12. General-purpose I/Os (GPIO)

12.1 Introduction

Each general-purpose I/O port has four 32-bit configuration registers (GPIOx_MODER, GPIOx_OTYPER, GPIOx_OSPEEDR, and GPIOx_PUPDR), two 32-bit data registers (GPIOx_IDR and GPIOx_ODR), a 16-bit reset register (GPIOx_BRR) and a 32-bit set/reset register (GPIOx_BSRR).

In addition, all GPIOs have a 32-bit locking register (GPIOx_LCKR), two 32-bit alternate function selection registers (GPIOx_AFRH and GPIOx_AFRL), a secure configuration register (GPIOx_SECCFGR) and a high-speed low-voltage register (GPIOx_HSLVR).

12.2 GPIO main features

• • Output states: push-pull or open drain + pull-up/down
• • Output data from output data register (GPIOx_ODR) or peripheral (alternate function output)
• • Speed selection for each I/O
• • Input states: floating, pull-up/down, analog
• • Configurable state of each I/O in Standby mode: floating, pull-up/down, analog
• • Input data to input data register (GPIOx_IDR) or peripheral (alternate function input)
• • Bit set and reset register (GPIOx_BSRR) for bitwise write access to GPIOx_ODR
• • Lock mechanism (GPIOx_LCKR) provided to freeze the I/O port configurations
• • Analog function
• • Alternate function selection registers
• • Fast toggle capable of changing every two clock cycles
• • Highly flexible pin multiplexing allows the use of I/O pins as GPIOs or as one of several peripheral functions
• • TrustZone security support

12.3 GPIO functional description

Subject to the specific hardware characteristics of each I/O port listed in the datasheet, each port bit of the general-purpose I/O (GPIO) ports can be individually configured by software in several modes:

• • Input floating
• • Input pull-up
• • Input pull-down
• • Analog
• • Output open-drain with pull-up or pull-down capability
• • Output push-pull with pull-up or pull-down capability
• • Alternate function push-pull with pull-up or pull-down capability
• • Alternate function open-drain with pull-up or pull-down capability

Each I/O port bit is freely programmable, however the I/O port registers must be accessed as 32-bit words, half-words or bytes. GPIOx_BSRR and GPIOx_BRR registers allow atomic read/modify accesses to any of the GPIOx_ODR registers. In this way, there is no risk of an IRQ occurring between the read and the modify access.

The figure below shows the basic structure of a three-volt or five-volt tolerant GPIO (TT or FT). Table 114 gives the possible port bit configurations.

Figure 36. Structure of three-volt or five-volt tolerant GPIO (TT or FT)

Note: On a TT GPIO, the analog switch is not present and replaced by a direct connection. The analog bloc parasitic circuitry does not allow five-volt tolerance.

Table 114. Port bit configuration ⁽¹⁾

1. GP = general-purpose, PP = push-pull, PU = pull-up, PD = pull-down, OD = open-drain, AF = alternate function.

12.3.1 General-purpose I/O (GPIO)

During and just after reset, the alternate functions are not active and most of the I/O ports are configured in analog mode.

The debug pins are in AF pull-up/pull-down after reset:

• • PA15: JTDI in pull-up
• • PA14: JTCK/SWCLK in pull-down
• • PA13: JTMS/SWDIO in pull-up
• • PB4: NJTRST in pull-up
• • PB3: JTDO/TRACEWO in floating state no pull-up/pull-down

BOOT0 is in input mode during the reset until at least the end of the option byte loading phase (see Section 12.3.15: Using BOOT0 pin as GPIO ).

When the pin is configured as output, the value written to the output data register (GPIOx_ODR) is output on the I/O pin. It is possible to use the output driver in push-pull mode or open-drain mode (only the low level is driven, high level is HI-Z).

The input data register (GPIOx_IDR) captures the data present on the I/O pin at every AHB clock cycle.

All GPIO pins have weak internal pull-up and pull-down resistors, that can be activated or not depending on the value in the GPIOx_PUPDR register.

12.3.2 I/O pin alternate function multiplexer and mapping

The device I/O pins are connected to on-board peripherals/modules through a multiplexer that allows only one peripheral alternate function (AF) connected to an I/O pin at a time. In this way, there is no conflict between peripherals available on the same I/O pin.

