GPIO Modes & Configuration

Embedded → Digital I/O & Timing (HAL)

Every pin starts as a blank slate

The mode you pick decides which internal circuit a pin connects to. On reset, every STM32 pin defaults to floating input — nothing drives it, nothing reads it usefully until configured.

The four pin modes

Mode	What it does
Input	voltage level read into a status register; pin drives nothing
Output	code drives the pin HIGH/LOW via a data register
Alternate function	pin handed to a peripheral (UART TX, PWM channel, I2C SCL) — one owner at a time
Analog	digital buffers disconnected so an ADC/DAC can use the pin without loading it

Push-pull vs open-drain — the output stage detail that trips people up

Push-pull is the default — both a pull-up and pull-down transistor, so writing 1/0 actively drives the pin to VDD/GND. Open-drainremoves the pull-up transistor — writing 0 pulls LOW, writing 1 just releases the pin to float (reads HIGH only via an external/internal pull-up). This lets multiple devices share one wire without fighting: I2C's SDA/SCL are open-drain so any device can pull LOW, and the shared pull-up brings it back HIGH once everyone releases. Two push-pull outputs driving opposite levels would short VDD to GND; two open-drain outputs never can.

Rule of thumb: use push-pull for a pin you fully own (an LED, a single-master SPI chip-select). Reach for open-drain only when the line is genuinely shared between multiple drivers — I2C being the textbook case.

Pull-up / pull-down resistors, and why floating inputs are dangerous

An undriven input floats, picking up noise and reading random glitches — a button wired with no resistor is exactly this trap. The fix: a weak resistor (tens of kΩ) tying the pin to a known rail. A pull-up ties to VDD (reads HIGH by default, button press reads clean LOW); pull-downis the mirror. Modern MCUs build a selectable weak pull-up/pull-down into every pin's config register, so an external resistor is rarely needed.

One pin, one register field, four settings

On an STM32: MODER (input/output/AF/analog), OTYPER (push-pull/open-drain), and PUPDR(none/pull-up/pull-down). Configuring PA5 as a push-pull output with no pull resistor — the classic "blink an LED" setup:

#define GPIOA_MODER   (*(volatile uint32_t *)0x40020000)
#define GPIOA_OTYPER  (*(volatile uint32_t *)0x40020004)
#define GPIOA_PUPDR   (*(volatile uint32_t *)0x4002000C)

GPIOA_MODER  &= ~(0x3u << (5 * 2));  // clear PA5's mode field
GPIOA_MODER  |=  (0x1u << (5 * 2));  // 01 = general-purpose output
GPIOA_OTYPER &= ~(0x1u << 5);        // 0 = push-pull (this is also the reset value)
GPIOA_PUPDR  &= ~(0x3u << (5 * 2));  // 00 = no pull-up/pull-down

Try it — pick a mode, see the register fields

Select a mode below and watch which MODER/OTYPER/PUPDR field values it maps to:

MODER5[1:0]

01

OTYPER5

0

PUPDR5[1:0]

00

Actively drives both HIGH and LOW. Use this for any pin you fully own — LEDs, chip-selects.

Practice

Open Wokwi and build a two-button circuit: one button using the MCU's internal pull-up (button pulls the pin LOW when pressed), one wired with no pull resistor at all. Watch the second pin's serial-monitor readout jitter between presses — that's the floating-input problem made visible, not theoretical.

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