GPIO internal peripheral
1 Article purpose
The purpose of this article is to
- briefly introduce the GPIO peripheral and its main features
- indicate the level of security supported by this hardware block
- explain how each instance can be allocated to the three runtime contexts and linked to the corresponding software components
- explain how to configure the GPIO peripheral.
2 Peripheral overview
The GPIO peripheral is used to configure the device IO ports, also called pins or pads.
Each GPIO instance controls 8 pins (for GPIOK and GPIOZ) or 16 pins (for GPIOA to GPIOJ).
Every IO port implements the logic shown in the image below, taken from STM32MP15 reference manuals:
- The IO pin (on the right) is the physical connection to a chip external ball, soldered on the PCB. The link between each GPIO pin and each ball of the package is given in the datasheet.
- The Read and Write accesses allow the processor (Arm® Cortex®-A7 or Arm® Cortex®-M4) to configure the peripheral, control the IO pin and get its status.
- Alternate function (AF) links allow to connect the IO port to an internal peripheral digital line. In such a case, the IO direction is given by the line purpose: for instance, UART transmit line (TX) is an output.
- Analog links allow to connect the IO port to an internal peripheral analog line. In such a case, the IO direction is given by the line purpose: for instance, ADC input line is an input.
- the pull-up and pull-down resistors are disabled (by hardware) in analog mode.
- at reset, all pins are set in analog input mode to protect the device and minimize the power consumption. All unused pins should be kept in this state.
The pin configuration done by the software consists in:
- setting the pin mode in the GPIOx_MODER register:
- input or output if the pin is used as general purpose (GPIO), controlled by software.
- alternate function (AF).
- selecting the alternate function in the GPIOx_AFRH/L register (only when the pin mode is AF):
- each IO port can support up to 16 alternate functions that are documented in the datasheet.
- setting the pin characteristics:
- no pull-up and no pull-down or pull-up or pull-down in the GPIOx_PUPDR register, needs to be selected to be coherent with the hardware schematics.
- push-pull or open-drain in the GPIOx_OTYPER register, needs to be selected to be coherent with the hardware schematics.
- output speed in the GPIOx_OSPEEDR register needs to be tuned to achieve the expected level of performance (rising and falling times) while limiting electromagnetic interferences (EMI) and overconsumption. As example, the table below summarizes the maximum achievable frequency for each supported IO voltage and a 30pF load:
GPIOx_OSPEEDR Meaning VDD=3v3 VDD=1v8
b00 Low speed 24 MHz 11 MHz 22 MHz b01 Medium speed 83 MHz 28 MHz 79 MHz b10 High speed 125 MHz 66 MHz 101 MHz b11 Very high speed 150 MHz 70 MHz 111 MHz
- There are different IO types with different characteristics: for instance, all pads are not able to achieve 150 MHz while supplied at 3v3. Refer to the datasheet to get the characteristics for each pin.
- When supplied with VDD=1v8, it is possible to enable the high speed low voltage (HSLV) pad mode for FTH (Five volt Tolerant High speed) and FTE (Five volt Tolerant Extended high speed) IO types on some peripherals (SPI, SDMMC, ETH, Dual QUADSPI and TRACE) via SYSCFG_IOCTRLR register in SYSCFG. Warning: As it could be destructive if used when VDD>2.7V, the HSLVEN_xx bits set in SYSCFG_IOCTRLR register are not taken account unless the OTP bit PRODUCT_BELOW_2V5 is set.
The table below shows all possible characteristics combinations for each pin mode:
pin mode GPIOx_PUPDR GPIOx_OTYPER GPIOx_OSPEEDR analog
Not applicable Not applicable Not applicable input (GPIO or AF)
no pull-up and no pull-down
Not applicable Not applicable output (GPIO or AF)
or bi-directional (AF)
cf. the table above
- 'Not applicable' means that setting this register has no effect but, in any case, there is no risk for the device.
- On the other hand, leaving a register not initialized whereas it should be, may lead to an unpredictable behavior!
Refer to STM32MP15 reference manuals for the complete features list, and to the software components, introduced below, to know which features are really implemented.
2.2 Security support
The GPIOA to GPIOK peripherals are non-secure.
The GPIOZ peripheral is secure aware.
