STM32 Programming· Live

GPIO input: read the button

In input mode the code reads the voltage on a pin. We set the button pin PC13 as an input with a pull-up resistor, and turn the LED on while the button is held.

In this lesson · 4 steps
  1. PC13 as an input, named BUTTON
  2. Turn on the pull-up
  3. Read the button in the loop
  4. Build and run

Input mode

The chip compares the pin voltage with a threshold: below it reads 0, above it reads 1. With a button, pressing it pulls the pin to ground, so it reads 0. When you let go, the voltage goes back up and it reads 1.

Input mode: the button sets the pin voltage, HAL_GPIO_ReadPin returns SET or RESET

Pull-up and pull-down resistors

When the button is not pressed, a pin with only a button on it is connected to nothing. It is floating, and it can read 0 or 1 at random.

A pull-up resistor connects the pin to 3.3 V, so it reads 1 by default and 0 when the button pulls it to ground. A pull-down resistor does the opposite. STM32 pins have both built in: you turn one on in CubeMX, no extra part needed.

Pull-up and pull-down resistors keep the pin from floating

1PC13 as an input, named BUTTON

  1. We stay in STM32_GPIO_PROGRAMMING. In CubeIDE's Project Explorer, double-click STM32_GPIO_PROGRAMMING.ioc: it opens in CubeMX (lesson 0.2 links the two; or use File → Load Project in CubeMX)
  2. Click PC13 → GPIO_Input, then right-click → Enter User Label → BUTTON

2Turn on the pull-up

  1. In the Pinout & Configuration tab, in the Categories list on the left: System Core → GPIO → select PC13 → GPIO Pull-up/Pull-down = Pull-up
  2. Generate Code. Your blink code is kept: it is inside a USER CODE section
  3. Back in CubeIDE, right-click the project → Refresh (F5), so it shows the new code
Why?
The button connects PC13 to ground when pressed. With the pull-up, the pin reads 1 at rest and 0 while pressed.

3Read the button in the loop

main.c · inside the braces of while (1): replace the blink code

If the pin reads RESET (0), the button is pressed: LED on. Otherwise LED off.Grey lines are already in the file: add only the coloured ones. Copy takes just those.

  /* USER CODE BEGIN WHILE */
  while (1)
  {
    if (HAL_GPIO_ReadPin(BUTTON_GPIO_Port, BUTTON_Pin) == GPIO_PIN_RESET)
    {
        /* button pressed: with the pull-up, the pin reads RESET when pressed */
        HAL_GPIO_WritePin(LED_CONTROL_GPIO_Port, LED_CONTROL_Pin, GPIO_PIN_SET);
    }
    else
    {
        HAL_GPIO_WritePin(LED_CONTROL_GPIO_Port, LED_CONTROL_Pin, GPIO_PIN_RESET);
    }
    /* USER CODE END WHILE */
Why?
HAL_GPIO_ReadPin returns GPIO_PIN_SET (1) or GPIO_PIN_RESET (0). Because of the pull-up, pressed means RESET.

4Build and run

  1. Terminate the last debug session if it still runs, then Debug and Resume
Check & troubleshoot

What you should see

  • Hold the blue button (B1): the green LED is on
  • Let go: the LED is off

If it doesn't work

  • LED is on when the button is not pressed: the comparison is the wrong way round; pressed is GPIO_PIN_RESET
  • Build error on BUTTON_Pin: the label on PC13 must be exactly BUTTON
  • Nothing happens: check PC13 is set to GPIO_Input and generate the code again

Good to know

  • The loop checks the button thousands of times per second, even when nothing changes. Chapter 2 fixes that with an interrupt.