ARM Cortex-M3 (STM32F103) Tutorial - GPIO - Read Push Button/Switch

In this tutorial, I will explain how to use STM32F103 GPIO peripheral for read input from push button. GPIO (General Purpose Input Output) on STM32F103 can be configure to 4 different modes (input mode, output mode, analog input mode, alternate function mode). For read input from push button, we need to configure GPIO in digital input mode.

STM32F013 has 3 input mode (digital input): input with internal pull-up, input with internal pull-down, and input floating. The logic voltage of STM32 microcontroller is 3.3V, so the logic voltage for GPIO input pins is also 3.3V, but there are several pins that have 5V tolerant. So, we can input 5V logic level to this 5V tolerant input pins. This table below shows I/O voltage level on STM32F103 input pins.

This is the characteristic of I/O port bit when configured as input mode:

• The output buffer is disabled.
• The schmitt trigger input is activated.
• The weak pull-up and pull-down resistors are activated or not depending on input
  configuration (pull-up, pull-down or floating).
• The data present on the I/O pin is sampled into the input data register every APB2
  clock cycle.
• A read access to the input data register obtains the I/O state.

This is the push button circuit for this tutorial. I also add a LED in this circuit just for show the state of button (is pressed or not).

For the GPIO, I choose input mode with internal pull-up. Therefore the button configuration is active low (when the button is pressed, the logic in data buffer is "0"). The output configuration for LED is open-drain mode and also active low.

This is the progam code. The purpose of this code is to turn on the LED when the button is pressed and turn off when it is not pressed.

#include "stm32f10x.h"
#include "stm32f10x_rcc.h"
#include "stm32f10x_gpio.h"

GPIO_InitTypeDef GPIO_InitStruct;

int main(void)
{
    // Enable clock for GPIOA
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_GPIOA, ENABLE);

    // Cofigure PA0 as open-drain output
    GPIO_InitStruct.GPIO_Pin = GPIO_Pin_0;
    GPIO_InitStruct.GPIO_Speed = GPIO_Speed_2MHz;
    GPIO_InitStruct.GPIO_Mode = GPIO_Mode_Out_OD;
    GPIO_Init(GPIOA, &GPIO_InitStruct);
 
    // Cofigure PA1 as input with internal pull-up resistor
    GPIO_InitStruct.GPIO_Pin = GPIO_Pin_1;
    GPIO_InitStruct.GPIO_Speed = GPIO_Speed_2MHz;
    GPIO_InitStruct.GPIO_Mode = GPIO_Mode_IPU;
    GPIO_Init(GPIOA, &GPIO_InitStruct);

    while (1)
    {
        // If button on PA1 is pressed (button circuit is active low)
        if (!(GPIO_ReadInputData(GPIOA) & GPIO_Pin_1))
        {
            // Turn on LED on PA0 (LED circuit is active low)
            GPIO_ResetBits(GPIOA, GPIO_Pin_0);
        }
        else
        {
            // Turn off LED on PA0
            GPIO_SetBits(GPIOA, GPIO_Pin_0);
        }
    }
}

You can use other input mode such as input with internal pull-down resistor or input floating. For input floating, you must add external pull-up or pull- down resistor. This is the example circuit for input with internal pull-down and input floating with external pull-up.

When using pull-down resistor, the logic is active high (when the button is pressed, the logic in data buffer is "1"). For more example code, you can visit here.

For STM32 tutorial with HAL library, you can click here.