Each I/O pin has a multiplexer with up to 16 alternate function inputs (AF0 to AF15) that can be configured through GPIOx_AFRL (for pin 0 to 7) and GPIOx_AFRH (for pin 8 to 15):

• • After reset, the multiplexer selection is alternate function 0 (AF0). The I/Os are configured in alternate function mode through GPIOx_MODER.
• • The specific alternate function assignments for each pin are detailed in the device datasheet.

In addition to this flexible I/O multiplexing architecture, each peripheral has alternate functions mapped onto different I/O pins to optimize the number of peripherals available in smaller packages.

To use an I/O in a given configuration, the user must proceed as follows:

• • Debug function: after each device reset these pins are assigned as alternate function pins immediately usable by the debugger host.
• • GPIO: configure the desired I/O as output, input or analog in GPIOx_MODER.
• • Alternate function:
  • – Connect the I/O to the desired AFx in one of GPIOx_AFRL or GPIOx_AFRH.
  • – Select the type, pull-up/pull-down and output speed via GPIOx_OTYPER, GPIOx_PUPDR, and GPIOx_OSPEEDR respectively.
  • – Configure the desired I/O as an alternate function in GPIOx_MODER.
  • – The Cortex-M33 output EVENTOUT signal can be output as alternate function on several I/Os. An event can be signaled through the configured pin after executing SEV instruction.
  • – The Cortex-M33 output EVENTOUT signal can be output as alternate function on several I/O pins. An event can be signaled through the configured pin after executing SEV instruction.
• • Additional functions:
  • – For the ADC, DAC, OPAMP, and COMP, configure the desired I/O in analog mode in GPIOx_MODER, and configure the required function in the ADC, DAC, OPAMP, and COMP registers.
  • – For the additional functions like RTC, WKUPx, USB, and oscillators, configure the required function in the related RTC, PWR, USB, and RCC registers. These functions have priority over the configuration in the standard GPIO registers.

Refer to the “Alternate function mapping” table in the device datasheet for the detailed mapping of the alternate function I/O pins.

12.3.3 I/O port control registers

Each of the GPIO ports has four 32-bit memory-mapped control registers (GPIOx_MODER, GPIOx_OTYPER, GPIOx_OSPEEDR, GPIOx_PUPDR) to configure up to 16 I/Os.

• • GPIOx_MODER is used to select the I/O mode (input, output, AF, analog).
• • GPIOx_OTYPER and GPIOx_OSPEEDR are used to select the output type (push-pull or open-drain) and speed.
• • GPIOx_PUPDR is used to select the pull-up/pull-down whatever the I/O direction.

12.3.4 I/O port state in low-power modes

In Standby and Shutdown modes, the GPIO peripheral is not active, and its configuration is forced externally by the PWR control.

Before entering a deep low-power mode, fix the GPIO pin states, using the PWR_PUCRx and PWR_PDCRx registers. This prevents disturbing external components and buses, and optimizes the power consumption during deep low-power modes.

Exiting stop mode causes the power supply to be reset, the PWR_PUCRx and PWR_PDCRx registers to be reset, and the GPIO to be set to floating mode until reset by the application.

12.3.5 I/O port data registers

Each GPIO has two 16-bit memory-mapped data registers: input and output data registers (GPIOx_IDR and GPIOx_ODR).

GPIOx_ODR stores the data to be output, it is read/write accessible. The data input through the I/O are stored into the input data register (GPIOx_IDR), a read-only register.

12.3.6 I/O data bitwise handling

The bit set reset register (GPIOx_BSRR) is a 32-bit register that allows the application to set and reset each individual bit in the output data register (GPIOx_ODR). The bit set reset register has twice the size of GPIOx_ODR.

Two control bits in GPIOx_BSRR, BS[i] and BR[i], correspond to each bit in GPIOx_ODR. When written to 1, BS[i] sets the corresponding ODR[i] bit. When written to 1, BR[i] resets the ODR[i] corresponding bit.

Writing any bit to 0 in GPIOx_BSRR does not have any effect on the corresponding bit in GPIOx_ODR. If there is an attempt to both set and reset a bit in GPIOx_BSRR, the set action takes priority.

