6. Reset and clock control (RCC)

6.1 Reset

There are three types of reset, defined as system reset, power reset and RTC domain reset.

6.1.1 Power reset

A power reset is generated when one of the following events occurs:

1. 1. Power-on/power-down reset (POR/PDR reset)
2. 2. When exiting Standby mode

A power reset sets all registers to their reset values except the RTC domain ( Figure 6: Power supply overview ).

In STM32F0x8 devices, the POR/PDR reset is not functional and the Standby mode is not available. Power reset must be provided from an external NPOR pin (active low and released by the application when all supply voltages are stabilized).

6.1.2 System reset

A system reset sets all registers to their reset values except the reset flags in the clock controller CSR register and the registers in the RTC domain (see Figure 6: Power supply overview ).

A system reset is generated when one of the following events occurs:

1. 1. A low level on the NRST pin (external reset)
2. 2. Window watchdog event (WWDG reset)
3. 3. Independent watchdog event (IWDG reset)
4. 4. A software reset (SW reset) (see Software reset )
5. 5. Low-power management reset (see Low-power management reset )
6. 6. Option byte loader reset (see Option byte loader reset )
7. 7. A power reset

The reset source can be identified by checking the reset flags in the Control/Status register, RCC_CSR (see Section 6.4.10: Control/status register (RCC_CSR) ).

These sources act on the NRST pin and it is always kept low during the delay phase. The RESET service routine vector is fixed at address 0x0000_0004 in the memory map.

The system reset signal provided to the device is output on the NRST pin. The pulse generator guarantees a minimum reset pulse duration of 20 µs for each internal reset source. In case of an external reset, the reset pulse is generated while the NRST pin is asserted low.

Figure 9. Simplified diagram of the reset circuit

Software reset

The SYSRESETREQ bit in Cortex ^® -M0 Application Interrupt and Reset Control Register must be set to force a software reset on the device. Refer to the Cortex ^™ -M0 technical reference manual for more details.

Low-power management reset

There are two ways to generate a low-power management reset:

1. 1. Reset generated when entering Standby mode:
   This type of reset is enabled by resetting nRST_STDBY bit in User Option Bytes. In this case, whenever a Standby mode entry sequence is successfully executed, the device is reset instead of entering Standby mode.
2. 2. Reset when entering Stop mode:
   This type of reset is enabled by resetting nRST_STOP bit in User Option Bytes. In this case, whenever a Stop mode entry sequence is successfully executed, the device is reset instead of entering Stop mode.

For further information on the User Option Bytes, refer to Section 4: Option bytes .

Option byte loader reset

The option byte loader reset is generated when the OBL_LAUNCH bit (bit 13) is set in the FLASH_CR register. This bit is used to launch the option byte loading by software.

6.1.3 RTC domain reset

The RTC domain has two specific resets that affect only the RTC domain ( Figure 6: Power supply overview ).

An RTC domain reset only affects the LSE oscillator, the RTC, the Backup registers and the RCC RTC domain control register (RCC_BDCR) . It is generated when one of the following events occurs.

1. 1. Software reset, triggered by setting the BDRST bit in the RTC domain control register (RCC_BDCR) .
2. 2. V _DD power-up if V _BAT has been disconnected when it was low.

The Backup registers are also reset when one of the following events occurs:

1. 1. RTC tamper detection event.
2. 2. Change of the read out protection from level 1 to level 0.

6.2 Clocks

Various clock sources can be used to drive the system clock (SYSCLK):

• • HSI 8 MHz RC oscillator clock
• • HSE oscillator clock
• • PLL clock
• • HSI48 48 MHz RC oscillator clock (available on STM32F04x, STM32F07x and STM32F09x devices only)

The devices have the following additional clock sources:

• • 40 kHz low speed internal RC (LSI RC) which drives the independent watchdog and optionally the RTC used for Auto-wake-up from Stop/Standby mode.
• • 32.768 kHz low speed external crystal (LSE crystal) which optionally drives the real-time clock (RTCCLK)
• • 14 MHz high speed internal RC (HSI14) dedicated for ADC.

Each clock source can be switched on or off independently when it is not used, to optimize power consumption.

Several prescalers can be used to configure the frequency of the AHB and the APB domains. The AHB and the APB domains maximum frequency is 48 MHz.

All the peripheral clocks are derived from their bus clock (HCLK for AHB or PCLK for APB) except:

• • The Flash memory programming interface clock (FLITFCLK) which is always the HSI clock.
• • The option byte loader clock which is always the HSI clock
• • The ADC clock which is derived (selected by software) from one of the two following sources:
  • – dedicated HSI14 clock, to run always at the maximum sampling rate
  • – APB clock (PCLK) divided by 2 or 4
• • The USART1 clock, USART2 clock (on STM32F07x and STM32F09x devices only) and USART3 clock (on STM32F09x devices only) which is derived (selected by software) from one of the four following sources:
  • – system clock
  • – HSI clock
  • – LSE clock
  • – APB clock (PCLK)
• • The I2C1 clock which is derived (selected by software) from one of the two following sources:
  • – system clock
  • – HSI clock
• • The USB clock which is derived (selected by software) from one of the two following sources:
  • – PLL clock
  • – HSI48 clock
• • The CEC clock which is derived from the HSI clock divided by 244 or from the LSE clock.
• • The I2S1 and I2S2 clock which is always the system clock.
• • The RTC clock which is derived from the LSE, LSI or from the HSE clock divided by 32.
• • The timer clock frequencies are automatically fixed by hardware. There are two cases:
  • – if the APB prescaler is 1, the timer clock frequencies are set to the same frequency as that of the APB domain;
  • – otherwise, they are set to twice (x2) the frequency of the APB domain.
• • The IWDG clock which is always the LSI clock.

The RCC feeds the Cortex System Timer (SysTick) external clock with the AHB clock (HCLK) divided by 8. The SysTick can work either with this clock or directly with the Cortex clock (HCLK), configurable in the SysTick Control and Status Register.

Figure 10. Clock tree (STM32F03x and STM32F05x devices)

1. Not available on STM32F05x devices.

Figure 11. Clock tree (STM32F04x, STM32F07x and STM32F09x devices)

MS31418V3

1. Not available on STM32F04x devices.

2. Not available on STM32F04x and STM32F07x devices

FCLK acts as Cortex ^® -M0's free-running clock. For more details refer to the Arm Cortex-M0 r0p0 technical reference manual (TRM) .

6.2.1 HSE clock

The high speed external clock signal (HSE) can be generated from two possible clock sources:

• • HSE external crystal/ceramic resonator
• • HSE user external clock

The resonator and the load capacitors have to be placed as close as possible to the oscillator pins in order to minimize output distortion and startup stabilization time. The loading capacitance values must be adjusted according to the selected oscillator.

Figure 12. HSE/ LSE clock sources

External crystal/ceramic resonator (HSE crystal)

The 4 to 32 MHz external oscillator has the advantage of producing a very accurate rate on the main clock.

The associated hardware configuration is shown in Figure 12 . Refer to the electrical characteristics section of the datasheet for more details.

The HSERDY flag in the Clock control register (RCC_CR) indicates if the HSE oscillator is stable or not. At startup, the clock is not released until this bit is set by hardware. An interrupt can be generated if enabled in the Clock interrupt register (RCC_CIR) .

The HSE Crystal can be switched on and off using the HSEON bit in the Clock control register (RCC_CR) .

For code example refer to the Appendix section A.3.1: HSE start sequence code example .

Caution: To switch ON the HSE oscillator, 512 HSE clock pulses need to be seen by an internal stabilization counter after the HSEON bit is set. Even in the case that no crystal or resonator is connected to the device, excessive external noise on the OSC_IN pin may still lead the oscillator to start. Once the oscillator is started, it needs another 6 HSE clock pulses to complete a switching OFF sequence. If for any reason the oscillations are no more present on the OSC_IN pin, the oscillator cannot be switched OFF, locking the OSC pins from any other use and introducing unwanted power consumption. To avoid such situation, it is strongly recommended to always enable the Clock Security System (CSS) which is able to switch OFF the oscillator even in this case.

