its own 7-bit or 10-bit address, and the general call address. The general call address detection can be enabled or disabled by software. The reserved SMBus addresses can also be enabled by software.

Data and addresses are transferred as 8-bit bytes, MSB first. The address is contained in the first byte (7-bit addressing) or in the first two bytes (10-bit addressing) following the START condition. The address is always transmitted in master mode.

The following figure shows the transmission of a single byte. The master generates nine SCL pulses. The transmitter sends the eight data bits to the receiver with the SCL pulses 1

to 8. Then the receiver sends the acknowledge bit to the transmitter with the ninth SCL pulse.

Figure 346. I ² C-bus protocol

The acknowledge can be enabled or disabled by software. The own addresses of the I2C peripheral can be selected by software.

33.4.5 I2C initialization

Enabling and disabling the peripheral

Before enabling the I2C peripheral, configure and enable its clock through the clock controller, and initialize its control registers.

The I2C peripheral can then be enabled by setting the PE bit of the I2C_CR1 register.

Disabling the I2C peripheral by clearing the PE bit resets the I2C peripheral. Refer to Section 33.4.6 for more details.

Noise filters

Before enabling the I2C peripheral by setting the PE bit of the I2C_CR1 register, the user must configure the analog and/or digital noise filters, as required.

The analog noise filter on the SDA and SCL inputs complies with the I ² C-bus specification which requires, in Fast-mode and Fast-mode Plus, the suppression of spikes shorter than 50 ns. Enabled by default, it can be disabled by setting the ANFOFF bit.

The digital filter is controlled through the DNF[3:0] bitfield of the I2C_CR1 register. When it is enabled, the internal SCL and SDA signals only take the level of their corresponding I ² C-bus line when remaining stable for more than DNF[3:0] periods of I2CCLK. This allows suppressing spikes shorter than the filtering capacity period programmable from one to fifteen I2CCLK periods.

The following table compares the two filters.

Table 211. Comparison of analog and digital filters

1. 1. Maximum duration of spikes that the filter can suppress

Caution: The filter configuration cannot be changed when the I2C peripheral is enabled.

I2C timings

To ensure correct data hold and setup times, the corresponding timings must be configured through the PRESC[3:0], SCLDEL[3:0], and SDADEL[3:0] bitfields of the I2C_TIMINGR register.

The STM32CubeMX tool calculates and provides the I2C_TIMINGR content in the I2C configuration window.

Figure 347. Setup and hold timings

When the SCL falling edge is internally detected, the delay \( t_{SDADEL} \) (impacting the hold time \( t_{HD;DAT} \) ) is inserted before sending SDA output:

The total SDA output delay is:

The \( t_{\text{SYNC1}} \) duration depends upon:

• • SCL falling slope
• • input delay \( t_{\text{AF(min)}} < t_{\text{AF}} < t_{\text{AF(max)}} \) introduced by the analog filter (if enabled)
• • input delay \( t_{\text{DNF}} = \text{DNF} \times t_{\text{I2CCLK}} \) introduced by the digital filter (if enabled)
• • delay due to SCL synchronization to I2CCLK clock (two to three I2CCLK periods)

To bridge the undefined region of the SCL falling edge, the user must set SDADEL[3:0] so as to fulfill the following condition:

Note: \( t_{\text{AF(min)}} \) and \( t_{\text{AF(max)}} \) are only part of the condition when the analog filter is enabled. Refer to the device datasheet for \( t_{\text{AF}} \) values.

The \( t_{\text{HD:DAT}} \) time can at maximum be 3.45 µs for Standard-mode, 0.9 µs for Fast-mode, and 0.45 µs for Fast-mode Plus. It must be lower than the maximum of \( t_{\text{VD:DAT}} \) by a transition time. This maximum must only be met if the device does not stretch the LOW period ( \( t_{\text{LOW}} \) ) of the SCL signal. When it stretches SCL, the data must be valid by the set-up time before it releases the clock.

The SDA rising edge is usually the worst case. The previous condition then becomes:

Note: This condition can be violated when NOSTRETCH = 0, because the device stretches SCL low to guarantee the set-up time, according to the SCLDEL[3:0] value.

After \( t_{\text{SDADEL}} \) , or after sending SDA output when the slave had to stretch the clock because the data was not yet written in I2C_TXDR register, the SCL line is kept at low level during the setup time. This setup time is \( t_{\text{SCLDEL}} = (\text{SCLDEL} + 1) \times t_{\text{PRESCL}} \) , where \( t_{\text{PRESCL}} = (\text{PRESC} + 1) \times t_{\text{I2CCLK}} \) . \( t_{\text{SCLDEL}} \) impacts the setup time \( t_{\text{SU:DAT}} \) .

To bridge the undefined region of the SDA transition (rising edge usually worst case), the user must program SCLDEL[3:0] so as to fulfill the following condition:

Refer to the following table for \( t_{\text{f}} \) , \( t_{\text{r}} \) , \( t_{\text{HD:DAT}} \) , \( t_{\text{VD:DAT}} \) , and \( t_{\text{SU:DAT}} \) standard values.

Use the SDA and SCL real transition time values measured in the application to widen the scope of allowed SDADEL[3:0] and SCLDEL[3:0] values. Use the maximum SDA and SCL transition time values defined in the standard to make the device work reliably regardless of the application.

Note: At every clock pulse, after SCL falling edge detection, I2C operating as master or slave stretches SCL low during at least \( [(\text{SDADEL} + \text{SCLDEL} + 1) \times (\text{PRESC} + 1) + 1] \times t_{\text{I2CCLK}} \) , in both transmission and reception modes. In transmission mode, if the data is not yet written in I2C_TXDR when SDA delay elapses, the I2C peripheral keeps stretching SCL low until the next data is written. Then new data MSB is sent on SDA output, and SCLDEL counter starts, continuing stretching SCL low to guarantee the data setup time.

When the NOSTRETCH bit is set in slave mode, the SCL is not stretched. The SDADEL[3:0] must then be programmed so that it ensures a sufficient setup time.

Additionally, in master mode, the SCL clock high and low levels must be configured by programming the PRESC[3:0], SCLH[7:0], and SCLL[7:0] bitfields of the I2C_TIMINGR register.

When the SCL falling edge is internally detected, the I2C peripheral releasing the SCL output after the delay \( t_{SCLL} = (SCLL + 1) \times t_{PRESCL} \) , where \( t_{PRESCL} = (PRESC + 1) \times t_{I2CCLK} \) . The \( t_{SCLL} \) delay impacts the SCL low time \( t_{LOW} \) .

When the SCL rising edge is internally detected, the I2C peripheral forces the SCL output to low level after the delay \( t_{SCLH} = (SCLH + 1) \times t_{PRESCL} \) , where \( t_{PRESCL} = (PRESC + 1) \times t_{I2CCLK} \) . The \( t_{SCLH} \) impacts the SCL high time \( t_{HIGH} \) .

Refer to I2C master initialization for more details.

Caution: Changing the timing configuration and the NOSTRETCH configuration is not allowed when the I2C peripheral is enabled. Like the timing settings, the slave NOSTRETCH settings must also be done before enabling the peripheral. Refer to I2C slave initialization for more details.

Figure 348. I2C initialization flow

graph TD; A([Initial settings]) --> B[Clear PE bit in I2C_CR1]; B --> C[Configure ANFOFF and DNF[3:0] in I2C_CR1]; C --> D["Configure PRESC[3:0], SDADEL[3:0], SCLDEL[3:0], SCLH[7:0], and SCLL[7:0] in I2C_TIMINGR"]; D --> E[Configure NOSTRETCH in I2C_CR1]; E --> F[Set PE bit in I2C_CR1]; F --> G([End]);

33.4.6 I2C reset

The reset of the I2C peripheral is performed by clearing the PE bit of the I2C_CR1 register. It has the effect of releasing the SCL and SDA lines. Internal state machines are reset and the communication control bits and the status bits revert to their reset values. This reset does not impact the configuration registers.

The impacted register bits are:

1. 1. I2C_CR2 register: START, STOP, PECBYTE, and NACK
2. 2. I2C_ISR register: BUSY, TXE, TXIS, RXNE, ADDR, NACKF, TCR, TC, STOPF, BERR, ARLO, PECERR, TIMEOUT, ALERT, and OVR

PE must be kept low during at least three APB clock cycles to perform the I2C reset. To ensure this, perform the following software sequence:

1. 1. Write PE = 0
2. 2. Check PE = 0
3. 3. Write PE = 1

33.4.7 I2C data transfer

The data transfer is managed through transmit and receive data registers and a shift register.

Reception

The SDA input fills the shift register. After the eighth SCL pulse (when the complete data byte is received), the shift register is copied into the I2C_RXDR register if it is empty (RXNE = 0). If RXNE = 1, which means that the previous received data byte has not yet

been read, the SCL line is stretched low until I2C_RXDR is read. The stretch occurs between the eighth and the ninth SCL pulse (before the acknowledge pulse).

Figure 349. Data reception

Transmission

If the I2C_TXDR register is not empty (TXE = 0), its content is copied into the shift register after the ninth SCL pulse (the acknowledge pulse). Then the shift register content is shifted out on the SDA line. If TXE = 1, which means that no data is written yet in I2C_TXDR, the SCL line is stretched low until I2C_TXDR is written. The stretch starts after the ninth SCL pulse.

Figure 350. Data transmission

Hardware transfer management

The I2C features an embedded byte counter to manage byte transfer and to close the communication in various modes, such as:

• – NACK, STOP and ReSTART generation in master mode
• – ACK control in slave receiver mode
• – PEC generation/checking

In master mode, the byte counter is always used. By default, it is disabled in slave mode. It can be enabled by software, by setting the SBC (slave byte control) bit of the I2C_CR1 register.

The number of bytes to transfer is programmed in the NBBYTES[7:0] bitfield of the I2C_CR2 register. If this number is greater than 255, or if a receiver wants to control the acknowledge value of a received data byte, the reload mode must be selected, by setting the RELOAD bit of the I2C_CR2 register. In this mode, the TCR flag is set when the number of bytes programmed in NBBYTES[7:0] is transferred (when the associated counter reaches zero), and an interrupt is generated if TCIE is set. SCL is stretched as long as the TCR flag is set. TCR is cleared by software when NBBYTES[7:0] is written to a non-zero value.

