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GPIO

Introduction to GPIO

General-Purpose Input/Output (also known as GPIO) is a simple yet essential component in embedded systems. These digital spacial pin can read or send discrete signals, typically represented as HIGH (1) or LOW (0).

Pins are the thin lines made of tin on the PCB, which connects the MCU to different ports and peripherals, facilitating communication between components.

  • Every pin on the PCB can function as a GPIO pin.
    • These pins are organised into blocks of 16, referred to by letter: GPIOA, GPIOB etc
      • Within each blocks, the pins are organised by numbers: GPIO_PIN_1, GPIO_PIN_2 etc
  • This way, every pin can be referred to by their block and number, such as PA1 or PB10.

For ease of use, you can define names for each pin inside the main.h file or directly within the CubeMX configuration.

Configuration

  1. Configuring the Macros:

Under Private defines:

/* Private defines -----------------------------------------------------------*/
#define BTN1_Pin GPIO_PIN_3
#define BTN1_GPIO_Port GPIOB
/* USER CODE BEGIN Private defines */

  1. Update the Pinout Name inside CubeMX

    We renamed PB3 to BTN1 under the pinout user menu, this way, CubeMX will automatically generate the Macros.

GPIO Read and Write Function

Using the HAL library and with the correct configuration, we can read inputs and send outputs.

GPIO Input

The HAL_GPIO_ReadPin function reads the GPIO input given the pin and block.

/* stm32f4xx_hal_gpio.h */
GPIO_PinState HAL_GPIO_ReadPin(GPIO_TypeDef GPIOx, uint16_t GPIO_Pin);
// usage
uint8_t state = HAL_GPIO_ReadPin(BTN1_GPIO_Port, BTN1_Pin); // returns 0 or 1

//To Improve the Readability, recalling we have
    #define BTN1_Pin GPIO_PIN_3
    #define BTN1_GPIO_Port GPIOB
// Therefore you can make it easier on yourself:
#define gpio_read(gpio) HAL_GPIO_ReadPin(gpio##_GPIO_Port, gpio##_Pin)
// usage
uint8_t state = gpio_read(BTN1);

GPIO Output

  • The gpio_set(gpio) macro sets the GPIO pin to be 1.
  • The gpio_reset(gpio) macro resets the GPIO pin to 0.
  • The gpio_toggle(gpio) macro toggles the GPIO pin. (0 -> 1, 1 -> 0)
/* main.h */
// Sets the GPIO pin to HIGH
#define gpio_set(gpio) HAL_GPIO_WritePin(gpio##_GPIO_Port, gpio##_Pin, GPIO_PIN_SET)
// Sets the GPIO pin to LOW
#define gpio_reset(gpio) HAL_GPIO_WritePin(gpio##_GPIO_Port, gpio##_Pin, GPIO_PIN_RESET)
// Toggles the GPIO pin
#define gpio_toggle(gpio) HAL_GPIO_TogglePin(gpio##_GPIO_Port, gpio##_Pin)

// usage
// Turns off the LED
gpio_set(LED1);

// Turns on the LED
gpio_reset(LED1);

// Toggles the LED
gpio_toggle(LED1);

Buttons and LEDs

While 0 and 1 might mean something intuitively, we cannot guarantee what 0 or 1 means/does. 0 and 1 are just the 2 states that the pin receives/outputs, and both 0 and 1 can mean on in different circumstances.

When you set the LED1 Pin to 1, the LED will actually turn off, and turns on when set to 0.

Similarly, for buttons, 1 is returned when released and 0 is returned when held.

To help reduce confusion and make your code more readable, we have provided the following:

/* main.h */
#define led_on(led) gpio_reset(led) // set pin to 0 -> turn on LED
#define led_off(led) gpio_set(led) // set pin to 1 -> turn off LED
#define led_toggle(led) gpio_toggle(led) // change state of LED

#define btn_read(btn) !gpio_read(btn)

/* main.c */
// NOTE: the following still works
// btn_read exists to help you but you can still use gpio_read
uint8_t btn1_state = !gpio_read(BTN1);

In cases where the wires were swapped, you can change the defines instead of changing every single line of code.

Examples

/*main.c*/
while(1){
    if (btn_read(BTN1)){
        led_on(LED1);
    }else{
        led_off(LED1);
    }
}

^ this code turns LED1 on when BTN1 is pressed, otherwise, LED1 is turned off.

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