External source (HSE bypass)

In this mode, an external clock source must be provided. It can have a frequency of up to 32 MHz. You select this mode by setting the HSEBYP and HSEON bits in the Clock control register (RCC_CR) . The external clock signal (square, sinus or triangle) with ~40-60% duty cycle depending on the frequency (refer to the datasheet ) has to drive the OSC_IN pin while the OSC_OUT pin can be used a GPIO. See Figure 12 .

6.2.2 HSI clock

The HSI clock signal is generated from an internal 8 MHz RC oscillator and can be used directly as a system clock or for PLL input

The HSI RC oscillator has the advantage of providing a clock source at low cost (no external components). It also has a faster startup time than the HSE crystal oscillator however, even with calibration the frequency is less accurate than an external crystal oscillator or ceramic resonator.

Calibration

RC oscillator frequencies can vary from one chip to another due to manufacturing process variations, this is why each device is factory calibrated by ST for 1% accuracy at \( T_A=25^\circ\text{C} \) .

After reset, the factory calibration value is loaded in the HSICAL[7:0] bits in the Clock control register (RCC_CR) .

If the application is subject to voltage or temperature variations this may affect the RC oscillator speed. You can trim the HSI frequency in the application using the HSITRIM[4:0] bits in the Clock control register (RCC_CR) .

For more details on how to measure the HSI frequency variation refer to Section 6.2.13: Internal/external clock measurement with TIM14 on page 107 .

The HSIRDY flag in the Clock control register (RCC_CR) indicates if the HSI RC is stable or not. At startup, the HSI RC output clock is not released until this bit is set by hardware.

The HSI RC can be switched on and off using the HSION bit in the Clock control register (RCC_CR) .

The HSI signal can also be used as a backup source (Auxiliary clock) if the HSE crystal oscillator fails. Refer to Section 6.2.8: Clock security system (CSS) on page 105 .

Furthermore it is possible to drive the HSI clock to the MCO multiplexer. Then the clock could be driven to the Timer 14 giving the ability to the user to calibrate the oscillator.

6.2.3 HSI48 clock

On STM32F04x, STM32F07x and STM32F09x devices only, the HSI48 clock signal is generated from an internal 48 MHz RC oscillator and can be used directly as a system clock or divided and be used as PLL input.

The internal 48MHz RC oscillator is mainly dedicated to provide a high precision clock to the USB peripheral by means of a special Clock recovery system (CRS) circuitry, which could use the USB SOF signal or the LSE or an external signal to automatically adjust the oscillator frequency on-fly, in a very small steps. This oscillator can also be used as a system clock source when the system is in run mode; it is disabled as soon as the system enters in Stop or Standby mode. When the CRS is not used, the HSI48 RC oscillator runs on its default frequency which is subject to manufacturing process variations, this is why each device is factory calibrated by ST for ~3% accuracy at \( T_A = 25\text{ }^\circ\text{C} \) .

For more details on how to configure and use the CRS peripheral refer to Section 7 .

The HSI48RDY flag in the Clock control register (RCC_CR) indicates if the HSI48 RC is stable or not. At startup, the HSI48 RC output clock is not released until this bit is set by hardware.

The HSI48 RC can be switched on and off using the HSI48ON bit in the Clock control register (RCC_CR). This oscillator is also automatically enabled (by hardware forcing HSI48ON bit to one) as soon as it is chosen as a clock source for the USB and the peripheral is enabled.

Furthermore it is possible to drive the HSI48 clock to the MCO multiplexer and use it as a clock source for other application components.

6.2.4 PLL

The internal PLL can be used to multiply the HSI, a divided HSI48 or the HSE output clock frequency. Refer to Figure 9: Simplified diagram of the reset circuit , Figure 12: HSE/ LSE clock sources and Clock control register (RCC_CR) .

The PLL configuration (selection of the input clock, predivider and multiplication factor) must be done before enabling the PLL. Once the PLL is enabled, these parameters cannot be changed.

To modify the PLL configuration, proceed as follows:

1. 1. Disable the PLL by setting PLLON to 0.
2. 2. Wait until PLLRDY is cleared. The PLL is now fully stopped.
3. 3. Change the desired parameter.
4. 4. Enable the PLL again by setting PLLON to 1.
5. 5. Wait until PLLRDY is set.

An interrupt can be generated when the PLL is ready, if enabled in the Clock interrupt register (RCC_CIR) .

The PLL output frequency must be set in the range 16-48 MHz.

For code example refer to the Appendix section A.3.2: PLL configuration modification code example .

6.2.5 LSE clock

The LSE crystal is a 32.768 kHz Low Speed External crystal or ceramic resonator. It has the advantage of providing a low-power but highly accurate clock source to the real-time clock peripheral (RTC) for clock/calendar or other timing functions.

The LSE crystal is switched on and off using the LSEON bit in RTC domain control register (RCC_BDCR) . The crystal oscillator driving strength can be changed at runtime using the LSEDRV[1:0] bits in the RTC domain control register (RCC_BDCR) to obtain the best compromise between robustness and short start-up time on one side and low-power consumption on the other.

The LSERDY flag in the RTC domain control register (RCC_BDCR) indicates whether the LSE crystal is stable or not. At startup, the LSE crystal output clock signal is not released until this bit is set by hardware. An interrupt can be generated if enabled in the Clock interrupt register (RCC_CIR) .

Caution: To switch ON the LSE oscillator, 4096 LSE clock pulses need to be seen by an internal stabilization counter after the LSEON bit is set. Even in the case that no crystal or resonator is connected to the device, excessive external noise on the OSC32_IN pin may still lead the oscillator to start. Once the oscillator is started, it needs another 6 LSE clock pulses to complete a switching OFF sequence. If for any reason the oscillations are no more present on the OSC_IN pin, the oscillator cannot be switched OFF, locking the OSC32 pins from any other use and introducing unwanted power consumption. The only way to recover such situation is to perform the RTC domain reset by software.

External source (LSE bypass)

In this mode, an external clock source must be provided. It can have a frequency of up to 1 MHz. You select this mode by setting the LSEBYP and LSEON bits in the RTC domain control register (RCC_BDCR) . The external clock signal (square, sinus or triangle) with ~50% duty cycle has to drive the OSC32_IN pin while the OSC32_OUT pin can be used as GPIO. See Figure 12 .

6.2.6 LSI clock

The LSI RC acts as a low-power clock source that can be kept running in Stop and Standby mode for the independent watchdog (IWDG) and RTC. The clock frequency is around 40 kHz. For more details, refer to the electrical characteristics section of the datasheets.

The LSI RC can be switched on and off using the LSION bit in the Control/status register (RCC_CSR) .

The LSIRDY flag in the Control/status register (RCC_CSR) indicates if the LSI oscillator is stable or not. At startup, the clock is not released until this bit is set by hardware. An interrupt can be generated if enabled in the Clock interrupt register (RCC_CIR) .

6.2.7 System clock (SYSCLK) selection

Various clock sources can be used to drive the system clock (SYSCLK):

• • HSI oscillator
• • HSE oscillator
• • PLL
• • HSI48 oscillator (available only on STM32F04x, STM32F07x and STM32F09x devices)

After a system reset, the HSI oscillator is selected as system clock. When a clock source is used directly or through the PLL as a system clock, it is not possible to stop it.

A switch from one clock source to another occurs only if the target clock source is ready (clock stable after startup delay or PLL locked). If a clock source which is not yet ready is selected, the switch will occur when the clock source becomes ready. Status bits in the Clock control register (RCC_CR) indicate which clock(s) is (are) ready and which clock is currently used as a system clock.

6.2.8 Clock security system (CSS)

Clock security system can be activated by software. In this case, the clock detector is enabled after the HSE oscillator startup delay, and disabled when this oscillator is stopped.

If a failure is detected on the HSE clock, the HSE oscillator is automatically disabled, a clock failure event is sent to the break input of the advanced-control timers (TIM1) and general-purpose timers (TIM15, TIM16 and TIM17) and an interrupt is generated to inform the software about the failure (clock security system interrupt, or CSSI), allowing the MCU to perform rescue operations. The CSSI is linked to the Cortex®-M0 NMI (Non-Maskable Interrupt) exception vector.

Note: Once the CSS is enabled and if the HSE clock fails, the CSS interrupt occurs and an NMI is automatically generated. The NMI is executed indefinitely unless the CSS interrupt pending bit is cleared. As a consequence, in the NMI ISR user must clear the CSS interrupt by setting the CSSC bit in the Clock interrupt register (RCC_CIR).