When NBBYTES[7:0] is reloaded with the last number of bytes to transfer, the RELOAD bit must be cleared.

When RELOAD = 0 in master mode, the counter can be used in two modes:

• • Automatic end (AUTOEND = 1 in the I2C_CR2 register). In this mode, the master automatically sends a STOP condition once the number of bytes programmed in the NBBYTES[7:0] bitfield is transferred.
• • Software end (AUTOEND = 0 in the I2C_CR2 register). In this mode, a software action is expected once the number of bytes programmed in the NBBYTES[7:0] bitfield is transferred; the TC flag is set and an interrupt is generated if the TCIE bit is set. The SCL signal is stretched as long as the TC flag is set. The TC flag is cleared by software when the START or STOP bit of the I2C_CR2 register is set. This mode must be used when the master wants to send a RESTART condition.

Caution: The AUTOEND bit has no effect when the RELOAD bit is set.

Table 213. I2C configuration

33.4.8 I2C slave mode

I2C slave initialization

To work in slave mode, the user must enable at least one slave address. The I2C_OAR1 and I2C_OAR2 registers are available to program the slave own addresses OA1 and OA2, respectively.

OA1 can be configured either in 7-bit (default) or in 10-bit addressing mode, by setting the OA1MODE bit of the I2C_OAR1 register.

OA1 is enabled by setting the OA1EN bit of the I2C_OAR1 register.

If an additional slave addresses are required, the second slave address OA2 can be configured. Up to seven OA2 LSBs can be masked, by configuring the OA2MSK[2:0] bitfield of the I2C_OAR2 register. Therefore, for OA2MSK[2:0] configured from 1 to 6, only OA2[7:2], OA2[7:3], OA2[7:4], OA2[7:5], OA2[7:6], or OA2[7] are compared with the received address. When OA2MSK[2:0] is other than 0, the address comparator for OA2 excludes the I2C reserved addresses (0000 XXX and 1111 XXX) and they are not acknowledged. If OA2MSK[2:0] = 7, all received 7-bit addresses are acknowledged (except reserved addresses). OA2 is always a 7-bit address.

When enabled through the specific bit, the reserved addresses can be acknowledged if they are programmed in the I2C_OAR1 or I2C_OAR2 register with OA2MSK[2:0] = 0.

OA2 is enabled by setting the OA2EN bit of the I2C_OAR2 register.

The general call address is enabled by setting the GCEN bit of the I2C_CR1 register.

When the I2C peripheral is selected by one of its enabled addresses, the ADDR interrupt status flag is set, and an interrupt is generated if the ADDRIE bit is set.

By default, the slave uses its clock stretching capability, which means that it stretches the SCL signal at low level when required, to perform software actions. If the master does not support clock stretching, I2C must be configured with NOSTRETCH = 1 in the I2C_CR1 register.

After receiving an ADDR interrupt, if several addresses are enabled, the user must read the ADDCODE[6:0] bitfield of the I2C_ISR register to check which address matched. The DIR flag must also be checked to know the transfer direction.

Slave with clock stretching

As long as the NOSTRETCH bit of the I2C_CR1 register is zero (default), the I2C peripheral operating as an I ² C-bus slave stretches the SCL signal in the following situations:

• • The ADDR flag is set and the received address matches with one of the enabled slave addresses.
  The stretch is released when the software clears the ADDR flag by setting the ADDRCF bit.
• • In transmission, the previous data transmission is completed and no new data is written in I2C_TXDR register, or the first data byte is not written when the ADDR flag is cleared (TXE = 1).
  The stretch is released when the data is written to the I2C_TXDR register.
• • In reception, the I2C_RXDR register is not read yet and a new data reception is completed.
  The stretch is released when I2C_RXDR is read.
• • In slave byte control mode (SBC bit set) with reload (RELOAD bit set), the last data byte transfer is finished (TCR bit set).
  The stretch is released when then TCR is cleared by writing a non-zero value in the NBYTES[7:0] bitfield.
• • After SCL falling edge detection.
  The stretch is released after \( [(\text{SDADEL} + \text{SCLDEL} + 1) \times (\text{PRESC} + 1) + 1] \times t_{\text{I2CCLK}} \) period.

Slave without clock stretching

As long as the NOSTRETCH bit of the I2C_CR1 register is set, the I2C peripheral operating as an I ² C-bus slave does not stretch the SCL signal.

The SCL clock is not stretched while the ADDR flag is set.

In transmission, the data must be written in the I2C_TXDR register before the first SCL pulse corresponding to its transfer occurs. If not, an underrun occurs, the OVR flag is set in the I2C_ISR register and an interrupt is generated if the ERRIE bit of the I2C_CR1 register is set. The OVR flag is also set when the first data transmission starts and the STOPF bit is still set (has not been cleared). Therefore, if the user clears the STOPF flag of the previous transfer only after writing the first data to be transmitted in the next transfer, it ensures that the OVR status is provided, even for the first data to be transmitted.

In reception, the data must be read from the I2C_RXDR register before the ninth SCL pulse (ACK pulse) of the next data byte occurs. If not, an overrun occurs, the OVR flag is set in the I2C_ISR register, and an interrupt is generated if the ERRIE bit of the I2C_CR1 register is set.

Slave byte control mode

To allow byte ACK control in slave reception mode, the slave byte control mode must be enabled, by setting the SBC bit of the I2C_CR1 register. This is required to comply with SMBus standards.

The reload mode must be selected to allow byte ACK control in slave reception mode (RELOAD = 1). To get control of each byte, NBBYTES[7:0] must be initialized to 0x1 in the ADDR interrupt subroutine, and reloaded to 0x1 after each received byte. When the byte is received, the TCR bit is set, stretching the SCL signal low between the eighth and the ninth SCL pulse. The user can read the data from the I2C_RXDR register, and then decide to acknowledge it or not by configuring the ACK bit of the I2C_CR2 register. The SCL stretch is released by programming NBBYTES to a non-zero value: the acknowledge or not-acknowledge is sent and the next byte can be received.

NBYTES[7:0] can be loaded with a value greater than 0x1. Receiving then continues until the corresponding number of bytes are received.

Note: The SBC bit must be configured when the I2C peripheral is disabled, when the slave is not addressed, or when ADDR = 1.

The RELOAD bit value can be changed when ADDR = 1, or when TCR = 1.

Caution: The slave byte control mode is not compatible with NOSTRETCH mode. Setting SBC when NOSTRETCH = 1 is not allowed.

Figure 351. Slave initialization flow

graph TD; A([Slave initialization]) --> B[Initial settings]; B --> C[Clear OA1EN and OA2EN in I2C_OAR1/I2C_OAR2]; C --> D["Configure OA1[9:0], OA1MODE, OA1EN, OA2[6:0], OA2MSK[2:0], OA2EN, and GCEN"]; D --> E["Optional: Configure SBC in I2C_CR1(1)"]; E --> F[Enable interrupts and/or DMA in I2C_CR1]; F --> G([End]);

1. 1. SBC must be set to support SMBus features.

Slave transmitter

A transmit interrupt status (TXIS) flag is generated when the I2C_TXDR register becomes empty. An interrupt is generated if the TXIE bit of the I2C_CR1 register is set.

The TXIS flag is cleared when the I2C_TXDR register is written with the next data byte to transmit.

When NACK is received, the NACKF flag is set in the I2C_ISR register and an interrupt is generated if the NACKIE bit of the I2C_CR1 register is set. The slave automatically releases the SCL and SDA lines to let the master perform a STOP or a RESTART condition. The TXIS bit is not set when a NACK is received.

When STOP is received and the STOPIE bit of the I2C_CR1 register is set, the STOPF flag of the I2C_ISR register is set and an interrupt is generated. In most applications, the SBC bit is usually programmed to 0. In this case, if TXE = 0 when the slave address is received (ADDR = 1), the user can choose either to send the content of the I2C_TXDR register as the first data byte, or to flush the I2C_TXDR register, by setting the TXE bit in order to program a new data byte.

In slave byte control mode (SBC = 1), the number of bytes to transmit must be programmed in NBBYTES[7:0] in the address match interrupt subroutine (ADDR = 1). In this case, the number of TXIS events during the transfer corresponds to the value programmed in NBBYTES[7:0].

Caution: When NOSTRETCH = 1, the SCL clock is not stretched while the ADDR flag is set, so the user cannot flush the I2C_TXDR register content in the ADDR subroutine to program the

first data byte. The first data byte to send must be previously programmed in the I2C_TXDR register:

• • This data can be the one written in the last TXIS event of the previous transmission message.
• • If this data byte is not the one to send, the I2C_TXDR register can be flushed, by setting the TXE bit, to program a new data byte. The STOPF bit must be cleared only after these actions. This guarantees that they are executed before the first data transmission starts, following the address acknowledge.

If STOPF is still set when the first data transmission starts, an underrun error is generated (the OVR flag is set).

If a TXIS event (transmit interrupt or transmit DMA request) is required, the user must set the TXIS bit in addition to the TXE bit, to generate the event.

Figure 352. Transfer sequence flow for I2C slave transmitter, NOSTRETCH = 0

graph TD; Start([Slave transmission]) --> Init[Slave initialization]; Init --> ADDR{I2C_ISR.ADDR = 1?}; ADDR -- No --> ADDR; ADDR -- Yes --> Read[Read ADDCODE and DIR in I2C_ISR<br/>Optional: Set I2C_ISR.TXE = 1<br/>Set I2C_ICR.ADDRCF]; Read --> TXIS{I2C_ISR.TXIS = 1?}; TXIS -- No --> TXIS; TXIS -- Yes --> Write[Write I2C_TXDR.TXDATA]; Write --> TXIS;

Figure 353. Transfer sequence flow for I2C slave transmitter, NOSTRETCH = 1

graph TD; Start([Slave transmission]) --> Init[Slave initialization]; Init --> LoopStart(( )); LoopStart --> TXIS{I2C_ISR.TXIS = 1?}; TXIS -- No --> STOPF{I2C_ISR.STOPF = 1?}; STOPF -- Yes --> Optional["Optional: Set I2C_ISR.TXE = 1 and I2C_ISR.TXIS=1"]; Optional --> STOPCF[Set I2C_ICR.STOPCF]; STOPCF --> LoopStart; TXIS -- Yes --> TXDATA[Write I2C_TXDR.TXDATA]; TXDATA --> LoopStart;

Figure 354. Transfer bus diagrams for I2C slave transmitter (mandatory events only)

Slave receiver

The RXNE bit of the I2C_ISR register is set when the I2C_RXDR is full, which generates an interrupt if the RXIE bit of the I2C_CR1 register is set. RXNE is cleared when I2C_RXDR is read.