Using GPIOx_BSRR to change the values of individual bits in GPIOx_ODR is a “one-shot” effect that does not lock the GPIOx_ODR bits. The GPIOx_ODR bits can always be accessed directly. GPIOx_BSRR provides a way of performing atomic bitwise handling.

There is no need for the software to disable interrupts when programming GPIOx_ODR at bit level: one or more bits can be modified in a single atomic AHB write access.

12.3.7 GPIO locking mechanism

GPIO control registers can be frozen by applying a specific write sequence to GPIOx_LCKR. The frozen registers are GPIOx_MODER, GPIOx_OTYPER, GPIOx_OSPEEDR, GPIOx_PUPDR, GPIOx_AFRL, GPIOx_AFRH and GPIOx_HSLVR.

To write GPIOx_LCKR, a specific write/read sequence must be applied. When the right LOCK sequence is applied to the bit 16 in this register, the value of LCKR[15:0] is used to lock the configuration of the I/Os (during the write sequence the LCKR[15:0] value must be the same). When the LOCK sequence is applied to a port bit, the value of the port bit can no longer be modified until the next MCU reset or peripheral reset. Each GPIOx_LCKR bit freezes the corresponding bit in the control registers (GPIOx_MODER, GPIOx_OTYPER, GPIOx_OSPEEDR, GPIOx_PUPDR, GPIOx_AFRL, GPIOx_AFRH, and GPIOx_HSLVR).

The LOCK sequence can only be performed using a word (32-bit long) access to GPIOx_LCKR due to the fact that GPIOx_LCKR bit 16 must be set at the same time as the [15:0] bits.

12.3.8 I/O alternate function input/output

Two registers are provided to select one of the alternate function inputs/outputs available for each I/O. With these registers, the user can connect an alternate function to some other pin as required by the application.

This means that a number of possible peripheral functions are multiplexed on each GPIO using GPIOx_AFRH and GPIOx_AFRH alternate function registers. The application can thus select any one of the possible functions for each I/O. The AF selection signal being common to the alternate function input and alternate function output, a single channel is selected for the alternate function input/output of a given I/O.

To know which functions are multiplexed on each GPIO pin, refer to the device datasheet.

12.3.9 Input configuration

When the I/O port is programmed as input:

• • The output buffer is disabled.
• • The Schmitt trigger input is activated.
• • The pull-up and pull-down resistors are activated depending on the value in the GPIOx_PUPDR register.
• • The data present on the I/O pin are sampled into the input data register every AHB clock cycle.
• • A read access to the input data register provides the I/O state.

The figure below shows the input configuration of the I/O port bit.

Figure 37. Input floating/pull-up/pull-down configurations

12.3.10 Output configuration

When the I/O port is programmed as output:

• • The output buffer is enabled:
  • – Open-drain mode: a 0 in the output register activates the N-MOS whereas a 1 in the output register leaves the port in Hi-Z (the P-MOS is never activated).
  • – Push-pull mode: a 0 in the output register activates the N-MOS whereas a 1 in the output register activates the P-MOS.
• • The Schmitt trigger input is activated.
• • The pull-up and pull-down resistors are activated depending on the value in the GPIOx_PUPDR register.
• • The data present on the I/O pin are sampled into the input data register every AHB clock cycle.
• • A read access to the input data register gets the I/O state.
• • A read access to the output data register gets the last written value.

The figure below shows the output configuration of the I/O port bit.

Figure 38. Output configuration

12.3.11 Alternate function configuration

When the I/O port is programmed as alternate function:

• • The output buffer can be configured in open-drain or push-pull mode.
• • The output buffer is driven by the signals coming from the peripheral (transmitter enable and data).
• • The Schmitt trigger input is activated.
• • The weak pull-up and pull-down resistors are activated or not depending on the value in the GPIOx_PUPDR register.
• • The data present on the I/O pin are sampled into the input data register every AHB clock cycle.
• • A read access to the input data register gets the I/O state.

The figure below shows the alternate function configuration of the I/O port bit.