If the HSE oscillator is used directly or indirectly as the system clock (indirectly means: it is used as PLL input clock, and the PLL clock is used as system clock), a detected failure causes a switch of the system clock to the HSI oscillator and the disabling of the HSE oscillator. If the HSE clock (divided or not) is the clock entry of the PLL used as system clock when the failure occurs, the PLL is disabled too.

6.2.9 ADC clock

The ADC clock selection is done inside the ADC_CFGR2 (refer to Section 13.11.5: ADC configuration register 2 (ADC_CFGR2) on page 271 ). It can be either the dedicated 14 MHz RC oscillator (HSI14) connected on the ADC asynchronous clock input or PCLK divided by 2 or 4. The 14 MHz RC oscillator can be configured by software either to be turned on/off (“auto-off mode”) by the ADC interface or to be always enabled. The HSI 14 MHz RC

oscillator cannot be turned on by ADC interface when the APB clock is selected as an ADC kernel clock.

6.2.10 RTC clock

The RTCCLK clock source can be either the HSE/32, LSE or LSI clocks. This is selected by programming the RTCSEL[1:0] bits in the RTC domain control register (RCC_BDCR) . This selection cannot be modified without resetting the RTC domain. The system must be always configured in a way that the PCLK frequency is greater than or equal to the RTCCLK frequency for proper operation of the RTC.

The LSE clock is in the RTC domain, whereas the HSE and LSI clocks are not. Consequently:

• • If LSE is selected as RTC clock:
  • – The RTC continues to work even if the \( V_{DD} \) supply is switched off, provided the \( V_{BAT} \) supply is maintained.
  • – The RTC remains clocked and functional under system reset
• • If LSI is selected as the RTC clock:
  • – The RTC state is not guaranteed if the \( V_{DD} \) supply is powered off. Refer to Section 6.2.6: LSI clock on page 104 for more details on LSI calibration.
• • If the HSE clock divided by 32 is used as the RTC clock:
  • – The RTC state is not guaranteed if the \( V_{DD} \) supply is powered off or if the internal voltage regulator is powered off (removing power from the 1.8 V domain).

When the RTC clock is LSE, the RTC remains clocked and functional under system reset.

6.2.11 Independent watchdog clock

If the Independent watchdog (IWDG) is started by either hardware option or software access, the LSI oscillator is forced ON and cannot be disabled. After the LSI oscillator temporization, the clock is provided to the IWDG.

6.2.12 Clock-out capability

The microcontroller clock output (MCO) capability allows the clock to be output onto the external MCO pin. The configuration registers of the corresponding GPIO port must be programmed in alternate function mode. One of the following clock signals can be selected as the MCO clock:

• • HSI14
• • SYSCLK
• • HSI
• • HSE
• • PLL clock divided by 2 or direct (direct connection is not available on STM32F05x devices)
• • LSE
• • LSI
• • HSI48 (on STM32F04x, STM32F07x and STM32F09x devices only)

The selection is controlled by the MCO[3:0] bits of the Clock configuration register (RCC_CFGR) .

For code example refer to the Appendix section A.3.3: MCO selection code example .

On STM32F03x, STM32F04x, STM32F07x and STM32F09x devices, the additional bit PLLNODIV of this register controls the divider bypass for a PLL clock input to MCO. The MCO frequency can be reduced by a configurable binary divider, controlled by the MCOPRE[2..0] bits of the Clock configuration register (RCC_CFGR) .

6.2.13 Internal/external clock measurement with TIM14

It is possible to indirectly measure the frequency of all on-board clock sources by mean of the TIM14 channel 1 input capture. As represented on Figure 13 .

Figure 13. Frequency measurement with TIM14 in capture mode

The input capture channel of the Timer 14 can be a GPIO line or an internal clock of the MCU. This selection is performed through the TI1_RMP [1:0] bits in the TIM14_OR register. The possibilities available are the following ones.

• • TIM14 Channel1 is connected to the GPIO. Refer to the alternate function mapping in the device datasheets.
• • TIM14 Channel1 is connected to the RTCCLK.
• • TIM14 Channel1 is connected to the HSE/32 Clock.
• • TIM14 Channel1 is connected to the microcontroller clock output (MCO). Refer to Section 6.2.12: Clock-out capability for MCO clock configuration.

For code example refer to the Appendix section A.3.4: Clock measurement configuration with TIM14 code example .

Calibration of the HSI

The primary purpose of connecting the LSE, through the MCO multiplexer, to the channel 1 input capture is to be able to precisely measure the HSI system clocks (for this, the HSI should be used as the system clock source). The number of HSI clock counts between consecutive edges of the LSE signal provides a measure of the internal clock period. Taking advantage of the high precision of LSE crystals (typically a few tens of ppm), it is possible to determine the internal clock frequency with the same resolution, and trim the source to compensate for manufacturing-process- and/or temperature- and voltage-related frequency deviations.

The HSI oscillator has dedicated user-accessible calibration bits for this purpose.

The basic concept consists in providing a relative measurement (e.g. the HSI/LSE ratio): the precision is therefore closely related to the ratio between the two clock sources. The higher the ratio is, the better the measurement is.

If LSE is not available, HSE/32 is the better option in order to reach the most precise calibration possible.

Calibration of the LSI

The calibration of the LSI will follow the same pattern that for the HSI, but changing the reference clock. It is necessary to connect LSI clock to the channel 1 input capture of the TIM14. Then define the HSE as system clock source, the number of its clock counts between consecutive edges of the LSI signal provides a measure of the internal low speed clock period.

The basic concept consists in providing a relative measurement (e.g. the HSE/LSI ratio): the precision is therefore closely related to the ratio between the two clock sources. The higher the ratio is, the better the measurement is.

Calibration of the HSI14

For the HSI14, because of its high frequency, it is not possible to have a precise resolution. However a solution could be to clock Timer 14 with HSE through PLL to reach 48 MHz, and to use the input capture line with the HSI14 and the capture prescaler defined to the higher value. In that configuration, we got a ratio of 27 events. It is still a bit low to have an accurate calibration. In order to increase the measure accuracy, it is advised to count the HSI periods after multiple cycles of Timer 14. Using polling to treat the capture event is necessary in this case.

6.3 Low-power modes

APB peripheral clocks and DMA clock can be disabled by software.

Sleep mode stops the CPU clock. The memory interface clocks (Flash and RAM interfaces) can be stopped by software during sleep mode. The AHB to APB bridge clocks are disabled by hardware during Sleep mode when all the clocks of the peripherals connected to them are disabled.

Stop mode stops all the clocks in the core supply domain and disables the PLL and the HSI, HSI48, HSI14 and HSE oscillators.

HDMI CEC, USART1, USART2 (only on STM32F07x and STM32F09x devices), USART3 (only on STM32F09x devices) and I2C1 have the capability to enable the HSI oscillator even when the MCU is in Stop mode (if HSI is selected as the clock source for that peripheral). When the system is in Stop mode, with the regulator in LP mode, the clock request coming from any of those three peripherals moves the regulator to MR mode in order to have the proper current drive capability for the core logic. The regulator moves back to LP mode once this request is removed without waking up the MCU.

HDMI CEC, USART1, USART2 (only on STM32F07x and STM32F09x devices) and USART3 (only on STM32F09x devices) can also be driven by the LSE oscillator when the system is in Stop mode (if LSE is selected as clock source for that peripheral) and the LSE oscillator is enabled (LSEON) but they do not have the capability to turn on the LSE oscillator.

Standby mode stops all the clocks in the core supply domain and disables the PLL and the HSI, HSI48, HSI14 and HSE oscillators.

The CPU's deepsleep mode can be overridden for debugging by setting the DBG_STOP or DBG_STANDBY bits in the DBGMCU_CR register.

When waking up from deepsleep after an interrupt (Stop mode) or reset (Standby mode), the HSI oscillator is selected as system clock.

If a Flash programming operation is on going, deepsleep mode entry is delayed until the Flash interface access is finished. If an access to the APB domain is ongoing, deepsleep mode entry is delayed until the APB access is finished.