When STOP condition is received and the STOPIE bit of the I2C_CR1 register is set, the STOPF flag in the I2C_ISR register is set and an interrupt is generated.

Figure 355. Transfer sequence flow for I2C slave receiver, NOSTRETCH = 0

graph TD; Start([Slave reception]) --> Init[Slave initialization]; Init --> ADDR{I2C_ISR.ADDR = 1?}; ADDR -- No --> Init; ADDR -- Yes --> Read[Read ADDCODE and DIR in I2C_ISR<br/>Set I2C_ICR.ADDRCF]; Read --> RXNE{I2C_ISR.RXNE = 1?}; RXNE -- No --> Init; RXNE -- Yes --> Write[Write I2C_RXDR.RXDATA]; Write --> Init;

Figure 356. Transfer sequence flow for I2C slave receiver, NOSTRETCH = 1

graph TD
    Start([Slave reception]) --> Init[Slave initialization]
    Init --> RXNE{I2C_ISR.RXNE = 1?}
    RXNE -- Yes --> Read[Read I2C_RXDR.RXDATA]
    Read --> RXNE
    RXNE -- No --> STOPF{I2C_ISR.STOPF = 1?}
    STOPF -- Yes --> StopCF[Set I2C_ICR.STOPCF]
    StopCF --> RXNE
    STOPF -- No --> RXNE
    

Figure 357. Transfer bus diagrams for I2C slave receiver (mandatory events only)

33.4.9 I2C master mode

I2C master initialization

Before enabling the peripheral, the I2C master clock must be configured, by setting the SCLH and SCLL bits in the I2C_TIMINGR register.

The STM32CubeMX tool calculates and provides the I2C_TIMINGR content in the I2C Configuration window.

A clock synchronization mechanism is implemented in order to support multi-master environment and slave clock stretching.

In order to allow clock synchronization:

• • The low level of the clock is counted using the SCLL counter, starting from the SCL low level internal detection.
• • The high level of the clock is counted using the SCLH counter, starting from the SCL high level internal detection.

I2C detects its own SCL low level after a \( t_{\text{SYNC1}} \) delay depending on the SCL falling edge, SCL input noise filters (analog and digital), and SCL synchronization to the I2CxCLK clock. I2C releases SCL to high level once the SCLL counter reaches the value programmed in the SCLL[7:0] bitfield of the I2C_TIMINGR register.

I2C detects its own SCL high level after a \( t_{\text{SYNC2}} \) delay depending on the SCL rising edge, SCL input noise filters (analog and digital), and SCL synchronization to the I2CxCLK clock. I2C ties SCL to low level once the SCLH counter reaches the value programmed in the SCLH[7:0] bitfield of the I2C_TIMINGR register.

Consequently the master clock period is:

The duration of \( t_{\text{SYNC1}} \) depends upon:

• • SCL falling slope
• • input delay induced by the analog filter (when enabled)
• • input delay induced by the digital filter (when enabled): \( \text{DNF}[3:0] \times t_{\text{I2CCLK}} \)
• • delay due to SCL synchronization with the I2CCLK clock (two to three I2CCLK periods)

The duration of \( t_{\text{SYNC2}} \) depends upon:

• • SCL rising slope
• • input delay induced by the analog filter (when enabled)
• • input delay induced by the digital filter (when enabled): \( \text{DNF}[3:0] \times t_{\text{I2CCLK}} \)
• • delay due to SCL synchronization with the I2CCLK clock (two to three I2CCLK periods)

Figure 358. Master clock generation

Caution: For compliance with the I ² C-bus or SMBus specification, the master clock must respect the timings in the following table.

Table 214. I ² C-bus and SMBus specification clock timings

Note: The SCLL[7:0] bitfield also determines the t _BUF and t _SU:STA timings and SCLH[7:0] the t _HD:STA and t _SU:STO timings.

Refer to Section 33.4.10 for examples of I2C_TIMINGR settings versus the I2CCLK frequency.

Master communication initialization (address phase)

To initiate the communication with a slave to address, set the following bitfields of the I2C_CR2 register:

• • ADD10: addressing mode (7-bit or 10-bit)
• • SADD[9:0]: slave address to send
• • RD_WRN: transfer direction
• • HEAD10R: in case of 10-bit address read, this bit determines whether the header only (for direction change) or the complete address sequence is sent.
• • NBYTES[7:0]: the number of bytes to transfer; if equal to or greater than 255 bytes, the bitfield must initially be set to 0xFF.

Note: Changing these bitfields is not allowed as long as the START bit is set.

Before launching the communication, make sure that the I ² C-bus is idle. This can be checked using the bus idle detection function or by verifying that the IDR bits of the GPIOs selected as SDA and SCL are set. Any low-level incident on the I ² C-bus lines that coincides with the START condition asserted by the I2C peripheral may cause its deadlock if not filtered out by the input filters. If such incidents cannot be prevented, design the software so that it restores the normal operation of the I2C peripheral in case of a deadlock, by toggling the PE bit of the I2C_CR1 register.

To launch the communication, set the START bit of the I2C_CR2 register. The master then automatically sends a START condition followed by the slave address, either immediately if the BUSY flag is low, or \( t_{BUF} \) time after the BUSY flag transits from high to low state. The BUSY flag is set upon sending the START condition.

In case of an arbitration loss, the master automatically switches back to slave mode and can acknowledge its own address if it is addressed as a slave.

Note: The START bit is reset by hardware when the slave address is sent on the bus, whatever the received acknowledge value. The START bit is also reset by hardware upon arbitration loss.

In 10-bit addressing mode, the master automatically keeps resending the slave address in a loop until the first address byte (first seven address bits) is acknowledged by the slave.

Setting the ADDRCF bit makes I2C quit that loop.

If the I2C peripheral is addressed as a slave (ADDR = 1) while the START bit is set, the I2C peripheral switches to slave mode and the START bit is cleared when the ADDRCF bit is set.

Note: The same procedure is applied for a repeated START condition. In this case, BUSY = 1.

Figure 359. Master initialization flow

graph TD
    A([Master initialization]) --> B[Initial settings]
    B --> C[Enable interrupts and/or DMA in I2C_CR1]
    C --> D([End])

Initialization of a master receiver addressing a 10-bit address slave

If the slave address is in 10-bit format, the user can choose to send the complete read sequence, by clearing the HEAD10R bit of the I2C_CR2 register. In this case, the master automatically sends the following complete sequence after the START bit is set:

(RE)START + Slave address 10-bit header Write + Slave address second byte + (RE)START + Slave address 10-bit header Read.

Figure 360. 10-bit address read access with HEAD10R = 0

If the master addresses a 10-bit address slave, transmits data to this slave and then reads data from the same slave, a master transmission flow must be done first. Then a repeated START is set with the 10-bit slave address configured with HEAD10R = 1. In this case, the master sends this sequence:

RESTART + Slave address 10-bit header Read.

Figure 361. 10-bit address read access with HEAD10R = 1

Master transmitter

In the case of a write transfer, the TXIS flag is set after each byte transmission, after the ninth SCL pulse when an ACK is received.

A TXIS event generates an interrupt if the TXIE bit of the I2C_CR1 register is set. The flag is cleared when the I2C_TXDR register is written with the next data byte to transmit.

The number of TXIS events during the transfer corresponds to the value programmed in NBBYTES[7:0]. If the total number of data bytes to transmit is greater than 255, the reload mode must be selected by setting the RELOAD bit in the I2C_CR2 register. In this case, when the NBBYTES[7:0] number of data bytes is transferred, the TCR flag is set and the SCL line is stretched low until NBBYTES[7:0] is written with a non-zero value.

When RELOAD = 0 and the number of data bytes defined in NBBYTES[7:0] is transferred:

• • In automatic end mode (AUTOEND = 1), a STOP condition is automatically sent.
• • In software end mode (AUTOEND = 0), the TC flag is set and the SCL line is stretched low, to perform software actions:
  • – A RESTART condition can be requested by setting the START bit of the I2C_CR2 register with the proper slave address configuration and the number of bytes to transfer. Setting the START bit clears the TC flag and sends the START condition on the bus.
  • – A STOP condition can be requested by setting the STOP bit of the I2C_CR2 register. This clears the TC flag and sends a STOP condition on the bus.

When a NACK is received, the TXIS flag is not set and a STOP condition is automatically sent. The NACKF flag of the I2C_ISR register is set. An interrupt is generated if the NACKIE bit is set.

Figure 362. Transfer sequence flow for I2C master transmitter, N ≤ 255 bytes

graph TD; Start([Master transmission]) --> Init[Master initialization]; Init --> Config["NBYS = N<br/>AUTOEND = 0 for RESTART; 1 for STOP<br/>Configure slave address<br/>Set I2C_CR2.START"]; Config --> LoopStart(( )); LoopStart --> NACKF{"I2C_ISR.NACKF = 1?"}; NACKF -- Yes --> End1([End]); NACKF -- No --> TXIS{"I2C_ISR.TXIS = 1?"}; TXIS -- No --> LoopStart; TXIS -- Yes --> Write[Write I2C_TXDR]; Write --> NBYSTrans{"NBYS transmitted?"}; NBYSTrans -- No --> LoopStart; NBYSTrans -- Yes --> TC{"I2C_ISR.TC = 1?"}; TC -- Yes --> SetStart["Set I2C_CR2.START with<br/>slave address NBYS ..."]; SetStart --> End2([End]); TC -- No --> End2; End2 -.-> MS19860V2[MS19860V2];

Figure 363. Transfer sequence flow for I2C master transmitter, N > 255 bytes

graph TD; Start([Master transmission]) --> Init[Master initialization]; Init --> Config["NBYTES = 0xFF; N=N-255<br/>RELOAD = 1<br/>Configure slave address<br/>Set I2C_CR2.START"]; Config --> LoopStart(( )); LoopStart --> NACKF{I2C_ISR.NACKF = 1?}; NACKF -- Yes --> End1([End]); NACKF -- No --> TXIS{I2C_ISR.TXIS = 1?}; TXIS -- No --> LoopStart; TXIS -- Yes --> Write[Write I2C_TXDR]; Write --> Transmitted{NBYTES transmitted ?}; Transmitted -- No --> LoopStart; Transmitted -- Yes --> TC{I2C_ISR.TC = 1?}; TC -- Yes --> SetStart["Set I2C_CR2.START<br/>with slave address<br/>NBYTES ..."]; SetStart -.-> Dots[...]; Dots --> TCR{I2C_ISR.TCR = 1?}; TC -- No --> TCR; TCR -- Yes --> End2([End]); TCR -- No --> Reconfig["IF N < 256<br/>NBYTES = N; N = 0; RELOAD = 0<br/>AUTOEND = 0 for RESTART; 1 for STOP<br/>ELSE<br/>NBYTES = 0xFF; N = N-255<br/>RELOAD = 1"]; Reconfig --> LoopStart;

MS19861V3

Figure 364. Transfer bus diagrams for I2C master transmitter (mandatory events only)

Master receiver

In the case of a read transfer, the RXNE flag is set after each byte reception, after the eighth SCL pulse. An RXNE event generates an interrupt if the RXIE bit of the I2C_CR1 register is set. The flag is cleared when I2C_RXDR is read.