Figure 39. Alternate function configuration

12.3.12 Analog configuration

When the I/O port is programmed as analog configuration:

• • The output buffer is disabled.
• • The Schmitt trigger input is deactivated, providing zero consumption for every analog value of the I/O pin. The output of the Schmitt trigger is forced to a constant value (0).
• • The weak pull-up is disabled by hardware. The weak pull-down is configurable.
• • Read access to the input data register gets the value 0.

The figure below shows the high-impedance, analog-input configuration of the I/O port bits.

Figure 40. High-impedance analog configuration

12.3.13 Using HSE or LSE oscillator pins as GPIOs

When the HSE or LSE oscillator is switched off (default state after reset), the related oscillator pins can be used as normal GPIOs.

When the HSE or LSE oscillator is switched on (by setting the HSEON or LSEON bit in RCC_CSR register), the oscillator takes control of its associated pins and the GPIO configuration of these pins has no effect.

When the oscillator is configured in a user external clock mode, only the pin is reserved for clock input, and the OSC_OUT or OSC32_OUT pin can still be used as normal GPIO.

12.3.14 Using the GPIO pins in the RTC supply domain

The PC13/PC14/PC15 GPIO functionality is lost when the core supply domain is powered off (when the device enters Standby mode). In this case, if their GPIO configuration is not bypassed by the RTC configuration, these pins are set in an analog input mode.

For details about I/O control by the RTC, refer to Section 46.3: RTC functional description .

12.3.15 Using BOOT0 pin as GPIO

PH3 may be used as boot pin (BOOT0) or as a GPIO. Depending on the NSWBOOT0 bit in the user option byte, PH3 switches from the input mode to the analog input mode:

• • After the option byte loading phase if NSWBOOT0 = 1.
• • After reset if NSWBOOT0 = 0.

Note: On some packages, PH3 is not available and PB7 is used as the BOOT0 pin instead. Refer to the device datasheet for more information.

12.3.16 TrustZone security

The TrustZone security is activated by the TZEN option bit in the FLASH_OPTR. When the TrustZone is enabled (TZEN = 1), each I/O pin of GPIO port can be individually configured as secure through GPIOx_SECCFGR. This register can be written only by a secure access, a nonsecure write access is ignored. Both secure and nonsecure reads return the register data, that is the security configuration of individual I/O pins.

When the selected I/O pin is configured as secure, its corresponding configuration bits for alternate function, mode selection, I/O data are secure against a nonsecure access. In case of nonsecure access, these bits are RAZ/WI. The GPIO clock and reset control bits in the RCC are automatically configured as secure as soon as at least one I/O in the GPIO is secure.

The I/Os with peripherals functions are also conditioned by the peripheral security configuration (see Section 5: Global TrustZone controller (GTZC) for more details):

• • For peripherals for which the I/O pin selection is done through alternate functions registers: if the peripheral is configured as secure, it cannot be connected to a nonsecure I/O pin. If, for peripherals other than LCD, this is not respected, the input data to the secure peripheral is forced to 0 (I/O input pin value is ignored) and the output pin value is forced to 0, thus avoiding any secure information leak through nonsecure I/Os. If this is not respected for the LCD peripheral, its I/O pins are left floating (LCD I/O switches remain open).

Note: The LCD is available on some STM32U3 series devices. Refer to the device datasheet for availability of associated peripherals.

• • For I/Os with analog switches, directly controlled by peripherals (such as ADC for instance): If the I/O is secure, the I/O analog switch cannot be controlled by a nonsecure peripheral. If this is not respected, the switch remains open. This prevent the redirection of secure data to a nonsecure peripheral or I/O through analog path. Refer to Table 9: Nonsecure peripherals not connectable to secure I/Os for more details.
• • Some of the paths between I/Os “additional functions” and peripherals are not blocked if the I/O is secure and the peripheral is nonsecure. Therefore it is recommended to configure those peripherals as secure even when not used by the application. Refer to Table 10: Nonsecure peripherals connectable to secure I/Os for the list of concerned

peripherals. When the path has a security control, it follows the same rule as I/O selection through alternate functions.

Refer to the device pins definition table in datasheet for more information about peripherals alternate functions and additional functions mapping.

After reset, all GPIO ports are secure.