6.4 RCC registers

Refer to Section 1.2 on page 42 for a list of abbreviations used in register descriptions.

6.4.1 Clock control register (RCC_CR)

Address offset: 0x00

Reset value: 0x0000 XX83 where X is undefined.

Access: no wait state, word, half-word and byte access

Bits 31:26 Reserved, must be kept at reset value.

Bit 25 PLL RDY : PLL clock ready flag

Set by hardware to indicate that the PLL is locked.

0: PLL unlocked

1: PLL locked

Bit 24 PLLON : PLL enable

Set and cleared by software to enable PLL.

Cleared by hardware when entering Stop or Standby mode. This bit can not be reset if the PLL clock is used as system clock or is selected to become the system clock.

0: PLL OFF

1: PLL ON

Bits 23:20 Reserved, must be kept at reset value.

Bit 19 CSSON : Clock security system enable

Set and cleared by software to enable the clock security system. When CSSON is set, the clock detector is enabled by hardware when the HSE oscillator is ready, and disabled by hardware if a HSE clock failure is detected.

0: Clock security system disabled (clock detector OFF).

1: Clock security system enabled (clock detector ON if the HSE is ready, OFF if not).

Bit 18 HSEBYP : HSE crystal oscillator bypass

Set and cleared by software to bypass the oscillator with an external clock. The external clock must be enabled with the HSEON bit set, to be used by the device. The HSEBYP bit can be written only if the HSE oscillator is disabled.

0: HSE crystal oscillator not bypassed

1: HSE crystal oscillator bypassed with external clock

Set by hardware to indicate that the HSE oscillator is stable. This bit needs 6 cycles of the HSE oscillator clock to fall down after HSEON reset.

0: HSE oscillator not ready

1: HSE oscillator ready

Set and cleared by software.

Cleared by hardware to stop the HSE oscillator when entering Stop or Standby mode. This bit cannot be reset if the HSE oscillator is used directly or indirectly as the system clock.

0: HSE oscillator OFF

1: HSE oscillator ON

These bits are initialized automatically at startup. They are adjusted by SW through the HSITRIM setting.

These bits provide an additional user-programmable trimming value that is added to the HSICAL[7:0] bits. It can be programmed to adjust to variations in voltage and temperature that influence the frequency of the HSI.

The default value is 16, which, when added to the HSICAL value, should trim the HSI to 8 MHz \( \pm \) 1%. The trimming step is around 40 kHz between two consecutive HSICAL steps.

Note: Increased value in the register results to higher clock frequency.

Bit 2 Reserved, must be kept at reset value.

Set by hardware to indicate that HSI oscillator is stable. After the HSION bit is cleared, HSIRDY goes low after 6 HSI oscillator clock cycles.

0: HSI oscillator not ready

1: HSI oscillator ready

Set and cleared by software.

Set by hardware to force the HSI oscillator ON when leaving Stop or Standby mode or in case of failure of the HSE crystal oscillator used directly or indirectly as system clock. This bit cannot be reset if the HSI is used directly or indirectly as system clock or is selected to become the system clock.

0: HSI oscillator OFF

1: HSI oscillator ON

6.4.2 Clock configuration register (RCC_CFGR)

Address offset: 0x04

Reset value: 0x0000 0000

Access: 0 \( \leq \) wait state \( \leq \) 2, word, half-word and byte access

1 or 2 wait states inserted only if the access occurs during clock source switch.

Bit 31 PLLNODIV : PLL clock not divided for MCO (not available on STM32F05x devices)

This bit is set and cleared by software. It switches off divider by 2 for PLL connection to MCO.

0: PLL is divided by 2 for MCO

1: PLL is not divided for MCO

Bits 30:28 MCOPRE[2:0] : Microcontroller clock output prescaler (not available on STM32F05x devices)

These bits are set and cleared by software to select the MCO prescaler division factor. To avoid glitches, it is highly recommended to change this prescaler only when the MCO output is disabled.

000: MCO is divided by 1

001: MCO is divided by 2

010: MCO is divided by 4

.....

111: MCO is divided by 128

Bits 27:24 MCO[3:0] : Microcontroller clock output

Set and cleared by software.

0000: MCO output disabled, no clock on MCO

0001: Internal RC 14 MHz (HSI14) oscillator clock selected

0010: Internal low speed (LSI) oscillator clock selected

0011: External low speed (LSE) oscillator clock selected

0100: System clock selected

0101: Internal RC 8 MHz (HSI) oscillator clock selected

0110: External 4-32 MHz (HSE) oscillator clock selected

0111: PLL clock selected (divided by 1 or 2, depending on PLLNODIV)

1000: Internal RC 48 MHz (HSI48) oscillator clock selected

Note: This clock output may have some truncated cycles at startup or during MCO clock source switching.

Bits 23:22 Reserved, must be kept at reset value.

These bits are written by software to define the PLL multiplication factor. These bits can be written only when PLL is disabled.

Caution: The PLL output frequency must not exceed 48 MHz.

• 0000: PLL input clock x 2
• 0001: PLL input clock x 3
• 0010: PLL input clock x 4
• 0011: PLL input clock x 5
• 0100: PLL input clock x 6
• 0101: PLL input clock x 7
• 0110: PLL input clock x 8
• 0111: PLL input clock x 9
• 1000: PLL input clock x 10
• 1001: PLL input clock x 11
• 1010: PLL input clock x 12
• 1011: PLL input clock x 13
• 1100: PLL input clock x 14
• 1101: PLL input clock x 15
• 1110: PLL input clock x 16
• 1111: PLL input clock x 16

This bit is the same bit as bit PREDIV[0] from RCC_CFGR2. Refer to RCC_CFGR2 PREDIV bits description for its meaning.

Set and cleared by software to select PLL or PREDIV clock source. These bits can be written only when PLL is disabled.

• 00: HSI/2 selected as PLL input clock (PREDIV forced to divide by 2 on STM32F04x, STM32F07x and STM32F09x devices)
• 01: HSI/PREDIV selected as PLL input clock
• 10: HSE/PREDIV selected as PLL input clock
• 11: HSI48/PREDIV selected as PLL input clock

Bit PLLSRC[0] is available only on STM32F04x, STM32F07x and STM32F09x devices, otherwise it is reserved (with value zero).

Obsolete setting. Proper ADC clock selection is done inside the ADC_CFGR2 (refer to Section 13.11.5: ADC configuration register 2 (ADC_CFGR2) on page 271 ).

Bits 13:11 Reserved, must be kept at reset value.

Set and cleared by software to control the division factor of the APB clock (PCLK).

• 0xx: HCLK not divided
• 100: HCLK divided by 2
• 101: HCLK divided by 4
• 110: HCLK divided by 8
• 111: HCLK divided by 16

Bits 7:4 HPRE[3:0] : HCLK prescaler

Set and cleared by software to control the division factor of the AHB clock.

• 0xxx: SYSCLK not divided
• 1000: SYSCLK divided by 2
• 1001: SYSCLK divided by 4
• 1010: SYSCLK divided by 8
• 1011: SYSCLK divided by 16
• 1100: SYSCLK divided by 64
• 1101: SYSCLK divided by 128
• 1110: SYSCLK divided by 256
• 1111: SYSCLK divided by 512

Bits 3:2 SWS[1:0] : System clock switch status

Set and cleared by hardware to indicate which clock source is used as system clock.

• 00: HSI oscillator used as system clock
• 01: HSE oscillator used as system clock
• 10: PLL used as system clock
• 11: HSI48 oscillator used as system clock (when available)

Bits 1:0 SW[1:0] : System clock switch

Set and cleared by software to select SYSCLK source.

Cleared by hardware to force HSI selection when leaving Stop and Standby mode or in case of failure of the HSE oscillator used directly or indirectly as system clock (if the Clock Security System is enabled).

• 00: HSI selected as system clock
• 01: HSE selected as system clock
• 10: PLL selected as system clock
• 11: HSI48 selected as system clock (when available)

6.4.3 Clock interrupt register (RCC_CIR)

Address offset: 0x08

Reset value: 0x0000 0000

Access: no wait state, word, half-word and byte access

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

Bit 23 CSSC : Clock security system interrupt clear

This bit is set by software to clear the CSSF flag.