If the total number of data bytes to receive is greater than 255, select the reload mode, by setting the RELOAD bit of the I2C_CR2 register. In this case, when the NBBYTES[7:0] number of data bytes is transferred, the TCR flag is set and the SCL line is stretched low until NBBYTES[7:0] is written with a non-zero value.

When RELOAD = 0 and the number of data bytes defined in NBBYTES[7:0] is transferred:

• • In automatic end mode (AUTOEND = 1), a NACK and a STOP are automatically sent after the last received byte.
• • In software end mode (AUTOEND = 0), a NACK is automatically sent after the last received byte. The TC flag is set and the SCL line is stretched low in order to allow software actions:
  • – A RESTART condition can be requested by setting the START bit of the I2C_CR2 register, with the proper slave address configuration and the number of bytes to transfer. Setting the START bit clears the TC flag and sends the START condition and the slave address on the bus.
  • – A STOP condition can be requested by setting the STOP bit of the I2C_CR2 register. This clears the TC flag and sends a STOP condition on the bus.

Figure 365. Transfer sequence flow for I2C master receiver, N ≤ 255 bytes

graph TD; Start([Master reception]) --> Init[Master initialization]; Init --> Config["NBYTES = N<br/>AUTOEND = 0 for RESTART; 1 for STOP<br/>Configure slave address<br/>Set I2C_CR2.START"]; Config --> RXNE{"I2C_ISR.RXNE<br/>= 1?"}; RXNE -- No --> RXNE; RXNE -- Yes --> Read[Read I2C_RXDR]; Read --> NBYTES{"NBYTES<br/>received?"}; NBYTES -- No --> RXNE; NBYTES -- Yes --> TC{"I2C_ISR.TC<br/>= 1?"}; TC -- Yes --> SetStart["Set I2C_CR2.START with<br/>slave address NBYTES ..."]; TC -- No --> End([End]); SetStart -.-> Dots[...];

MS19863V2

Figure 366. Transfer sequence flow for I2C master receiver, N > 255 bytes

graph TD; Start([Master reception]) --> Init[Master initialization]; Init --> Config["NBYTES = 0xFF; N=N-255<br>RELOAD =1<br>Configure slave address<br>Set I2C_CR2.START"]; Config --> RXNE{I2C_ISR.RXNE =1?}; RXNE -- No --> End([End]); RXNE -- Yes --> Read[Read I2C_RXDR]; Read --> NBytes{NBYTES received?}; NBytes -- No --> End; NBytes -- Yes --> TC{I2C_ISR.TC = 1?}; TC -- Yes --> SetStart["Set I2C_CR2.START with<br>slave address NBYTES ..."]; SetStart --> Dots[...]; Dots --> TCR{I2C_ISR.TCR = 1?}; TC -- No --> TCR; TCR -- Yes --> IFN["IF N< 256<br>NBYTES =N; N=0;RELOAD=0<br>AUTOEND=0 for RESTART; 1 for STOP<br>ELSE<br>NBYTES =0xFF;N=N-255<br>RELOAD=1"]; TCR -- No --> End; IFN --> End; IFN --> Config;

MS19864V2

Figure 367. Transfer bus diagrams for I2C master receiver (mandatory events only)

33.4.10 I2C_TIMINGR register configuration examples

The following tables provide examples of how to program the I2C_TIMINGR register to obtain timings compliant with the I2C-bus specification. To get more accurate configuration values, use the STM32CubeMX tool (I2C Configuration window).

Table 215. Timing settings for \( f_{I2CCLK} \) of 8 MHz

1. 1. \( t_{SCL} \) is greater than \( t_{SCLL} + t_{SCLH} \) due to SCL internal detection delay. Values provided for \( t_{SCL} \) are examples only.
2. 2. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times f_{I2CCLK} = 500 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 1000 \) ns.
3. 3. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times f_{I2CCLK} = 500 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 750 \) ns.
4. 4. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times f_{I2CCLK} = 500 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 655 \) ns.

Table 216. Timing settings for \( f_{I2CCLK} \) of 16 MHz

1. 1. \( t_{SCL} \) is greater than \( t_{SCLL} + t_{SCLH} \) due to SCL internal detection delay. Values provided for \( t_{SCL} \) are examples only.
2. 2. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times f_{I2CCLK} = 250 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 1000 \) ns.
3. 3. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times f_{I2CCLK} = 250 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 750 \) ns.
4. 4. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times f_{I2CCLK} = 250 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 500 \) ns.

1. \( t_{SCL} \) is greater than \( t_{SCLL} + t_{SCLH} \) due to the SCL internal detection delay. Values provided for \( t_{SCL} \) are only examples.

2. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times t_{I2CCLK} = 83.3 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 1000 \) ns

3. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times t_{I2CCLK} = 83.3 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 750 \) ns

4. \( t_{SYNC1} + t_{SYNC2} \) minimum value is \( 4 \times t_{I2CCLK} = 83.3 \) ns. Example with \( t_{SYNC1} + t_{SYNC2} = 250 \) ns

33.4.11 SMBus specific features

Introduction

The system management bus (SMBus) is a two-wire interface through which various devices can communicate with each other and with the rest of the system. It is based on operation principles of the I ² C-bus. The SMBus provides a control bus for system and power management related tasks.

The I2C peripheral is compatible with the SMBus specification ( http://smbus.org ).

The system management bus specification refers to three types of devices:

• • Slave is a device that receives or responds to a command.
• • Master is a device that issues commands, generates clocks, and terminates the transfer.
• • Host is a specialized master that provides the main interface to the system CPU. A host must be a master-slave and must support the SMBus host notify protocol. Only one host is allowed in a system.

The I2C peripheral can be configured as a master or a slave device, and also as a host.

Bus protocols

There are eleven possible command protocols for any given device. The device can use any or all of them to communicate. These are: Quick Command , Send Byte , Receive Byte , Write Byte , Write Word , Read Byte , Read Word , Process Call , Block Read , Block Write , and Block Write-Block Read Process Call . The protocols must be implemented by the user software.

For more details on these protocols, refer to the SMBus specification ( http://smbus.org ). STM32CubeMX implements an SMBus stack thanks to X-CUBE-SMBUS, a downloadable software pack that allows basic SMBus configuration per I2C instance.

Address resolution protocol (ARP)

SMBus slave address conflicts can be resolved by dynamically assigning a new unique address to each slave device. To provide a mechanism to isolate each device for the purpose of address assignment, each device must implement a unique 128-bit device identifier (UDID). In the I2C peripheral, it is implemented by software.

The I2C peripheral supports the Address resolution protocol (ARP). The SMBus device default address (0b1100 001) is enabled by setting the SMBDEN bit of the I2C_CR1 register. The ARP commands must be implemented by the user software.

Arbitration is also performed in slave mode for ARP support.

For more details on the SMBus address resolution protocol, refer to the SMBus specification ( http://smbus.org ).

Received command and data acknowledge control

An SMBus receiver must be able to NACK each received command or data. In order to allow the ACK control in slave mode, the slave byte control mode must be enabled, by setting the SBC bit of the I2C_CR1 register. Refer to Slave byte control mode for more details.

Host notify protocol

To enable the host notify protocol, set the SMBHEN bit of the I2C_CR1 register. The I2C peripheral then acknowledges the SMBus host address (0b0001 000).

When this protocol is used, the device acts as a master and the host as a slave.

SMBus alert

The I2C peripheral supports the SMBALERT# optional signal through the SMBA pin. With the SMBALERT# signal, an SMBus slave device can signal to the SMBus host that it wants to talk. The host processes the interrupt and simultaneously accesses all SMBALERT# devices through the alert response address (0b0001 100). Only the device/devices which pulled SMBALERT# low acknowledges/acknowledge the alert response address.

When the I2C peripheral is configured as an SMBus slave device (SMBHEN = 0), the SMBA pin is pulled low by setting the ALERTEN bit of the I2C_CR1 register. The alert response address is enabled at the same time.

When the I2C peripheral is configured as an SMBus host (SMBHEN = 1), the ALERT flag of the I2C_ISR register is set when a falling edge is detected on the SMBA pin and ALERTEN = 1. An interrupt is generated if the ERRIE bit of the I2C_CR1 register is set. When ALERTEN = 0, the alert line is considered high even if the external SMBA pin is low.

Note: If the SMBus alert pin is not required, keep the ALERTEN bit cleared. The SMBA pin can then be used as a standard GPIO.

Packet error checking

A packet error checking mechanism introduced in the SMBus specification improves reliability and communication robustness. The packet error checking is implemented by

appending a packet error code (PEC) at the end of each message transfer. The PEC is calculated by using the \( C(x) = x^8 + x^2 + x + 1 \) CRC-8 polynomial on all the message bytes (including addresses and read/write bits).

The I2C peripheral embeds a hardware PEC calculator and allows a not acknowledge to be sent automatically when the received byte does not match the hardware calculated PEC.

Timeouts

To comply with the SMBus timeout specifications, the I2C peripheral embeds hardware timers.