For the impacted registers, Table 115 gives a summary of the secured bits following the individual security configuration bits in the GPIOx_SECCFGR register. When a secure configuration bit is set:

• • Secured bits: read and write operations are only allowed by a secure access. Nonsecure read or write accesses on secured bits are RAZ/WI. There is no illegal access event generated.
• • Nonsecure bits: no restriction. Read and write operations are allowed by both secure and nonsecure accesses.

When the TrustZone security is disabled (TZEN = 0 in FLASH_OPTR), all registers bits are nonsecure. The GPIOx_SECCFGR register is RAZ/WI.

Table 115. GPIO secured bits

12.3.17 Privileged and unprivileged modes

All GPIO registers can be read and written by privileged and unprivileged accesses, whatever the security state (secure or nonsecure).

12.3.18 High-speed low-voltage mode (HSLV)

Some I/Os have the capability to increase their maximum speed at low voltage by configuring them in HSLV mode. The I/O HSLV bit controls whether the I/O output speed is optimized to operate at 3.3 V (default setting) or at 1.8 V (HSLV = 1).

Note: The HSLV feature is not available on STM32U356/366.

Caution: The I/O HSLV configuration bit must not be set if the I/O supply ( \( V_{DD} \) or \( V_{DDIO2} \) ) is above 2.7 V. Setting it while the voltage is higher than 2.7 V can damage the device. The I/O HSLV bit can be set only when the corresponding option bit is activated (IO_VDD_HSLV or IO_VDDIO2_HSLV depending on the I/O supply, refer to Section 7: Embedded flash memory (FLASH) ). There is no hardware protection associated to this feature so it is recommended to use it only as a static configuration for fixed I/O supply.

Caution: Setting this bit when the I/O is configured in Fm+ mode is forbidden. An I/O is in Fm+ mode when it is configured as I2C alternate function, with FMP=1 in I2C_CR1 register. PB6, PB7, PB8, PB9 can also be in Fm+ mode when PB6_FMP, PB7_FMP, PB8_FMP, PB9_FMP, respectively, is set in SYSCFG_CFGR1 register.

12.3.19 I/O compensation cell

The I/O commutation slew rate ( \( t_{fall}/t_{rise} \) ) can be adapted by software depending on Process, Voltage and Temperatures conditions, in order to reduce the I/O noise on power supply. Refer to Section 13: System configuration controller (SYSCFG) for more details.

12.4 GPIO in low-power modes

Table 116. Effect of low-power modes on the GPIO

12.5 GPIO interrupts

All ports have external interrupt capability. To use external interrupt lines, the port can be configured in input, output or alternate function mode (the port must not be configured in analog mode). Refer to Section 17: Extended interrupts and event controller (EXTI) .

12.6 GPIO registers

The peripheral registers can be written in word, half-word, or byte mode.

The number of GPIOx ports varies in STM32U3 series devices. Refer to the product datasheet for availability of a particular port. If not present, consider the associated bits as reserved and keep them at the reset value.

12.6.1 GPIO port mode register (GPIOx_MODER) (x = A to H)

Address offset: 0x00

Reset value: 0xABFF FFFF (for port A)

Reset value: 0xFFFF FEBF (for port B)

Reset value: 0xFFFF FFFF (for ports C to E)

Reset value: 0xFFFF FFFF (for port F, port F is only available on STM32U3B5/3C5)

Reset value: 0xFFFF FFFF (for port G, port G is not available on STM32U356/366)

Reset value: 0x0000 00CF (for port H)

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Bits 31:0 MODEy[1:0] : Port x I/O pin y mode configuration (y = 15 to 0)

These bits are written by software to configure the I/O mode.

00: Input mode

01: General purpose output mode

10: Alternate function mode

11: Analog mode (reset state)

Note: This bitfield is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.2 GPIO port output type register (GPIOx_OTYPER) (x = A to H)

Address offset: 0x04

Reset value: 0x0000 0000

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

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

Bits 15:0 OTy : Port x I/O pin y output type configuration (y = 15 to 0)

These bits are written by software to configure the I/O output type.