• 0: No effect
• 1: Clear CSSF flag

Bit 22 HSI48RDYC : HSI48 Ready Interrupt Clear

This bit is set by software to clear the HSI48RDYF flag.

0: No effect

1: Clear HSI48RDYF flag

Bit 21 HSI14RDYC : HSI14 ready interrupt clear

This bit is set by software to clear the HSI14RDYF flag.

0: No effect

1: Clear HSI14RDYF flag

Bit 20 PLLRDYC : PLL ready interrupt clear

This bit is set by software to clear the PLLRDYF flag.

0: No effect

1: Clear PLLRDYF flag

Bit 19 HSERDYC : HSE ready interrupt clear

This bit is set by software to clear the HSERDYF flag.

0: No effect

1: Clear HSERDYF flag

Bit 18 HSIRDYC : HSI ready interrupt clear

This bit is set software to clear the HSIRDYF flag.

0: No effect

1: Clear HSIRDYF flag

Bit 17 LSERDYC : LSE ready interrupt clear

This bit is set by software to clear the LSERDYF flag.

0: No effect

1: LSERDYF cleared

Bit 16 LSIRDYC : LSI ready interrupt clear

This bit is set by software to clear the LSIRDYF flag.

0: No effect

1: LSIRDYF cleared

Bit 15 Reserved, must be kept at reset value.

Bit 14 HSI48RDYIE : HSI48 ready interrupt enable

Set and cleared by software to enable/disable interrupt caused by the HSI48 oscillator stabilization.

0: HSI48 ready interrupt disabled

1: HSI48 ready interrupt enabled

Bit 13 HSI14RDYIE : HSI14 ready interrupt enable

Set and cleared by software to enable/disable interrupt caused by the HSI14 oscillator stabilization.

0: HSI14 ready interrupt disabled

1: HSI14 ready interrupt enabled

Bit 12 PLLRDYIE : PLL ready interrupt enable

Set and cleared by software to enable/disable interrupt caused by PLL lock.

0: PLL lock interrupt disabled

1: PLL lock interrupt enabled

Set and cleared by software to enable/disable interrupt caused by the HSE oscillator stabilization.

• 0: HSE ready interrupt disabled
• 1: HSE ready interrupt enabled

Set and cleared by software to enable/disable interrupt caused by the HSI oscillator stabilization.

• 0: HSI ready interrupt disabled
• 1: HSI ready interrupt enabled

Set and cleared by software to enable/disable interrupt caused by the LSE oscillator stabilization.

• 0: LSE ready interrupt disabled
• 1: LSE ready interrupt enabled

Set and cleared by software to enable/disable interrupt caused by the LSI oscillator stabilization.

• 0: LSI ready interrupt disabled
• 1: LSI ready interrupt enabled

Set by hardware when a failure is detected in the HSE oscillator.

Cleared by software setting the CSSC bit.

• 0: No clock security interrupt caused by HSE clock failure
• 1: Clock security interrupt caused by HSE clock failure

Set by hardware when the HSI48 becomes stable and HSI48RDYDIE is set in a response to setting the HSI48ON bit in Clock control register 2 (RCC_CR2). When HSI48ON is not set but the HSI48 oscillator is enabled by the peripheral through a clock request, this bit is not set and no interrupt is generated.

Cleared by software setting the HSI48RDYC bit.

• 0: No clock ready interrupt caused by the HSI48 oscillator
• 1: Clock ready interrupt caused by the HSI48 oscillator

Set by hardware when the HSI14 becomes stable and HSI14RDYDIE is set in a response to setting the HSI14ON bit in Clock control register 2 (RCC_CR2) . When HSI14ON is not set but the HSI14 oscillator is enabled by the peripheral through a clock request, this bit is not set and no interrupt is generated.

Cleared by software setting the HSI14RDYC bit.

• 0: No clock ready interrupt caused by the HSI14 oscillator
• 1: Clock ready interrupt caused by the HSI14 oscillator

Set by hardware when the PLL locks and PLLRDYDIE is set.

Cleared by software setting the PLLRDYC bit.

• 0: No clock ready interrupt caused by PLL lock
• 1: Clock ready interrupt caused by PLL lock

Bit 3 HSERDYF : HSE ready interrupt flag

Set by hardware when the HSE clock becomes stable and HSERDYDIE is set.

Cleared by software setting the HSERDYC bit.

0: No clock ready interrupt caused by the HSE oscillator

1: Clock ready interrupt caused by the HSE oscillator

Bit 2 HSIRDYF : HSI ready interrupt flag

Set by hardware when the HSI clock becomes stable and HSIRDYDIE is set in a response to setting the HSION (refer to Clock control register (RCC_CR) ). When HSION is not set but the HSI oscillator is enabled by the peripheral through a clock request, this bit is not set and no interrupt is generated.

Cleared by software setting the HSIRDYC bit.

0: No clock ready interrupt caused by the HSI oscillator

1: Clock ready interrupt caused by the HSI oscillator

Bit 1 LSERDYF : LSE ready interrupt flag

Set by hardware when the LSE clock becomes stable and LSERDYDIE is set.

Cleared by software setting the LSERDYC bit.

0: No clock ready interrupt caused by the LSE oscillator

1: Clock ready interrupt caused by the LSE oscillator

Bit 0 LSIRDYF : LSI ready interrupt flag

Set by hardware when the LSI clock becomes stable and LSIRDYDIE is set.

Cleared by software setting the LSIRDYC bit.

0: No clock ready interrupt caused by the LSI oscillator

1: Clock ready interrupt caused by the LSI oscillator

6.4.4 APB peripheral reset register 2 (RCC_APB2RSTR)

Address offset: 0x0C

Reset value: 0x00000 0000

Access: no wait state, word, half-word and byte access

Bits 31:23 Reserved, must be kept at reset value.

Bit 22 DBGMCURST : Debug MCU reset

Set and cleared by software.

0: No effect

1: Reset Debug MCU

Bits 21:19 Reserved, must be kept at reset value.

1. Bit 18 TIM17RST : TIM17 timer reset
   Set and cleared by software.
   0: No effect
   1: Reset TIM17 timer
2. Bit 17 TIM16RST : TIM16 timer reset
   Set and cleared by software.
   0: No effect
   1: Reset TIM16 timer
3. Bit 16 TIM15RST : TIM15 timer reset
   Set and cleared by software.
   0: No effect
   1: Reset TIM15 timer
4. Bit 15 Reserved, must be kept at reset value.
5. Bit 14 USART1RST : USART1 reset
   Set and cleared by software.
   0: No effect
   1: Reset USART1
6. Bit 13 Reserved, must be kept at reset value.
7. Bit 12 SPI1RST : SPI1 reset
   Set and cleared by software.
   0: No effect
   1: Reset SPI1
8. Bit 11 TIM1RST : TIM1 timer reset
   Set and cleared by software.
   0: No effect
   1: Reset TIM1 timer
9. Bit 10 Reserved, must be kept at reset value.
10. Bit 9 ADCIRST : ADC interface reset
    Set and cleared by software.
    0: No effect
    1: Reset ADC interface
11. Bit 8 Reserved, must be kept at reset value.
12. Bit 7 USART8RST : USART8 reset
    Set and cleared by software
    0: No effect
    1: Reset USART8
13. Bit 6 USART7RST : USART7 reset
    Set and cleared by software
    0: No effect
    1: Reset USART7
14. Bit 5 USART6RST : USART6 reset
    Set and cleared by software
    0: No effect
    1: Reset USART6

Bits 4:1 Reserved, must be kept at reset value.

Bit 0 SYSCFGRST : SYSCFG reset

Set and cleared by software.

0: No effect

1: Reset SYSCFG

6.4.5 APB peripheral reset register 1 (RCC_APB1RSTR)

Address offset: 0x10

Reset value: 0x0000 0000

Access: no wait state, word, half-word and byte access

Bit 31 Reserved, must be kept at reset value.

Bit 30 CECRST : HDMI CEC reset

Set and cleared by software.

0: No effect

1: Reset HDMI CEC

Bit 29 DACRST : DAC interface reset

Set and cleared by software.

0: No effect

1: Reset DAC interface

Bit 28 PWRRST : Power interface reset

Set and cleared by software.

0: No effect

1: Reset power interface

Bit 27 CRSRST : Clock recovery system interface reset

Set and cleared by software.