Table 218. SMBus timeout specifications

1. 1. \( t_{LOW:SEXT} \) is the cumulative time a given slave device is allowed to extend the clock cycles in one message from the initial START to the STOP. It is possible that another slave device or the master also extends the clock causing the combined clock low extend time to be greater than \( t_{LOW:SEXT} \) . Therefore, this parameter is measured with the slave device as the sole target of a full-speed master.
2. 2. \( t_{LOW:MEXT} \) is the cumulative time a master device is allowed to extend its clock cycles within each byte of a message as defined from START-to-ACK, ACK-to-ACK, or ACK-to-STOP. It is possible that a slave device or another master also extends the clock, causing the combined clock low time to be greater than \( t_{LOW:MEXT} \) on a given byte. Therefore, this parameter is measured with a full speed slave device as the sole target of the master.

Figure 368. Timeout intervals for \( t_{LOW:SEXT} \) , \( t_{LOW:MEXT} \)

Bus idle detection

A master can assume that the bus is free if it detects that the clock and data signals have been high for \( t_{\text{IDLE}} > t_{\text{HIGH}}(\text{max}) \) (refer to the table in Section 33.4.9 ).

This timing parameter covers the condition where a master is dynamically added to the bus, and may not have detected a state transition on the SMBCLK or SMBDAT lines. In this case, the master must wait long enough to ensure that a transfer is not currently in progress. The I2C peripheral supports a hardware bus idle detection.

33.4.12 SMBus initialization

In addition to the I2C initialization for the I ² C-bus, the use of the peripheral for the SMBus communication requires some extra initialization steps.

Received command and data acknowledge control (slave mode)

An SMBus receiver must be able to NACK each received command or data. To allow ACK control in slave mode, the slave byte control mode must be enabled, by setting the SBC bit of the I2C_CR1 register. Refer to Slave byte control mode for more details.

Specific addresses (slave mode)

The specific SMBus addresses must be enabled if required. Refer to Bus idle detection for more details.

The SMBus device default address (0b1100 001) is enabled by setting the SMBDEN bit of the I2C_CR1 register.

The SMBus host address (0b0001 000) is enabled by setting the SMBHEN bit of the I2C_CR1 register.

The alert response address (0b0001100) is enabled by setting the ALERTEN bit of the I2C_CR1 register.

Packet error checking

PEC calculation is enabled by setting the PECEN bit of the I2C_CR1 register. Then the PEC transfer is managed with the help of the hardware byte counter associated with the NBYTES[7:0] bitfield of the I2C_CR2 register. The PECEN bit must be configured before enabling the I2C.

The PEC transfer is managed with the hardware byte counter, so the SBC bit must be set when interfacing the SMBus in slave mode. The PEC is transferred after transferring NBYTES[7:0] - 1 data bytes, if the PECBYTE bit is set and the RELOAD bit is cleared. If RELOAD is set, PECBYTE has no effect.

Caution: Changing the PECEN configuration is not allowed when the I2C peripheral is enabled.

Table 219. SMBus with PEC configuration

Timeout detection

The timeout detection is enabled by setting the TIMOUTEN and TEXTEN bits of the I2C_TIMEOUTR register. The timers must be programmed in such a way that they detect a timeout before the maximum time given in the SMBus specification.

\( t_{\text{TIMEOUT}} \) check

To check the \( t_{\text{TIMEOUT}} \) parameter, load the 12-bit TIMEOUTA[11:0] bitfield with the timer reload value. Keep the TIDLE bit at 0 to detect the SCL low level timeout.

Then set the TIMOUTEN bit of the I2C_TIMEOUTR register, to enable the timer.

If SCL is tied low for longer than the \( (\text{TIMEOUTA} + 1) \times 2048 \times t_{\text{I2CCLK}} \) period, the TIMEOUT flag of the I2C_ISR register is set.

Refer to Table 220 .

Caution: Changing the TIMEOUTA[11:0] bitfield and the TIDLE bit values is not allowed when the TIMOUTEN bit is set.

\( t_{\text{LOW:SEXT}} \) and \( t_{\text{LOW:MEXT}} \) check

A 12-bit timer associated with the TIMEOUTB[11:0] bitfield allows checking \( t_{\text{LOW:SEXT}} \) for the I2C peripheral operating as a slave, or \( t_{\text{LOW:MEXT}} \) when it operates as a master. As the standard only specifies a maximum, the user can choose the same value for both. The timer is then enabled by setting the TEXTEN bit in the I2C_TIMEOUTR register.

If the SMBus peripheral performs a cumulative SCL stretch for longer than the \( (\text{TIMEOUTB} + 1) \times 2048 \times t_{\text{I2CCLK}} \) period, and within the timeout interval described in Bus idle detection section, the TIMEOUT flag of the I2C_ISR register is set.

Refer to Table 221 .

Caution: Changing the TIMEOUTB[11:0] bitfield value is not allowed when the TEXTEN bit is set.

Bus idle detection

To check the \( t_{\text{IDLE}} \) period, the TIMEOUTA[11:0] bitfield associated with 12-bit timer must be loaded with the timer reload value. Keep the TIDLE bit at 1 to detect both SCL and SDA high level timeout. Then set the TIMOUTEN bit of the I2C_TIMEOUTR register to enable the timer.

If both the SCL and SDA lines remain high for longer than the \( (\text{TIMEOUTA} + 1) \times 4 \times t_{\text{I2CCLK}} \) period, the TIMEOUT flag of the I2C_ISR register is set.

Refer to Table 222 .

Caution: Changing the TIMEOUTA[11:0] bitfield and the TIDLE bit values is not allowed when the TIMOUTEN bit is set.

33.4.13 SMBus I2C_TIMEOUTR register configuration examples

The following tables provide examples of settings to reach target \( t_{TIMEOUT} \) , \( t_{LOW:SEXT} \) , \( t_{LOW:MEXT} \) , and \( t_{IDLE} \) timings at different \( f_{I2CCLK} \) frequencies.

Table 220. TIMEOUTA[11:0] for maximum \( t_{TIMEOUT} \) of 25 ms

Table 221. TIMEOUTB[11:0] for maximum \( t_{LOW:SEXT} \) and \( t_{LOW:MEXT} \) of 8 ms

Table 222. TIMEOUTA[11:0] for maximum \( t_{IDLE} \) of 50 \( \mu\text{s} \)

33.4.14 SMBus slave mode

In addition to I2C slave transfer management (refer to Section 33.4.8: I2C slave mode ), this section provides extra software flowcharts to support SMBus.

SMBus slave transmitter

When using the I2C peripheral in SMBus mode, set the SBC bit to enable the PEC transmission at the end of the programmed number of data bytes. When the PECBYTE bit is set, the number of bytes programmed in NBBYTES[7:0] includes the PEC transmission. In that case, the total number of TXIS interrupts is NBBYTES[7:0] - 1, and the content of the I2C_PECR register is automatically transmitted if the master requests an extra byte after the transfer of the NBBYTES[7:0] - 1 data bytes.

Caution: The PECBYTE bit has no effect when the RELOAD bit is set.

Figure 369. Transfer sequence flow for SMBus slave transmitter N bytes + PEC

graph TD
    Start([SMBus slave transmission]) --> Init[Slave initialization]
    Init --> ADDR_Check{I2C_ISR.ADDR = 1?}
    ADDR_Check -- No --> ADDR_Check
    ADDR_Check -- Yes --> Read_ADDR[Read ADDCODE and DIR in I2C_ISR
I2C_CR2.NBYTES = N + 1
PECBYTE=1
Set I2C_ICR.ADDRCF]
    Read_ADDR --> TXIS_Check{I2C_ISR.TXIS = 1?}
    TXIS_Check -- No --> TXIS_Check
    TXIS_Check -- Yes --> Write_TXDATA[Write I2C_TXDR.TXDATA]
    Write_TXDATA --> TXIS_Check

Figure 370. Transfer bus diagram for SMBus slave transmitter (SBC = 1)

SMBus slave receiver

When using the I2C peripheral in SMBus mode, set the SBC bit to enable the PEC checking at the end of the programmed number of data bytes. To allow the ACK control of each byte, the reload mode must be selected (RELOAD = 1). Refer to Slave byte control mode for more details.

To check the PEC byte, the RELOAD bit must be cleared and the PECBYTE bit must be set. In this case, after the receipt of NBBYTES[7:0] - 1 data bytes, the next received byte is compared with the internal I2C_PECR register content. A NACK is automatically generated if the comparison does not match, and an ACK is automatically generated if the comparison matches, whatever the ACK bit value. Once the PEC byte is received, it is copied into the I2C_RXDR register like any other data, and the RXNE flag is set.

Upon a PEC mismatch, the PECERR flag is set and an interrupt is generated if the ERRIE bit of the I2C_CR1 register is set.

If no ACK software control is required, the user can set the PECBYTE bit and, in the same write operation, load NBBYTES[7:0] with the number of bytes to receive in a continuous flow. After the receipt of NBBYTES[7:0] - 1 bytes, the next received byte is checked as being the PEC.

Caution: The PECBYTE bit has no effect when the RELOAD bit is set.

Figure 371. Transfer sequence flow for SMBus slave receiver N bytes + PEC

graph TD
    Start([SMBus slave reception]) --> Init[Slave initialization]
    Init --> AddrCheck{I2C_ISR.ADDR = 1?}
    AddrCheck -- No --> Init
    AddrCheck -- Yes --> AddrProc[Read ADDCODE and DIR in I2C_ISR
I2C_CR2.NBYTES = 1, RELOAD = 1
PECBYTE = 1
Set I2C_ICR.ADDRCF]
    AddrProc --> DataCheck1{I2C_ISR.RXNE = 1?
I2C_ISR.TCR = 1?}
    DataCheck1 -- No --> DataCheck1
    DataCheck1 -- Yes --> DataProc1[Read I2C_RXDR.RXDATA
Program I2C_CR2.NACK = 0
I2C_CR2.NBYTES = 1
N = N - 1]
    DataProc1 --> NCheck{N = 1?}
    NCheck -- No --> DataCheck1
    NCheck -- Yes --> DataProc2[Read I2C_RXDR.RXDATA
Program RELOAD = 0
NACK = 0 and NBYTES = 1]
    DataProc2 --> DataCheck2{I2C_ISR.RXNE = 1?}
    DataCheck2 -- No --> DataCheck2
    DataCheck2 -- Yes --> DataProc3[Read I2C_RXDR.RXDATA]
    DataProc3 --> End([End])
    

Figure 372. Bus transfer diagrams for SMBus slave receiver (SBC = 1)

33.4.15 SMBus master mode

In addition to I2C master transfer management (refer to Section 33.4.9: I2C master mode ), this section provides extra software flowcharts to support SMBus.