0: Output push-pull (reset state)

1: Output open-drain

Note: This bit is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.3 GPIO port output speed register (GPIOx_OSPEEDR) (x = A to H)

Address offset: 0x08

Reset value: 0x0C00 0000 (for port A)

Reset value: 0x0000 00C0 (for port B)

Reset value: 0x0000 0000 (for the other ports)

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

Bits 31:0 OSPEEDy[1:0] : Port x I/O pin y output speed configuration (y = 15 to 0)

These bits are written by software to configure the I/O output speed.

00: Low speed

01: Medium speed

10: High speed

11: Very-high speed

Note: Refer to the device datasheet for the frequency specifications, and the power supply and load conditions for each speed.

This bitfield is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.4 GPIO port pull-up/pull-down register (GPIOx_PUPDR) (x = A to H)

Address offset: 0x0C

Reset value: 0x6400 0000 (for port A)

Reset value: 0x0000 0100 (for port B)

Reset value: 0x0000 0000 (for the other ports)

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

Bits 31:0 PUPDy[1:0] : Port x I/O pin y pull-up/pull-down configuration (y = 15 to 0)

These bits are written by software to configure the I/O pull-up or pull-down

00: No pull-up or pull-down

01: Pull-up

10: Pull-down

11: Reserved

Note: This bitfield is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.5 GPIO port input data register (GPIOx_IDR) (x = A to H)

Address offset: 0x10

Reset value: 0x0000 XXXX

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

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

Bits 15:0 IDy : Port x I/O pin y input data (y = 15 to 0)

These bits are read-only. They contain the input value of the corresponding I/O port.

Note: This bit is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.6 GPIO port output data register (GPIOx_ODR) (x = A to H)

Address offset: 0x14

Reset value: 0x0000 0000

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

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

Bits 15:0 ODy : Port x I/O pin y output data (y = 15 to 0)

These bits can be read and written by software.

Note: For atomic bit set/reset, these bits can be individually set and/or reset by writing to GPIOx_BSRR or GPIOx_BRR.

This bit is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.7 GPIO port bit set/reset register (GPIOx_BSRR) (x = A to H)

Address offset: 0x18

Reset value: 0x0000 0000

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

Bits 31:16 BRy : Port x I/O pin y output reset (y = 15 to 0)

These bits are write-only. A read to these bits returns the value 0x0000.

0: No action on the corresponding ODy bit

1: Resets the corresponding ODy bit

Note: If both BSy and BRy are set, BSy has priority.

This bit is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

Bits 15:0 BSy : Port x I/O pin y output set (y = 15 to 0)

These bits are write-only. A read to these bits returns the value 0x0000.

0: No action on the corresponding ODy bit

1: Sets the corresponding ODy bit

Note: The bit is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.8 GPIO port configuration lock register (GPIOx_LCKR) (x = A to H)

Address offset: 0x1C

Reset value: 0x0000 0000

This register is used to lock the configuration of the port bits when a correct write sequence is applied to bit 16 (LCKK). The value of bits [15:0] is used to lock the configuration of the GPIO.

During the write sequence, LCKR[15:0] value must not change. When the LOCK sequence has been applied on a port bit, the value of this port bit can no longer be modified until the next MCU reset or peripheral reset.

Each lock bit freezes corresponding configuration bits in configuration and alternate function registers.

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: A specific write sequence is used to write to this GPIOx_LCKR register. Only word access (32-bit long) is allowed during this locking sequence.

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

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

This bit can be read any time. It can only be modified by using the lock key write sequence.

0: Port configuration lock key not active

1: Port configuration lock key active. The GPIOx_LCKR register is locked until the next MCU reset or peripheral reset.

• – LOCK key write sequence:
• WR LCKR[16] = 1 + LCKR[15:0]
• WR LCKR[16] = 0 + LCKR[15:0]
• WR LCKR[16] = 1 + LCKR[15:0]
• – LOCK key read
• RD LCKR[16] = 1 (this read operation is optional but it confirms that the lock is active)

Note: During the lock key write sequence, the value of LCK[15:0] must not change.

Any error in the lock sequence aborts the LOCK.

After the first lock sequence on any bit of the port, any read access on the LCKK bit returns 1 until the next MCU reset or peripheral reset.

These bits are read/write but can only be written when LCKK = 0.

