GPIO and Digital IO

General-Purpose Input/Output — the most basic peripheral on any MCU. Each GPIO pin can be configured as a digital input or output, and most pins have alternate functions (UART TX, SPI clock, PWM output, etc.) selected through a multiplexer.

Why It Matters

GPIO is how the MCU touches the physical world. LEDs, buttons, relays, chip-select lines, bit-banged protocols — all GPIO. Understanding output modes, pull resistors, and alternate functions is prerequisite to using every other peripheral.

How It Works

Pin Multiplexing

Each physical pin connects to multiple internal peripherals through a mux. Only one function is active at a time.

                   ┌─────────┐
  Physical Pin <---┤   MUX   │<-- GPIO output
                   │         │<-- UART TX (AF7)
                   │         │<-- SPI MOSI (AF5)
                   │         │<-- TIM PWM (AF1)
                   └─────────┘
                     ^
               AFR register
              selects which AF

On STM32, the GPIOx->AFR[0] (pins 0-7) and AFR[1] (pins 8-15) registers select the alternate function. Each pin gets 4 bits (AF0-AF15).

Output Modes

Push-Pull:                    Open-Drain:
  VDD                           VDD
   |                             |
  [P-FET] -- ON for HIGH       [pull-up R]
   |                             |
   +---- Pin                     +---- Pin
   |                             |
  [N-FET] -- ON for LOW        [N-FET] -- ON for LOW
   |                             |
  GND                           GND

  Can drive HIGH and LOW        Can only pull LOW
  Most common mode              HIGH = floating (needs pull-up)
                                Used for I2C, level shifting,
                                wired-OR buses

Push-pull: actively drives both HIGH and LOW. Default for most outputs (LEDs, SPI, UART TX).

Open-drain: only pulls LOW; HIGH state is floating. Requires external or internal pull-up resistor. Essential for I2C (SDA/SCL are open-drain by spec) and any shared bus where multiple devices must drive the same line.

Input Modes

Mode	PUPDR bits	Behavior
Floating	00	No pull resistor. Pin voltage undefined when disconnected. Use when external driver always provides a signal.
Pull-up	01	Internal ~40k resistor to VDD. Default HIGH, reads LOW when grounded.
Pull-down	10	Internal ~40k resistor to GND. Default LOW, reads HIGH when driven.

For buttons, always use a pull-up or pull-down. Floating inputs pick up noise and read randomly.

GPIO Speed Setting

The OSPEEDR register controls output slew rate (how fast the pin transitions):

Setting	Typical max freq	Use case
Low	2 MHz	LEDs, relays
Medium	25 MHz	General IO
High	50 MHz	SPI, UART
Very High	100 MHz	SDIO, high-speed SPI

Higher speed = faster edges = more EMI and power draw. Use the lowest speed that meets your timing requirement. Relates to Signal Integrity.

STM32 GPIO Registers

Register	Name	Purpose
MODER	Mode	Input/Output/AF/Analog (2 bits per pin)
OTYPER	Output type	Push-pull / Open-drain (1 bit per pin)
OSPEEDR	Speed	Slew rate (2 bits per pin)
PUPDR	Pull-up/down	None/Up/Down (2 bits per pin)
IDR	Input data	Read pin state (read-only)
ODR	Output data	Set pin output level
BSRR	Bit set/reset	Atomic set/clear (no read-modify-write needed)
AFR[2]	Alternate function	Select AF0-AF15 (4 bits per pin)

Code Example

LED on PA5 (output) + button on PA0 (input with pull-up, active low):

// Enable GPIOA clock
RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;
 
// PA5: output, push-pull, low speed
GPIOA->MODER  &= ~(3 << (5 * 2));
GPIOA->MODER  |=  (1 << (5 * 2));   // 01 = output
GPIOA->OTYPER &= ~(1 << 5);         // 0  = push-pull
GPIOA->OSPEEDR &= ~(3 << (5 * 2));  // 00 = low speed
 
// PA0: input with pull-up
GPIOA->MODER  &= ~(3 << (0 * 2));   // 00 = input
GPIOA->PUPDR  &= ~(3 << (0 * 2));
GPIOA->PUPDR  |=  (1 << (0 * 2));   // 01 = pull-up
 
// Toggle LED when button pressed
while (1) {
    if (!(GPIOA->IDR & (1 << 0))) {        // button grounds PA0
        GPIOA->BSRR = (1 << 5);            // set PA5 (LED on)
    } else {
        GPIOA->BSRR = (1 << (5 + 16));     // reset PA5 (LED off)
    }
}

Note: BSRR is preferred over ODR |= because it is atomic — bits 0-15 set, bits 16-31 clear. No read-modify-write race condition with ISRs.

Software Debounce

Mechanical buttons bounce for 1-10ms, producing multiple edges on a single press:

uint8_t read_button(void) {
    static uint32_t last_time = 0;
    uint32_t now = HAL_GetTick();
    if (now - last_time < 50) return 0;     // 50ms debounce window
    if (!(GPIOA->IDR & (1 << 0))) {         // active low
        last_time = now;
        return 1;
    }
    return 0;
}

Related

• Microcontroller Architecture — GPIO sits on the AHB bus
• Memory-Mapped IO — register access patterns for GPIO
• Interrupts and Timers — EXTI for GPIO edge/level interrupts
• UART SPI I2C — alternate function peripherals that share GPIO pins
• Transistors as Switches — what push-pull and open-drain actually mean at transistor level
• Signal Integrity — why GPIO speed settings matter