Electronics - STM32 - GPIO overview with the STM32F3-Discovery board

GPIO is one of the most easy peripheral to understand in a microcontroller.

But of course it demands a bit of practice to handle it with ease.

That's the purpose of this STM32F303-Discovery board tutorial.

We are going to blink 4 LEDS in order to get how the basics work.

GitHub

Retrieve this example with full code on my GitHub:

https://github.com/badprog/badprog-stm32f303vct6-blinking-led

Registers

In the STM32F303VCT6, there are of course some GPIOs.

These GPIOs are made of several different registers.

Configuration

• GPIOx->MODER (mode register)
• GPIOx->OTYPER (output type register)
• GPIOx->OSPEEDR (output speed register)
• GPIOx->PUPDR (pull-up/pull-down register)

Data

• GPIOx->IDR (input data register)
• GPIOx->ODR (output data register)

Set/Reset

• GPIOx->BSRR (bit set/reset register)

Locking

• GPIOx->LCKR (lock register)

Alternate function selection

• GPIOx->AFRL (alternate function register low)
• GPIOx->AFRH (alternate function register high)

Reset

• GPIOx->BRR (bit reset register)

GPIO_InitTypedef

In order to use the corresponding configuration, we can use the STM32 libraries to set the GPIO_InitTypeDef structure.

This structure is made of 5 elements:

• Alternate
• Mode
• Pin
• Pull
• Speed

To set it, it's necessary to pass the adress of this structure to the following function:

• void HAL_GPIO_Init(GPIO_TypeDef  *GPIOx, GPIO_InitTypeDef *GPIO_Init)

There are 2 ways of using MCU, libraries and modification by hand.

Let's get coded

To have a LED switched on, there are essentiel steps.

Settings oscillator and clocks

This is done with the following function:

• void SystemClock_Config(void);

Settings the correspondings port(s) and pins

Of course, in this step, we have to enable the clock of each port we want to use in the program.

There is a function for that:

• __HAL_RCC_GPIOE_CLK_ENABLE()

To be simple, this function calls:

• SET_BIT(RCC->AHBENR, RCC_AHBENR_GPIOEEN);

This enable de RCC->AHBENR register with the value RCC_AHBENR_GPIOEEN (0x00200000).

All that to change the bit 21 of RCC->AHBENR from 0 to 1.

Handling the correponding pins

For example, each LED has its own pin.

Onto this board we have:

• LED_4  = GPIO_E_PIN_8   (top left   - Blue)
• LED_3  = GPIO_E_PIN_9   (top        - Red)
• LED_5  = GPIO_E_PIN_10  (top right  - Orange)
• LED_7  = GPIO_E_PIN_11  (right      - Green)

• LED_9  = GPIO_E_PIN_12  (bottom right  - Blue)
• LED_10 = GPIO_E_PIN_13  (bottom  - Red)
• LED_8  = GPIO_E_PIN_14  (bottom left  - Orange)
• LED_6  = GPIO_E_PIN_15  (left  - Green)

Alternate function

Each pin can be used by many peripherals.

For example, on the pin PE15 (Port E and pin 15), there are:

• ADC1_EXTI15
• ADC2_EXTI15
• ADC4_IN2
• TIM1_BKIN
• USART3_RX
• GPIO_Input
• GPIO_Output
• GPIO_Analog
• GPIO_EXT15

So to use the PE15 as a GPIO_Output to blink a LED.

If we wanted to use the ADC4 on this pin, we would have set this PE15 as alternate function through the GPIOx_MODER register.

Indeed, the GPIOx_MODER register is used to configure the I/O mode, like:

• Input
• Output
• Alternate Function (AF)
• Analog

So for ADC, it'll be the Analog mode.

Indeed, each I/O pin has a multiplexer that allows only one peripheral alternate function to be used at a time.

It means that once a pin has been set, if you want to use it as another peripheral, it's necessary to set it again with the corresponding alternate function.

The GPIOx_AFRL and GPIOx_AFRH are also the famous registers to select the appropriate peripherals and have to be modified consequently.

Using the GPIO inside your own code

• GPIOE->ODR ^= MY_PINS;

The final code

#include "stm32f3xx_hal.h"

void SystemClock_Config(void);

/**
 * LED_4  = GPIO_E_PIN_8   (top left   - Blue)
 * LED_3  = GPIO_E_PIN_9   (top        - Red)
 * LED_5  = GPIO_E_PIN_10  (top right  - Orange)
 * LED_7  = GPIO_E_PIN_11  (right      - Green)
 *
 * LED_9  = GPIO_E_PIN_12  (bottom right  - Blue)
 * LED_10 = GPIO_E_PIN_13  (bottom  - Red)
 * LED_8  = GPIO_E_PIN_14  (bottom left  - Orange)
 * LED_6  = GPIO_E_PIN_15  (left  - Green)
 *
 */
#define MY_PINS GPIO_PIN_8 | GPIO_PIN_9 | GPIO_PIN_10 | GPIO_PIN_11

/**
 * GPIO
 *
 * The Alternate element allows to set the GPIO as an alternate function,
 * for example GPIO_AF10_TIM17 or GPIO_AF4_I2C1 to set this GPIO as a timer pin
 * or an I2C pin.
 * In this configuration, the mode has to be set as GPIO_MODE_AF_xx.
 * But in this example, we won't change anything, because we just want to blink
 * some LEDs.
 */
void badprog_gpio() {

  GPIO_InitTypeDef GPIO_InitStruct;

  __HAL_RCC_GPIOE_CLK_ENABLE()
  ;
  GPIO_InitStruct.Alternate = 0; //
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pin = MY_PINS;              // set all pins needed
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOE, &GPIO_InitStruct);

}

/**
 * Main, the famous one!
 */
int main(void) {
  HAL_Init();
  SystemClock_Config();
  badprog_gpio();
  while (1) {
    GPIOE->ODR ^= MY_PINS;    // let's blink
    HAL_Delay(1000);
  }
}

/** System Clock Configuration
 */
void SystemClock_Config(void) {

  RCC_OscInitTypeDef RCC_OscInitStruct;
  RCC_ClkInitTypeDef RCC_ClkInitStruct;

  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
  RCC_OscInitStruct.HSICalibrationValue = 16;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
  RCC_OscInitStruct.PLL.PLLMUL = RCC_PLL_MUL12;
  HAL_RCC_OscConfig(&RCC_OscInitStruct);

  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK | RCC_CLOCKTYPE_SYSCLK
      | RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV2;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
  HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_1);

  HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq() / 1000);

  HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);

  /* SysTick_IRQn interrupt configuration */
  HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
}

Conclusion

You are now able to understand a bit more than earlier about GPIO and especially with the STM32F3 microcontroller.

Retrieve exhaustive explanation in the reference manual RM0316 from page 228 to 247:

http://www.keil.com/dd/docs/datashts/st/stm32f3xx/dm00043574.pdf

Good job, you know more now!