0: Port configuration not locked

1: Port configuration locked

Note: This bit is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.9 GPIO alternate function low register (GPIOx_AFRL) (x = A to H)

Address offset: 0x20

Reset value: 0x0000 0000

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

Bits 31:0 AFSELy[3:0] : Port x I/O pin y alternate function selection for (y = 7 to 0)

These bits are written by software to configure alternate function I/Os.

0000: AF0
0001: AF1
0010: AF2
0011: AF3
0100: AF4
0101: AF5
0110: AF6
0111: AF7
1000: AF8
1001: AF9
1010: AF10
1011: AF11
1100: AF12
1101: AF13
1110: AF14
1111: AF15

Note: This bitfield is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.10 GPIO alternate function high register (GPIOx_AFRH)
(x = A to H)

Address offset: 0x24

Reset value: 0x0000 0000

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

Bits 31:0 AFSELY[3:0] : Port x I/O pin y alternate function selection (y = 15 to 8)

These bits are written by the software to configure alternate function I/Os.

0000: AF0

0001: AF1

0010: AF2

0011: AF3

0100: AF4

0101: AF5

0110: AF6

0111: AF7

1000: AF8

1001: AF9

1010: AF10

1011: AF11

1100: AF12

1101: AF13

1110: AF14

1111: AF15

Note: This bitfield is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.11 GPIO port bit reset register (GPIOx_BRR) (x = A to H)

Address offset: 0x28

Reset value: 0x0000 0000

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

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

Bits 15:0 BRy : Port x I/O pin y output reset (y = 15 to 0)

These bits are write-only. A read to these bits returns the value 0x0000.

0: No action on the corresponding ODy bit

1: Reset the corresponding ODy bit

Note: This bit is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.12 GPIO high-speed low-voltage register (GPIOx_HSLVR) (x = A to H)

Address offset: 0x2C

Reset value: 0x0000 0000

Individual bits of this register are protected against nonsecure access when corresponding SECy = 1 in GPIO_SECCFGR and the system is secure (TZEN = 1).

Note: Port F is only available on STM32U3B5/3C5.

Note: This register is reserved on STM32U356/366.

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

Bits 15:0 HSLVy : Port x I/O pin y high-speed low-voltage configuration (y = 15 to 0)

These bits are written by software to optimize the I/O speed when the I/O supply is low.

Each bit is active only if the corresponding IO_VDD_HSLV/IO_VDDIO2_HSLV user option bit is set. It must be used only if the I/O supply voltage is below 2.7 V.

Setting these bits when the I/O supply ( \( V_{DD} \) or \( V_{DDIO2} \) ) is higher than 2.7 V may be destructive. Setting this bit when the I/O is configured in Fm+ mode is forbidden.

0: I/O speed optimization disabled

1: I/O speed optimization enabled

Note: Not all I/Os support the HSLV mode. Refer to the I/O structure in the corresponding datasheet for the list of I/Os supporting this feature. HSLV configuration for the other I/Os must be kept at reset value.

This bit is reserved and must be kept at reset value when the corresponding I/O is not available on the selected package.

12.6.13 GPIO secure configuration register (GPIOx_SECCFGR) (x = A to H)

Address offset: 0x30

Reset value: 0x0000 FFFF (for ports A to F)

Reset value: 0x0000 000B (for port H)

When the system is secure (TZEN = 1), this register is used to configure selected I/O pin as secure. It can be written only by a secure access. A nonsecure write access to this register is ignored. Both nonsecure and secure reads return the register data.

When the system is not secure (TZEN = 0), this register is RAZ/WI.

Note: Port F is only available on STM32U3B5/3C5.

Note: Port G is not available on STM32U356/366.

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

Bits 15:0 SECy : Port x I/O pin y security configuration (y = 15 to 0)

These bits are written by software to enable or disable the I/O port pin security.

0: The I/O pin is nonsecure

1: The I/O pin is secure. Refer to Table 115: GPIO secured bits for all corresponding secured bits.

Note: The bit is reserved and must be kept to reset value when the corresponding I/O is not available on the selected package.

12.6.14 GPIO register map

Table 117. GPIO register map and reset values

Table 117. GPIO register map and reset values (continued)

Refer to Section 2.3: Memory organization for the register boundary addresses.