SMBus master transmitter

When the SMBus master wants to transmit the PEC, the PECBYTE bit must be set and the number of bytes must be loaded in the NBYTES[7:0] bitfield, before setting the START bit. In this case, the total number of TXIS interrupts is NBYTES[7:0] - 1. So if the PECBYTE bit is set when NBYTES[7:0] = 0x1, the content of the I2C_PECR register is automatically transmitted.

If the SMBus master wants to send a STOP condition after the PEC, the automatic end mode must be selected (AUTOEND = 1). In this case, the STOP condition automatically follows the PEC transmission.

When the SMBus master wants to send a RESTART condition after the PEC, the software mode must be selected (AUTOEND = 0). In this case, once NBYTES[7:0] - 1 are transmitted, the I2C_PECR register content is transmitted. The TC flag is set after the PEC transmission, stretching the SCL line low. The RESTART condition must be programmed in the TC interrupt subroutine.

Caution: The PECBYTE bit has no effect when the RELOAD bit is set.

Figure 373. Bus transfer diagrams for SMBus master transmitter

SMBus master receiver

When the SMBus master wants to receive, at the end of the transfer, the PEC followed by a STOP condition, the automatic end mode can be selected (AUTOEND = 1). The PECBYTE bit must be set and the slave address programmed before setting the START bit. In this case, after the receipt of NBBYTES[7:0] - 1 data bytes, the next received byte is automatically checked versus the I2C_PECR register content. A NACK response is given to the PEC byte, followed by a STOP condition.

When the SMBus master receiver wants to receive, at the end of the transfer, the PEC byte followed by a RESTART condition, the software mode must be selected (AUTOEND = 0). The PECBYTE bit must be set and the slave address programmed before setting the START bit. In this case, after the receipt of NBBYTES[7:0] - 1 data bytes, the next received byte is automatically checked versus the I2C_PECR register content. The TC flag is set after the PEC byte reception, stretching the SCL line low. The RESTART condition can be programmed in the TC interrupt subroutine.

Caution: The PECBYTE bit has no effect when the RELOAD bit is set.

Figure 374. Bus transfer diagrams for SMBus master receiver

33.4.16 Error conditions

The following errors are the conditions that can cause the communication to fail.

Bus error (BERR)

A bus error is detected when a START or a STOP condition is detected and is not located after a multiple of nine SCL clock pulses. START or STOP condition is detected when an SDA edge occurs while SCL is high.

The bus error flag is set only if the I2C peripheral is involved in the transfer as master or addressed slave (that is, not during the address phase in slave mode).

In case of a misplaced START or RESTART detection in slave mode, the I2C peripheral enters address recognition state like for a correct START condition.

When a bus error is detected, the BERR flag of the I2C_ISR register is set, and an interrupt is generated if the ERRRIE bit of the I2C_CR1 register is set.

Arbitration loss (ARLO)

An arbitration loss is detected when a high level is sent on the SDA line, but a low level is sampled on the SCL rising edge.

In master mode, arbitration loss is detected during the address phase, data phase and data acknowledge phase. In this case, the SDA and SCL lines are released, the START control bit is cleared by hardware and the master switches automatically to slave mode.

In slave mode, arbitration loss is detected during data phase and data acknowledge phase. In this case, the transfer is stopped and the SCL and SDA lines are released.

When an arbitration loss is detected, the ARLO flag of the I2C_ISR register is set and an interrupt is generated if the ERRRIE bit of the I2C_CR1 register is set.

Overrun/underrun error (OVR)

An overrun or underrun error is detected in slave mode when NOSTRETCH = 1 and:

• • In reception when a new byte is received and the RXDR register has not been read yet. The new received byte is lost, and a NACK is automatically sent as a response to the new byte.
• • In transmission:
  • – When STOPF = 1 and the first data byte must be sent. The content of the I2C_TXDR register is sent if TXE = 0, 0xFF if not.
  • – When a new byte must be sent and the I2C_TXDR register has not been written yet, 0xFF is sent.

When an overrun or underrun error is detected, the OVR flag of the I2C_ISR register is set and an interrupt is generated if the ERRRIE bit of the I2C_CR1 register is set.

Packet error checking error (PECERR)

A PEC error is detected when the received PEC byte does not match the I2C_PECR register content. A NACK is automatically sent after the wrong PEC reception.

When a PEC error is detected, the PECERR flag of the I2C_ISR register is set and an interrupt is generated if the ERRRIE bit of the I2C_CR1 register is set.

Timeout error (TIMEOUT)

A timeout error occurs for any of these conditions:

• • TIDLE = 0 and SCL remains low for the time defined in the TIMEOUTA[11:0] bitfield: this is used to detect an SMBus timeout.
• • TIDLE = 1 and both SDA and SCL remains high for the time defined in the TIMEOUTA[11:0] bitfield: this is used to detect a bus idle condition.
• • Master cumulative clock low extend time reaches the time defined in the TIMEOUTB[11:0] bitfield (SMBus \( t_{LOW:MEXT} \) parameter).
• • Slave cumulative clock low extend time reaches the time defined in the TIMEOUTB[11:0] bitfield (SMBus \( t_{LOW:SEXT} \) parameter).

When a timeout violation is detected in master mode, a STOP condition is automatically sent.

When a timeout violation is detected in slave mode, the SDA and SCL lines are automatically released.

When a timeout error is detected, the TIMEOUT flag is set in the I2C_ISR register and an interrupt is generated if the ERRIE bit of the I2C_CR1 register is set.

Alert (ALERT)

The ALERT flag is set when the I2C peripheral is configured as a host (SMBHEN = 1), the SMBALERT# signal detection is enabled (ALERTEN = 1), and a falling edge is detected on the SMBA pin. An interrupt is generated if the ERRIE bit of the I2C_CR1 register is set.

33.5 I2C in low-power modes

Table 223. Effect of low-power modes to I2C

33.6 I2C interrupts

The following table gives the list of I2C interrupt requests.

Table 224. I2C interrupt requests

Table 224. I2C interrupt requests (continued)

33.7 I2C DMA requests

33.7.1 Transmission using DMA

DMA (direct memory access) can be enabled for transmission by setting the TXDMAEN bit of the I2C_CR1 register. Data is loaded from an SRAM area configured through the DMA peripheral (see Section 8: Direct memory access controller (DMA) ) to the I2C_TXDR register whenever the TXIS bit is set.

Only the data are transferred with DMA.

In master mode, the initialization, the slave address, direction, number of bytes and START bit are programmed by software (the transmitted slave address cannot be transferred with DMA). When all data are transferred using DMA, DMA must be initialized before setting the START bit. The end of transfer is managed with the NBBYTES counter. Refer to Master transmitter .

In slave mode:

• • With NOSTRETCH = 0, when all data are transferred using DMA, DMA must be initialized before the address match event, or in ADDR interrupt subroutine, before clearing ADDR.
• • With NOSTRETCH = 1, the DMA must be initialized before the address match event.

The PEC transfer is managed with the counter associated to the NBBYTES[7:0] bitfield. Refer to SMBus slave transmitter and SMBus master transmitter .

Note: If DMA is used for transmission, it is not required to set the TXIE bit.

33.7.2 Reception using DMA

DMA (direct memory access) can be enabled for reception by setting the RXDMAEN bit of the I2C_CR1 register. Data is loaded from the I2C_RXDR register to an SRAM area configured through the DMA peripheral (refer to Section 8: Direct memory access controller (DMA) ) whenever the RXNE bit is set. Only the data (including PEC) are transferred with DMA.

In master mode, the initialization, the slave address, direction, number of bytes and START bit are programmed by software. When all data are transferred using DMA, DMA must be

initialized before setting the START bit. The end of transfer is managed with the NBYTES counter.

In slave mode with NOSTRETCH = 0, when all data are transferred using DMA, DMA must be initialized before the address match event, or in the ADDR interrupt subroutine, before clearing the ADDR flag.

The PEC transfer is managed with the counter associated to the NBYTES[7:0] bitfield. Refer to SMBus slave receiver and SMBus master receiver .

Note: If DMA is used for reception, it is not required to set the RXIE bit.

33.8 I2C debug modes

When the device enters debug mode (core halted), the SMBus timeout either continues working normally or stops, depending on the DBG_I2Cx_SMBUS_TIMEOUT bits in the DBG block.

33.9 I2C registers

Refer to Section 1.2 for the list of abbreviations used in register descriptions.

The registers are accessed by words (32-bit).

33.9.1 I2C control register 1 (I2C_CR1)

Address offset: 0x00

Reset value: 0x0000 0000

Access: no wait states, except if a write access occurs while a write access is ongoing. In this case, wait states are inserted in the second write access, until the previous one is completed. The latency of the second write access can be up to \( 2 \times PCLK1 + 6 \times I2CCLK \) .

Bits 31:24 Reserved, must be kept at reset value.

Bit 23 PECEN : PEC enable

• 0: PEC calculation disabled
• 1: PEC calculation enabled

0: The SMBALERT# signal on SMBA pin is not supported in host mode (SMBHEN = 1). In device mode (SMBHEN = 0), the SMBA pin is released and the alert response address header is disabled (0001100x followed by NACK).

1: The SMBALERT# signal on SMBA pin is supported in host mode (SMBHEN = 1). In device mode (SMBHEN = 0), the SMBA pin is driven low and the alert response address header is enabled (0001100x followed by ACK).

Note: When ALERTEN = 0, the SMBA pin can be used as a standard GPIO.

0: Device default address disabled. Address 0b1100001x is NACKed.

1: Device default address enabled. Address 0b1100001x is ACKed.

0: Host address disabled. Address 0b0001000x is NACKed.

1: Host address enabled. Address 0b0001000x is ACKed.

0: General call disabled. Address 0b00000000 is NACKed.

1: General call enabled. Address 0b00000000 is ACKed.

Bit 18 Reserved, must be kept at reset value.

This bit is used to disable clock stretching in slave mode. It must be kept cleared in master mode.