0: No effect

1: Reset CRS interface

Bit 26 Reserved, must be kept at reset value.

Bit 25 CANRST : CAN interface reset

Set and cleared by software.

0: No effect

1: Reset CAN interface

Bit 24 Reserved, must be kept at reset value.

Bit 23 USBRST : USB interface reset

Set and cleared by software.

0: No effect

1: Reset USB interface

Bit 22 I2C2RST : I2C2 reset

Set and cleared by software.

0: No effect

1: Reset I2C2

Bit 21 I2C1RST : I2C1 reset

Set and cleared by software.

0: No effect

1: Reset I2C1

Bit 20 USART5RST : USART5 reset

Set and cleared by software.

0: No effect

1: Reset USART4

Bit 19 USART4RST : USART4 reset

Set and cleared by software.

0: No effect

1: Reset USART4

Bit 18 USART3RST : USART3 reset

Set and cleared by software.

0: No effect

1: Reset USART3

Bit 17 USART2RST : USART2 reset

Set and cleared by software.

0: No effect

1: Reset USART2

Bits 16:15 Reserved, must be kept at reset value.

Bit 14 SPI2RST : SPI2 reset

Set and cleared by software.

0: No effect

1: Reset SPI2

Bits 13:12 Reserved, must be kept at reset value.

Bit 11 WWDGRST : Window watchdog reset

Set and cleared by software.

0: No effect

1: Reset window watchdog

Bits 10:9 Reserved, must be kept at reset value.

Bit 8 TIM14RST : TIM14 timer reset

Set and cleared by software.

0: No effect

1: Reset TIM14

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

Bit 5 TIM7RST : TIM7 timer reset
Set and cleared by software.

0: No effect
1: Reset TIM7

Bit 4 TIM6RST : TIM6 timer reset
Set and cleared by software.

0: No effect
1: Reset TIM6

Bits 3:2 Reserved, must be kept at reset value.

Bit 1 TIM3RST : TIM3 timer reset
Set and cleared by software.

0: No effect
1: Reset TIM3

Bit 0 TIM2RST : TIM2 timer reset
Set and cleared by software.

0: No effect
1: Reset TIM2

6.4.6 AHB peripheral clock enable register (RCC_AHBENR)

Address offset: 0x14

Reset value: 0x0000 0014

Access: no wait state, word, half-word and byte access

Note: When the peripheral clock is not active, the peripheral register values may not be readable by software and the returned value is always 0x0.

Bits 31:25 Reserved, must be kept at reset value.

Bit 24 TSCEN : Touch sensing controller clock enable

Set and cleared by software.

0: TSC clock disabled
1: TSC clock enabled

Bit 23 Reserved, must be kept at reset value.

Bit 22 IOPFEN : I/O port F clock enable

Set and cleared by software.

0: I/O port F clock disabled
1: I/O port F clock enabled

1. Bit 21 IOPEN : I/O port E clock enable
   Set and cleared by software.
   0: I/O port E clock disabled
   1: I/O port E clock enabled
2. Bit 20 IOPDEN : I/O port D clock enable
   Set and cleared by software.
   0: I/O port D clock disabled
   1: I/O port D clock enabled
3. Bit 19 IOPCEN : I/O port C clock enable
   Set and cleared by software.
   0: I/O port C clock disabled
   1: I/O port C clock enabled
4. Bit 18 IOPBEN : I/O port B clock enable
   Set and cleared by software.
   0: I/O port B clock disabled
   1: I/O port B clock enabled
5. Bit 17 IOPAEN : I/O port A clock enable
   Set and cleared by software.
   0: I/O port A clock disabled
   1: I/O port A clock enabled
6. Bits 16:7 Reserved, must be kept at reset value.
7. Bit 6 CRCEN : CRC clock enable
   Set and cleared by software.
   0: CRC clock disabled
   1: CRC clock enabled
8. Bit 5 Reserved, must be kept at reset value.
9. Bit 4 FLITFEN : FLITF clock enable
   Set and cleared by software to disable/enable FLITF clock during Sleep mode.
   0: FLITF clock disabled during Sleep mode
   1: FLITF clock enabled during Sleep mode
10. Bit 3 Reserved, must be kept at reset value.
11. Bit 2 SRAMEN : SRAM interface clock enable
    Set and cleared by software to disable/enable SRAM interface clock during Sleep mode.
    0: SRAM interface clock disabled during Sleep mode.
    1: SRAM interface clock enabled during Sleep mode
12. Bit 1 DMA2EN : DMA2 clock enable
    Set and cleared by software.
    0: DMA2 clock disabled
    1: DMA2 clock enabled
13. Bit 0 DMAEN : DMA clock enable
    Set and cleared by software.
    0: DMA clock disabled
    1: DMA clock enabled

6.4.7 APB peripheral clock enable register 2 (RCC_APB2ENR)

Address: 0x18

Reset value: 0x0000 0000

Access: word, half-word and byte access

No wait states, except if the access occurs while an access to a peripheral in the APB domain is on going. In this case, wait states are inserted until the access to APB peripheral is finished.

Note: When the peripheral clock is not active, the peripheral register values may not be readable by software and the returned value is always 0x0.

Bits 31:23 Reserved, must be kept at reset value.

Bit 22 DBGMCUEN MCU debug module clock enable

Set and reset by software.

0: MCU debug module clock disabled

1: MCU debug module enabled

Bits 21:19 Reserved, must be kept at reset value.

Bit 18 TIM17EN : TIM17 timer clock enable

Set and cleared by software.

0: TIM17 timer clock disabled

1: TIM17 timer clock enabled

Bit 17 TIM16EN : TIM16 timer clock enable

Set and cleared by software.

0: TIM16 timer clock disabled

1: TIM16 timer clock enabled

Bit 16 TIM15EN : TIM15 timer clock enable

Set and cleared by software.

0: TIM15 timer clock disabled

1: TIM15 timer clock enabled

Bit 15 Reserved, must be kept at reset value.

Bit 14 USART1EN : USART1 clock enable

Set and cleared by software.

0: USART1 clock disabled

1: USART1 clock enabled

Bit 13 Reserved, must be kept at reset value.

1. Bit 12 SPI1EN : SPI1 clock enable
   Set and cleared by software.
   0: SPI1 clock disabled
   1: SPI1 clock enabled
2. Bit 11 TIM1EN : TIM1 timer clock enable
   Set and cleared by software.
   0: TIM1 timer clock disabled
   1: TIM1P timer clock enabled
3. Bit 10 Reserved, must be kept at reset value.
4. Bit 9 ADCEN : ADC interface clock enable
   Set and cleared by software.
   0: ADC interface disabled
   1: ADC interface clock enabled
5. Bit 8 Reserved, must be kept at reset value.
6. Bit 7 USART8EN : USART8 clock enable
   Set and cleared by software.
   0: USART8clock disabled
   1: USART8clock enabled
7. Bit 6 USART7EN : USART7 clock enable
   Set and cleared by software.
   0: USART7clock disabled
   1: USART7clock enabled
8. Bit 5 USART6EN : USART6 clock enable
   Set and cleared by software.
   0: USART6clock disabled
   1: USART6clock enabled
9. Bits 4:1 Reserved, must be kept at reset value.
10. Bit 0 SYSCFGCOMPEN : SYSCFG & COMP clock enable
    Set and cleared by software.
    0: SYSCFG & COMP clock disabled
    1: SYSCFG & COMP clock enabled

6.4.8 APB peripheral clock enable register 1 (RCC_APB1ENR)

Address: 0x1C

Reset value: 0x0000 0000

Access: word, half-word and byte access

No wait state, except if the access occurs while an access to a peripheral on APB domain is on going. In this case, wait states are inserted until this access to APB peripheral is finished.

Note: When the peripheral clock is not active, the peripheral register values may not be readable by software and the returned value is always 0x0.

Bit 31 Reserved, must be kept at reset value.

Bit 30 CECEN : HDMI CEC clock enable

Set and cleared by software.

0: HDMI CEC clock disabled

1: HDMI CEC clock enabled

Bit 29 DACEN : DAC interface clock enable

Set and cleared by software.

0: DAC interface clock disabled

1: DAC interface clock enabled

Bit 28 PWREN : Power interface clock enable

Set and cleared by software.