3 Peripheral usage and associated software
3.1 Boot time
The STM32CubeMX tool allows to configure in one place the GPIO configuration that is applied at boot time and used at runtime, so it is highly recommended to use it to generate your device tree. Moreover, STM32CubeMX integrates all the information documented in the datasheet, making this configuration step straightforward.
Since a GPIO configuration is done via atomic registers read and write, concurrent accesses from different cores must be avoided and that is why all GPIO configurations are done by the Arm® Cortex®-A7. The strategy is to progressively initialize the GPIO all along the boot chain, as soon as one boot component needs to use them:
- Most of the GPIOs used by the ROM code are directly defined in the ROM code but it is possible to change some pins muxing via dedicated words in BSEC.
- The other boot components are relying on a common binding in the device tree to get the pins configuration:
- The FSBL configures both secure (GPIOZ) and non-secure (GPIOA to GPIOK) instances.
- The SSBL and Linux pinctrl configure only non-secure instances, so GPIOA to GPIOK, and only the pins left non-secure in GPIOZ by the FSBL. Linux also initializes the GPIO used by the coprocessor, via its resource manager.
The GPIO configuration must not be done from different cores to avoid concurrent accesses, but this is not the case for the GPIO using: each core can manipulate IO on its own since dedicated set/clear registers are available for that.
Nevertheless, beyond the boot time, the GPIO configuration also evolves at runtime: while entering in low power mode, some GPIOs may be put back to analog input mode in order to reduce the power consumption. This is done in two times:
- the Arm® Cortex®-A7 non-secure takes care of the non-secure GPIO with Linux IOs pins frameworks.
- the Arm® Cortex®-A7 secure takes care of the secure pins of GPIOZ behind PSCI secure services.
On wakeup, the boot chain restores the GPIO configuration similarly to what is done at boot time.
Let's come back to the runtime allocation...
The GPIOZ pins can individually be:
- set secure and assigned to the Arm® Cortex®-A7 secure for using with OP-TEE
- set non-secure and assigned to the Arm® Cortex®-A7 non-secure for using in Linux® with the IOs pins frameworks
- set non-secure and assigned to the Arm® Cortex®-M4 for using in STM32Cube with GPIO HAL driver
The GPIOA to GPIOK pins can individually be:
- assigned to the Arm® Cortex®-A7 non-secure for using in Linux® with the IOs pins frameworks
- assigned to the Arm® Cortex®-M4 for using in STM32Cube with GPIO HAL driver
3.2.2 Software frameworks
|Core/IOs||GPIO||OP-TEE GPIO driver||Linux IOs pins overview||STM32Cube GPIO driver|
3.2.3 Peripheral configuration
The configuration is applied by the firmware running in the context to which the peripheral is assigned. The configuration by itself can be done via STM32CubeMX tool for all internal peripheral, then it can manually be completed (especially for external peripherals) according to the information given in the corresponding software framework article.
In Linux kernel, each GPIO bank is declared as a "gpio-controller" in the device tree and each pin can then be used via two different consumer frameworks:
- Pinctrl framework is used to control the alternate function (AF) selection for a given device driver, via the Pinctrl device tree configuration.
- Gpiolib framework is used to control a pin in GPIO mode from another device driver or a user space application: refer to GPIO device tree configuration for further details.
3.2.4 Peripheral assignment
Check boxes illustrate the possible peripheral allocations supported by STM32 MPU Embedded Software:
- ☐ means that the peripheral can be assigned (☑) to the given runtime context.
- ✓ is used for system peripherals that cannot be unchecked because they are statically connected in the device.
Refer to How to assign an internal peripheral to a runtime context for more information on how to assign peripherals manually or via STM32CubeMX.
The present chapter describes STMicroelectronics recommendations or choice of implementation. Additional possiblities might be described in STM32MP15 reference manuals.
|Core/IOs||GPIO||GPIOA (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOB (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOC (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOD (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOE (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOF (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOG (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOH (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOI (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOJ (16 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOK (8 pins)||☐||☐||Shareable (with pin granularity)|
|GPIOZ (8 pins)||☐||☐||☐||Shareable (with pin granularity)|
4 How to go further
Printed Circuit Board
GPIO alternate function
External memory interface
High Speed Low Voltage pin mode
One Time Programmed
Read Only Memory
Open Portable Trusted Execution Environment