0: Clock stretching enabled

1: Clock stretching disabled

Note: This bit can be programmed only when the I2C peripheral is disabled (PE = 0).

This bit is used to enable hardware byte control in slave mode.

0: Slave byte control disabled

1: Slave byte control enabled

0: DMA mode disabled for reception

1: DMA mode enabled for reception

0: DMA mode disabled for transmission

1: DMA mode enabled for transmission

Bit 13 Reserved, must be kept at reset value.

0: Analog noise filter enabled

1: Analog noise filter disabled

Note: This bit can be programmed only when the I2C peripheral is disabled (PE = 0).

These bits are used to configure the digital noise filter on SDA and SCL input. The digital filter, filters spikes with a length of up to \( DNF[3:0] * t_{I2CCLK} \)

0000: Digital filter disabled

0001: Digital filter enabled and filtering capability up to one \( t_{I2CCLK} \)

...

1111: digital filter enabled and filtering capability up to fifteen \( t_{I2CCLK} \)

Note: If the analog filter is enabled, the digital filter is added to it. This filter can be programmed only when the I2C peripheral is disabled (PE = 0).

0: Error detection interrupts disabled

1: Error detection interrupts enabled

Note: Any of these errors generates an interrupt:

• - arbitration loss (ARLO)
• - bus error detection (BERR)
• - overrun/underrun (OVR)
• - timeout detection (TIMEOUT)
• - PEC error detection (PECERR)
• - alert pin event detection (ALERT)

0: Transfer complete interrupt disabled

1: Transfer complete interrupt enabled

Note: Any of these events generates an interrupt:

• Transfer complete (TC)
• Transfer complete reload (TCR)

0: STOP detection (STOPF) interrupt disabled

1: STOP detection (STOPF) interrupt enabled

0: Not acknowledge (NACKF) received interrupts disabled

1: Not acknowledge (NACKF) received interrupts enabled

0: Address match (ADDR) interrupts disabled

1: Address match (ADDR) interrupts enabled

0: Receive (RXNE) interrupt disabled

1: Receive (RXNE) interrupt enabled

0: Transmit (TXIS) interrupt disabled

1: Transmit (TXIS) interrupt enabled

0: Peripheral disabled

1: Peripheral enabled

Note: When PE = 0, the I2C SCL and SDA lines are released. Internal state machines and status bits are put back to their reset value. When cleared, PE must be kept low for at least three APB clock cycles.

33.9.2 I2C control register 2 (I2C_CR2)

Address offset: 0x04

Reset value: 0x0000 0000

Access: no wait states, except if a write access occurs while a write access is ongoing. In this case, wait states are inserted in the second write access until the previous one is completed. The latency of the second write access can be up to \( 2 \times PCLK1 + 6 \times I2CCLK \) .

Bits 31:27 Reserved, must be kept at reset value.

Bit 26 PECBYTE : Packet error checking byte

This bit is set by software, and cleared by hardware when the PEC is transferred, or when a STOP condition or an Address matched is received, also when PE = 0.

0: No PEC transfer

1: PEC transmission/reception is requested

Note: Writing 0 to this bit has no effect.

This bit has no effect when RELOAD is set, and in slave mode when SBC = 0.

Bit 25 AUTOEND : Automatic end mode (master mode)

This bit is set and cleared by software.

0: software end mode: TC flag is set when NBYPES data are transferred, stretching SCL low.

1: Automatic end mode: a STOP condition is automatically sent when NBYPES data are transferred.

Note: This bit has no effect in slave mode or when the RELOAD bit is set.

Bit 24 RELOAD : NBYPES reload mode

This bit is set and cleared by software.

0: The transfer is completed after the NBYPES data transfer (STOP or RESTART follows).

1: The transfer is not completed after the NBYPES data transfer (NBYPES is reloaded). TCR flag is set when NBYPES data are transferred, stretching SCL low.

Bits 23:16 NBYTES[7:0] : Number of bytes

The number of bytes to be transmitted/received is programmed there. This field is don't care in slave mode with SBC = 0.

Note: Changing these bits when the START bit is set is not allowed.

The bit is set by software, cleared by hardware when the NACK is sent, or when a STOP condition or an Address matched is received, or when PE = 0.

0: an ACK is sent after current received byte.

1: a NACK is sent after current received byte.

Note: Writing 0 to this bit has no effect.

This bit is used only in slave mode: in master receiver mode, NACK is automatically generated after last byte preceding STOP or RESTART condition, whatever the NACK bit value.

When an overrun occurs in slave receiver NOSTRETCH mode, a NACK is automatically generated, whatever the NACK bit value.

When hardware PEC checking is enabled (PECBYTE = 1), the PEC acknowledge value does not depend on the NACK value.

This bit only pertains to master mode. It is set by software and cleared by hardware when a STOP condition is detected or when PE = 0.

0: No STOP generation

1: STOP generation after current byte transfer

Note: Writing 0 to this bit has no effect.

This bit is set by software. It is cleared by hardware after the START condition followed by the address sequence is sent, by an arbitration loss, by a timeout error detection, or when PE = 0. It can also be cleared by software, by setting the ADDRCF bit of the I2C_ICR register.

0: No START generation

1: RESTART/START generation:

If the I2C is already in master mode with AUTOEND = 0, setting this bit generates a repeated START condition when RELOAD = 0, after the end of the NBBYTES transfer.

Otherwise, setting this bit generates a START condition once the bus is free.

Note: Writing 0 to this bit has no effect.

The START bit can be set even if the bus is BUSY or I2C is in slave mode.

This bit has no effect when RELOAD is set.

0: The master sends the complete 10-bit slave address read sequence: START + 2 bytes 10-bit address in write direction + RESTART + first seven bits of the 10-bit address in read direction.

1: The master sends only the first seven bits of the 10-bit address, followed by read direction.

Note: Changing this bit when the START bit is set is not allowed.

0: The master operates in 7-bit addressing mode

1: The master operates in 10-bit addressing mode

Note: Changing this bit when the START bit is set is not allowed.

0: Master requests a write transfer

1: Master requests a read transfer

Note: Changing this bit when the START bit is set is not allowed.

Bits 9:0 SADD[9:0] : Slave address (master mode)

Condition: In 7-bit addressing mode (ADD10 = 0):

SADD[7:1] must be written with the 7-bit slave address to be sent. Bits SADD[9], SADD[8] and SADD[0] are don't care.

Condition: In 10-bit addressing mode (ADD10 = 1):

SADD[9:0] must be written with the 10-bit slave address to be sent.

Note: Changing these bits when the START bit is set is not allowed.

33.9.3 I2C own address 1 register (I2C_OAR1)

Address offset: 0x08

Reset value: 0x0000 0000

Access: no wait states, except if a write access occurs while a write access is ongoing. In this case, wait states are inserted in the second write access until the previous one is completed. The latency of the second write access can be up to \( 2 \times PCLK1 + 6 \times I2CCLK \) .

Bits 31:16 Reserved, must be kept at reset value.

Bit 15 OA1EN : Own address 1 enable

0: Own address 1 disabled. The received slave address OA1 is NACKed.

1: Own address 1 enabled. The received slave address OA1 is ACKed.

Bits 14:11 Reserved, must be kept at reset value.

Bit 10 OA1MODE : Own address 1 10-bit mode

0: Own address 1 is a 7-bit address.

1: Own address 1 is a 10-bit address.

Note: This bit can be written only when OA1EN = 0.

Bits 9:0 OA1[9:0] : Interface own slave address

7-bit addressing mode: OA1[7:1] contains the 7-bit own slave address. Bits OA1[9], OA1[8] and OA1[0] are don't care.

10-bit addressing mode: OA1[9:0] contains the 10-bit own slave address.

Note: These bits can be written only when OA1EN = 0.

33.9.4 I2C own address 2 register (I2C_OAR2)

Address offset: 0x0C

Reset value: 0x0000 0000

Access: no wait states, except if a write access occurs while a write access is ongoing. In this case, wait states are inserted in the second write access, until the previous one is completed. The latency of the second write access can be up to \( 2 \times PCLK1 + 6 \times I2CCLK \) .

Bits 31:16 Reserved, must be kept at reset value.

Bit 15 OA2EN : Own address 2 enable

• 0: Own address 2 disabled. The received slave address OA2 is NACKed.
• 1: Own address 2 enabled. The received slave address OA2 is ACKed.

Bits 14:11 Reserved, must be kept at reset value.

Bits 10:8 OA2MSK[2:0] : Own address 2 masks

• 000: No mask
• 001: OA2[1] is masked and don't care. Only OA2[7:2] are compared.
• 010: OA2[2:1] are masked and don't care. Only OA2[7:3] are compared.
• 011: OA2[3:1] are masked and don't care. Only OA2[7:4] are compared.
• 100: OA2[4:1] are masked and don't care. Only OA2[7:5] are compared.
• 101: OA2[5:1] are masked and don't care. Only OA2[7:6] are compared.
• 110: OA2[6:1] are masked and don't care. Only OA2[7] is compared.
• 111: OA2[7:1] are masked and don't care. No comparison is done, and all (except reserved) 7-bit received addresses are acknowledged.

Note: These bits can be written only when OA2EN = 0.

As soon as OA2MSK ≠ 0, the reserved I2C addresses (0b0000xxx and 0b1111xxx) are not acknowledged, even if the comparison matches.

Bits 7:1 OA2[7:1] : Interface address

7-bit addressing mode: 7-bit address

Note: These bits can be written only when OA2EN = 0.

Bit 0 Reserved, must be kept at reset value.

33.9.5 I2C timing register (I2C_TIMINGR)

Address offset: 0x10

Reset value: 0x0000 0000

Access: no wait states

Bits 31:28 PRESC[3:0] : Timing prescaler

This field is used to prescale I2CCLK to generate the clock period \( t_{\text{PRESC}} \) used for data setup and hold counters (refer to section I2C timings ), and for SCL high and low level counters (refer to section I2C master initialization ).

Bits 27:24 Reserved, must be kept at reset value.

Bits 23:20 SCLDEL[3:0] : Data setup time

This field is used to generate a delay \( t_{\text{SCLDEL}} = (\text{SCLDEL} + 1) \times t_{\text{PRESC}} \) between SDA edge and SCL rising edge. In master and in slave modes with NOSTRETCH = 0, the SCL line is stretched low during \( t_{\text{SCLDEL}} \) .