0: Power interface clock disabled

1: Power interface clock enabled

Bit 27 CRSEN : Clock recovery system interface clock enable

Set and cleared by software.

0: CRS interface clock disabled

1: CRS interface clock enabled

Bit 26 Reserved, must be kept at reset value.

Bit 25 CANEN : CAN interface clock enable

Set and cleared by software.

0: CAN interface clock disabled

1: CAN interface clock enabled

Bit 24 Reserved, must be kept at reset value.

Bit 23 USBEN : USB interface clock enable

Set and cleared by software.

0: USB interface clock disabled

1: USB interface clock enabled

Bit 22 I2C2EN : I2C2 clock enable

Set and cleared by software.

0: I2C2 clock disabled

1: I2C2 clock enabled

Bit 21 I2C1EN : I2C1 clock enable

Set and cleared by software.

0: I2C1 clock disabled

1: I2C1 clock enabled

1. Bit 20 USART5EN : USART5 clock enable
   Set and cleared by software.
   0: USART5 clock disabled
   1: USART5 clock enabled
2. Bit 19 USART4EN : USART4 clock enable
   Set and cleared by software.
   0: USART4 clock disabled
   1: USART4 clock enabled
3. Bit 18 USART3EN : USART3 clock enable
   Set and cleared by software.
   0: USART3 clock disabled
   1: USART3 clock enabled
4. Bit 17 USART2EN : USART2 clock enable
   Set and cleared by software.
   0: USART2 clock disabled
   1: USART2 clock enabled
5. Bits 16:15 Reserved, must be kept at reset value.
6. Bit 14 SPI2EN : SPI2 clock enable
   Set and cleared by software.
   0: SPI2 clock disabled
   1: SPI2 clock enabled
7. Bits 13:12 Reserved, must be kept at reset value.
8. Bit 11 WWDGEN : Window watchdog clock enable
   Set and cleared by software.
   0: Window watchdog clock disabled
   1: Window watchdog clock enabled
9. Bits 10:9 Reserved, must be kept at reset value.
10. Bit 8 TIM14EN : TIM14 timer clock enable
    Set and cleared by software.
    0: TIM14 clock disabled
    1: TIM14 clock enabled
11. Bits 7:6 Reserved, must be kept at reset value.
12. Bit 5 TIM7EN : TIM7 timer clock enable
    Set and cleared by software.
    0: TIM7 clock disabled
    1: TIM7 clock enabled
13. Bit 4 TIM6EN : TIM6 timer clock enable
    Set and cleared by software.
    0: TIM6 clock disabled
    1: TIM6 clock enabled
14. Bits 3:2 Reserved, must be kept at reset value.

Bit 1 TIM3EN : TIM3 timer clock enable

Set and cleared by software.

0: TIM3 clock disabled

1: TIM3 clock enabled

Bit 0 TIM2EN : TIM2 timer clock enable

Set and cleared by software.

0: TIM2 clock disabled

1: TIM2 clock enabled

6.4.9 RTC domain control register (RCC_BDCR)

Address offset: 0x20

Reset value: 0x0000 0018, reset by RTC domain reset.

Access: 0 ≤ wait state ≤ 3, word, half-word and byte access

Wait states are inserted in case of successive accesses to this register.

Note: The LSEON , LSEBYP , RTCSEL and RTCEN bits of the RTC domain control register (RCC_BDCR) are in the RTC domain. As a result, after reset, these bits are write-protected and the DBP bit in the Power control register (PWR_CR) has to be set before they can be modified. Refer to Section 5.1.3: Battery backup domain for further information. These bits are only reset after a RTC domain reset (see Section 6.1.3: RTC domain reset ). Any internal or external reset does not have any effect on these bits.

Bits 31:17 Reserved, must be kept at reset value.

Bit 16 BDRST : RTC domain software reset

Set and cleared by software.

0: Reset not activated

1: Resets the entire RTC domain

Bit 15 RTCEN : RTC clock enable

Set and cleared by software.

0: RTC clock disabled

1: RTC clock enabled

Bits 14:10 Reserved, must be kept at reset value.

Bits 9:8 RTCSEL[1:0] : RTC clock source selection

Set by software to select the clock source for the RTC. Once the RTC clock source has been selected, it cannot be changed anymore unless the RTC domain is reset. The BDRST bit can be used to reset them.

• 00: No clock
• 01: LSE oscillator clock used as RTC clock
• 10: LSI oscillator clock used as RTC clock
• 11: HSE oscillator clock divided by 32 used as RTC clock

Bits 7:5 Reserved, must be kept at reset value.

Bits 4:3 LSEDRV LSE oscillator drive capability

Set and reset by software to modulate the LSE oscillator’s drive capability. A reset of the RTC domain restores the default value.

• 00: ‘Xtal mode’ low drive capability
• 01: ‘Xtal mode’ medium-high drive capability
• 10: ‘Xtal mode’ medium-low drive capability
• 11: ‘Xtal mode’ high drive capability (reset value)

Note: The oscillator is in Xtal mode when it is not in bypass mode.

Bit 2 LSEBYP : LSE oscillator bypass

Set and cleared by software to bypass oscillator in debug mode. This bit can be written only when the external 32 kHz oscillator is disabled.

• 0: LSE oscillator not bypassed
• 1: LSE oscillator bypassed

Bit 1 LSERDY : LSE oscillator ready

Set and cleared by hardware to indicate when the external 32 kHz oscillator is stable. After the LSEON bit is cleared, LSERDY goes low after 6 external low-speed oscillator clock cycles.

• 0: LSE oscillator not ready
• 1: LSE oscillator ready

Bit 0 LSEON : LSE oscillator enable

Set and cleared by software.

• 0: LSE oscillator OFF
• 1: LSE oscillator ON

6.4.10 Control/status register (RCC_CSR)

Address: 0x24

Reset value: 0xXXX0 0000, reset by system Reset, except reset flags by power Reset only.

Access: 0 ≤ wait state ≤ 3, word, half-word and byte access

Wait states are inserted in case of successive accesses to this register.

Bit 31 LPWRRSTF : Low-power reset flag

Set by hardware when a Low-power management reset occurs.

Cleared by writing to the RMVF bit.

0: No Low-power management reset occurred

1: Low-power management reset occurred

For further information refer to Low-power management reset .

Bit 30 WWDGIRSTF : Window watchdog reset flag

Set by hardware when a window watchdog reset occurs.

Cleared by writing to the RMVF bit.

0: No window watchdog reset occurred

1: Window watchdog reset occurred

Bit 29 IWDGIRSTF : Independent watchdog reset flag

Set by hardware when an independent watchdog reset from V _DD domain occurs.

Cleared by writing to the RMVF bit.

0: No watchdog reset occurred

1: Watchdog reset occurred

Bit 28 SFTRSTF : Software reset flag

Set by hardware when a software reset occurs.

Cleared by writing to the RMVF bit.

0: No software reset occurred

1: Software reset occurred

Bit 27 PORRSTF : POR/PDR reset flag

Set by hardware when a POR/PDR reset occurs.

Cleared by writing to the RMVF bit.

0: No POR/PDR reset occurred

1: POR/PDR reset occurred

Bit 26 PINRSTF : PIN reset flag

Set by hardware when a reset from the NRST pin occurs.

Cleared by writing to the RMVF bit.

0: No reset from NRST pin occurred

1: Reset from NRST pin occurred

Bit 25 OBLRSTF : Option byte loader reset flag

Set by hardware when a reset from the OBL occurs.

Cleared by writing to the RMVF bit.

0: No reset from OBL occurred

1: Reset from OBL occurred

Bit 24 RMVF : Remove reset flag

Set by software to clear the reset flags including RMVF.

0: No effect

1: Clear the reset flags

Bit 23 V18PWRIRSTF : Reset flag of the 1.8 V domain.

Set by hardware when a POR/PDR of the 1.8 V domain occurred.

Cleared by writing to the RMVF bit.

0: No POR/PDR reset of the 1.8 V domain occurred

1: POR/PDR reset of the 1.8 V domain occurred

Caution: On the STM32F0x8 family, this flag must be read as reserved.

Bits 22:2 Reserved, must be kept at reset value.