Note: \( t_{\text{SCLDEL}} \) is used to generate \( t_{\text{SU:DAT}} \) timing.

Bits 19:16 SDADEL[3:0] : Data hold time

This field is used to generate the delay \( t_{\text{SDADEL}} \) between SCL falling edge and SDA edge. In master and in slave modes with NOSTRETCH = 0, the SCL line is stretched low during \( t_{\text{SDADEL}} \) .

Note: \( t_{\text{SDADEL}} \) is used to generate \( t_{\text{HD:DAT}} \) timing.

Bits 15:8 SCLH[7:0] : SCL high period (master mode)

This field is used to generate the SCL high period in master mode.

Note: SCLH is also used to generate \( t_{\text{SU:STO}} \) and \( t_{\text{HD:STA}} \) timing.

Bits 7:0 SCLL[7:0] : SCL low period (master mode)

This field is used to generate the SCL low period in master mode.

Note: SCLL is also used to generate \( t_{\text{BUF}} \) and \( t_{\text{SU:STA}} \) timings.

Note: This register must be configured when the I2C peripheral is disabled (PE = 0).

Note: The STM32CubeMX tool calculates and provides the I2C_TIMINGR content in the I2C Configuration window.

33.9.6 I2C timeout register (I2C_TIMEOUTR)

Address offset: 0x14

Reset value: 0x0000 0000

Access: no wait states, except if a write access occurs while a write access is ongoing. In this case, wait states are inserted in the second write access until the previous one is completed. The latency of the second write access can be up to \( 2 \times PCLK1 + 6 \times I2CCLK \) .

Bit 31 TEXTEN : Extended clock timeout enable

0: Extended clock timeout detection is disabled

1: Extended clock timeout detection is enabled. When a cumulative SCL stretch for more than \( t_{LOW:EXT} \) is done by the I2C interface, a timeout error is detected (TIMEOUT = 1).

Bits 30:28 Reserved, must be kept at reset value.

Bits 27:16 TIMEOUTB[11:0] : Bus timeout B

This field is used to configure the cumulative clock extension timeout:

• – Master mode: the master cumulative clock low extend time ( \( t_{LOW:MEXT} \) ) is detected
• – Slave mode: the slave cumulative clock low extend time ( \( t_{LOW:SEXT} \) ) is detected

Note: These bits can be written only when TEXTEN = 0.

Bit 15 TIMOUTEN : Clock timeout enable

0: SCL timeout detection is disabled

1: SCL timeout detection is enabled. When SCL is low for more than \( t_{TIMEOUT} \) (TIDLE = 0) or high for more than \( t_{IDLE} \) (TIDLE = 1), a timeout error is detected (TIMEOUT = 1).

Bits 14:13 Reserved, must be kept at reset value.

Bit 12 TIDLE : Idle clock timeout detection

0: TIMEOUTA is used to detect SCL low timeout

1: TIMEOUTA is used to detect both SCL and SDA high timeout (bus idle condition)

Note: This bit can be written only when TIMOUTEN = 0.

Bits 11:0 TIMEOUTA[11:0] : Bus timeout A

This field is used to configure:

The SCL low timeout condition \( t_{TIMEOUT} \) when TIDLE = 0

The bus idle condition (both SCL and SDA high) when TIDLE = 1

Note: These bits can be written only when TIMOUTEN = 0.

33.9.7 I2C interrupt and status register (I2C_ISR)

Address offset: 0x18

Reset value: 0x0000 0001

Access: no wait states

Bits 31:24 Reserved, must be kept at reset value.

Bits 23:17 ADDCODE[6:0] : Address match code (slave mode)

These bits are updated with the received address when an address match event occurs (ADDR = 1). In the case of a 10-bit address, ADDCODE provides the 10-bit header followed by the two MSBs of the address.

Bit 16 DIR : Transfer direction (slave mode)

This flag is updated when an address match event occurs (ADDR = 1).

0: Write transfer, slave enters receiver mode.

1: Read transfer, slave enters transmitter mode.

Bit 15 BUSY : Bus busy

This flag indicates that a communication is in progress on the bus. It is set by hardware when a START condition is detected, and cleared by hardware when a STOP condition is detected, or when PE = 0.

Bit 14 Reserved, must be kept at reset value.

Bit 13 ALERT : SMBus alert

This flag is set by hardware when SMBHEN = 1 (SMBus host configuration), ALERTEN = 1 and an SMBALERT# event (falling edge) is detected on SMBA pin. It is cleared by software by setting the ALERTCF bit.

Note: This bit is cleared by hardware when PE = 0.

Bit 12 TIMEOUT : Timeout or \( t_{LOW} \) detection flag

This flag is set by hardware when a timeout or extended clock timeout occurred. It is cleared by software by setting the TIMEOUTCF bit.

Note: This bit is cleared by hardware when PE = 0.

Bit 11 PECERR : PEC error in reception

This flag is set by hardware when the received PEC does not match with the PEC register content. A NACK is automatically sent after the wrong PEC reception. It is cleared by software by setting the PECCF bit.

Note: This bit is cleared by hardware when PE = 0.

Bit 10 OVR : Overrun/underrun (slave mode)

This flag is set by hardware in slave mode with NOSTRETCH = 1, when an overrun/underrun error occurs. It is cleared by software by setting the OVRCF bit.

Note: This bit is cleared by hardware when PE = 0.

This flag is set by hardware in case of arbitration loss. It is cleared by software by setting the ARLOCF bit.

Note: This bit is cleared by hardware when PE = 0.

This flag is set by hardware when a misplaced START or STOP condition is detected whereas the peripheral is involved in the transfer. The flag is not set during the address phase in slave mode. It is cleared by software by setting the BERRCF bit.

Note: This bit is cleared by hardware when PE = 0.

This flag is set by hardware when RELOAD = 1 and NBYTES data have been transferred. It is cleared by software when NBYTES is written to a non-zero value.

Note: This bit is cleared by hardware when PE = 0.

This flag is only for master mode, or for slave mode when the SBC bit is set.

This flag is set by hardware when RELOAD = 0, AUTOEND = 0 and NBYTES data have been transferred. It is cleared by software when START bit or STOP bit is set.

Note: This bit is cleared by hardware when PE = 0.

This flag is set by hardware when a STOP condition is detected on the bus and the peripheral is involved in this transfer:

• – as a master, provided that the STOP condition is generated by the peripheral.
• – as a slave, provided that the peripheral has been addressed previously during this transfer.

It is cleared by software by setting the STOPCF bit.

Note: This bit is cleared by hardware when PE = 0.

This flag is set by hardware when a NACK is received after a byte transmission. It is cleared by software by setting the NACKCF bit.

Note: This bit is cleared by hardware when PE = 0.

This bit is set by hardware as soon as the received slave address matched with one of the enabled slave addresses. It is cleared by software by setting ADDRCF bit.

Note: This bit is cleared by hardware when PE = 0.

This bit is set by hardware when the received data is copied into the I2C_RXDR register, and is ready to be read. It is cleared when I2C_RXDR is read.

Note: This bit is cleared by hardware when PE = 0.

This bit is set by hardware when the I2C_TXDR register is empty and the data to be transmitted must be written in the I2C_TXDR register. It is cleared when the next data to be sent is written in the I2C_TXDR register.

This bit can be written to 1 by software only when NOSTRETCH = 1, to generate a TXIS event (interrupt if TXIE = 1 or DMA request if TXDMAEN = 1).

Note: This bit is cleared by hardware when PE = 0.

Bit 0 TXE : Transmit data register empty (transmitters)

This bit is set by hardware when the I2C_TXDR register is empty. It is cleared when the next data to be sent is written in the I2C_TXDR register.

This bit can be written to 1 by software in order to flush the transmit data register I2C_TXDR.

Note: This bit is set by hardware when PE = 0.

33.9.8 I2C interrupt clear register (I2C_ICR)

Address offset: 0x1C

Reset value: 0x0000 0000

Access: no wait states

Bits 31:14 Reserved, must be kept at reset value.

Bit 13 ALERTCF : Alert flag clear

Writing 1 to this bit clears the ALERT flag in the I2C_ISR register.

Bit 12 TIMOUTCF : Timeout detection flag clear

Writing 1 to this bit clears the TIMEOUT flag in the I2C_ISR register.

Bit 11 PEC CF : PEC error flag clear

Writing 1 to this bit clears the PECERR flag in the I2C_ISR register.

Bit 10 OVR CF : Overrun/underrun flag clear

Writing 1 to this bit clears the OVR flag in the I2C_ISR register.

Bit 9 ARLOCF : Arbitration lost flag clear

Writing 1 to this bit clears the ARLO flag in the I2C_ISR register.

Bit 8 BERRCF : Bus error flag clear

Writing 1 to this bit clears the BERRF flag in the I2C_ISR register.

Bits 7:6 Reserved, must be kept at reset value.

Bit 5 STOPCF : STOP detection flag clear

Writing 1 to this bit clears the STOPF flag in the I2C_ISR register.

Bit 4 NACKCF : Not acknowledge flag clear

Writing 1 to this bit clears the NACKF flag in I2C_ISR register.

Bit 3 ADDRCF : Address matched flag clear

Writing 1 to this bit clears the ADDR flag in the I2C_ISR register. Writing 1 to this bit also clears the START bit in the I2C_CR2 register.

Bits 2:0 Reserved, must be kept at reset value.

33.9.9 I2C PEC register (I2C_PECR)

Address offset: 0x20

Reset value: 0x0000 0000

Access: no wait states

Bits 31:8 Reserved, must be kept at reset value.

Bits 7:0 PEC[7:0] : Packet error checking register

This field contains the internal PEC when PECEN=1.

The PEC is cleared by hardware when PE = 0.

33.9.10 I2C receive data register (I2C_RXDR)

Address offset: 0x24

Reset value: 0x0000 0000

Access: no wait states

Bits 31:8 Reserved, must be kept at reset value.

Bits 7:0 RXDATA[7:0] : 8-bit receive data

Data byte received from the I ² C-bus.

Address offset: 0x28

Reset value: 0x0000 0000

Access: no wait states

Bits 31:8 Reserved, must be kept at reset value.

33.9.12 I2C register map

The table below provides the I2C register map and the reset values.

Table 225. I2C register map and reset values

Refer to Section 2.2 for the register boundary addresses.