Bit 1 LSIRDY : LSI oscillator ready

Set and cleared by hardware to indicate when the LSI oscillator is stable. After the LSION bit is cleared, LSIRDY goes low after 3 LSI oscillator clock cycles.

0: LSI oscillator not ready

1: LSI oscillator ready

Bit 0 LSION : LSI oscillator enable

Set and cleared by software.

0: LSI oscillator OFF

1: LSI oscillator ON

6.4.11 AHB peripheral reset register (RCC_AHBRSTR)

Address: 0x28

Reset value: 0x0000 0000

Access: no wait states, word, half-word and byte access

Bits 31:25 Reserved, must be kept at reset value.

Bit 24 TSCRST : Touch sensing controller reset

Set and cleared by software.

0: No effect

1: Reset TSC

Bit 23 Reserved, must be kept at reset value.

Bit 22 IOPFRST : I/O port F reset

Set and cleared by software.

0: No effect

1: Reset I/O port F

Bit 21 IOPERST : I/O port E reset

Set and cleared by software.

0: No effect

1: Reset I/O port E

Bit 20 IOPDRST : I/O port D reset

Set and cleared by software.

0: No effect

1: Reset I/O port D

Bit 19 IOPCRST : I/O port C reset
Set and cleared by software.

0: No effect
1: Reset I/O port C

Bit 18 IOPBRST : I/O port B reset
Set and cleared by software.

0: No effect
1: Reset I/O port B

Bit 17 IOPARST : I/O port A reset
Set and cleared by software.

0: No effect
1: Reset I/O port A

Bits 16:0 Reserved, must be kept at reset value.

6.4.12 Clock configuration register 2 (RCC_CFGR2)

Address: 0x2C

Reset value: 0x0000 0000

Access: no wait states, word, half-word and byte access

Bits 31:4 Reserved, must be kept at reset value.

Bits 3:0 PREDIV[3:0] PREDIV division factor

These bits are set and cleared by software to select PREDIV division factor. They can be written only when the PLL is disabled.

Note: Bit 0 is the same bit as bit 17 in Clock configuration register (RCC_CFGR) , so modifying bit 17 in Clock configuration register (RCC_CFGR) also modifies bit 0 in Clock configuration register 2 (RCC_CFGR2) (for compatibility with other STM32 products)

0000: PREDIV input clock not divided

0001: PREDIV input clock divided by 2

0010: PREDIV input clock divided by 3

0011: PREDIV input clock divided by 4

0100: PREDIV input clock divided by 5

0101: PREDIV input clock divided by 6

0110: PREDIV input clock divided by 7

0111: PREDIV input clock divided by 8

1000: PREDIV input clock divided by 9

1001: PREDIV input clock divided by 10

1010: PREDIV input clock divided by 11

1011: PREDIV input clock divided by 12

1100: PREDIV input clock divided by 13

1101: PREDIV input clock divided by 14

1110: PREDIV input clock divided by 15

1111: PREDIV input clock divided by 16

6.4.13 Clock configuration register 3 (RCC_CFGR3)

Address: 0x30

Reset value: 0x0000 0000

Access: no wait states, word, half-word and byte access

Bits 31:20 Reserved, must be kept at reset value.

Bits 19:18 USART3SW[1:0] : USART3 clock source selection (available only on STM32F09x devices)

This bit is set and cleared by software to select the USART3 clock source.

00: PCLK selected as USART3 clock source (default)

01: System clock (SYSCLK) selected as USART3 clock

10: LSE clock selected as USART3 clock

11: HSI clock selected as USART3 clock

Bits 17:16 USART2SW[1:0] : USART2 clock source selection (available only on STM32F07x and STM32F09x devices)

This bit is set and cleared by software to select the USART2 clock source.

• 00: PCLK selected as USART2 clock source (default)
• 01: System clock (SYSCLK) selected as USART2 clock
• 10: LSE clock selected as USART2 clock
• 11: HSI clock selected as USART2 clock

Bits 15:9 Reserved, must be kept at reset value.

Bit 8 ADC SW : ADC clock source selection

Obsolete setting. To be kept at reset value, connecting the HSI14 clock to the ADC asynchronous clock input. Proper ADC clock selection is done inside the ADC_CFGR2 (refer to Section 13.11.5: ADC configuration register 2 (ADC_CFGR2) on page 271 ).

Bit 7 USBSW : USB clock source selection

This bit is set and cleared by software to select the USB clock source.

• 0: HSI48 clock selected as USB clock source (default)
• 1: PLL clock (PLLCLK) selected as USB clock

Bit 6 CECSW : HDMI CEC clock source selection

This bit is set and cleared by software to select the CEC clock source.

• 0: HSI clock, divided by 244, selected as CEC clock (default)
• 1: LSE clock selected as CEC clock

Bit 5 Reserved, must be kept at reset value.

Bit 4 I2C1SW : I2C1 clock source selection

This bit is set and cleared by software to select the I2C1 clock source.

• 0: HSI clock selected as I2C1 clock source (default)
• 1: System clock (SYSCLK) selected as I2C1 clock

Bits 3:2 Reserved, must be kept at reset value.

Bits 1:0 USART1SW[1:0] : USART1 clock source selection

This bit is set and cleared by software to select the USART1 clock source.

• 00: PCLK selected as USART1 clock source (default)
• 01: System clock (SYSCLK) selected as USART1 clock
• 10: LSE clock selected as USART1 clock
• 11: HSI clock selected as USART1 clock

6.4.14 Clock control register 2 (RCC_CR2)

Address: 0x34

Reset value: 0xXX00 XX80, where X is undefined.

Access: no wait states, word, half-word and byte access

Bits 31:24 HSI48CAL[7:0] : HSI48 factory clock calibration

These bits are initialized automatically at startup and are read-only.

Bits 23:18 Reserved, must be kept at reset value.

Bit 17 HSI48RDY : HSI48 clock ready flag

Set by hardware to indicate that HSI48 oscillator is stable. After the HSI48ON bit is cleared, HSI48RDY goes low after 6 HSI48 oscillator clock cycles.

0: HSI48 oscillator not ready

1: HSI48 oscillator ready

Bit 16 HSI48ON : HSI48 clock enable

Set and cleared either by software or by hardware. Set by hardware when the USB peripheral is enabled and switched on this source; reset by hardware to stop the oscillator when entering in Stop or Standby mode. This bit cannot be reset if the HSI48 is used directly or indirectly as system clock or is selected to become the system clock.

0: HSI48 oscillator OFF

1: HSI48 oscillator ON

Bits 15:8 HSI14CAL[7:0] : HSI14 clock calibration

These bits are initialized automatically at startup.

Bits 7:3 HSI14TRIM[4:0] : HSI14 clock trimming

These bits provide an additional user-programmable trimming value that is added to the HSI14CAL[7:0] bits. It can be programmed to adjust to variations in voltage and temperature that influence the frequency of the HSI14.

The default value is 16, which, when added to the HSI14CAL value, should trim the HSI14 to 14 MHz \( \pm \) 1%. The trimming step is around 50 kHz between two consecutive HSI14CAL steps.

Bit 2 HSI14DIS : HSI14 clock request from ADC disable

Set and cleared by software.

When set this bit prevents the ADC interface from enabling the HSI14 oscillator.

0: ADC interface can turn on the HSI14 oscillator

1: ADC interface can not turn on the HSI14 oscillator

Bit 1 HSI14RDY : HSI14 clock ready flag

Set by hardware to indicate that HSI14 oscillator is stable. After the HSI14ON bit is cleared, HSI14RDY goes low after 6 HSI14 oscillator clock cycles. When HSI14ON is not set but the HSI14 oscillator is enabled by the peripheral through a clock request, this bit is not set.

0: HSI14 oscillator not ready

1: HSI14 oscillator ready

Bit 0 HSI14ON : HSI14 clock enable

Set and cleared by software. When the HSI14 oscillator is enabled by the peripheral through a clock request, this bit is not set and resetting it does not stop the HSI14 oscillator.

0: HSI14 oscillator OFF

1: HSI14 oscillator ON

6.4.15 RCC register map

The following table gives the RCC register map and the reset values.

Table 19. RCC register map and reset values

Table 19. RCC register map and reset values (continued)

Refer to Section 2.2 on page 46 for the register boundary addresses.