🟠 What is a general purpose input output (GPIO) of the MCU

GPIO

GPIO stands for general purpose input/output. It is a type of pin found on an integrated circuit that does not have a specific function.
While most pins have a dedicated purpose, such as sending a signal to a certain component, the function of a GPIO pin is customizable and can be controlled by the software.

It can work as: 📥 Input (reads logical signals, such as from a button)
📤 Output (controls something - LED, relay, motor)
🔄 Sometimes - both as an input and as an output (push-pull)

Pin

Pin Mode : Each port bit of the general-purpose I/O (GPIO) ports can be individually configured by software in several modes:

  • input or output
  • analog
  • alternate function (AF). Pin characteristics :
  • Input : no pull-up and no pull-down or pull-up or pull-down
  • Output : push-pull or open-drain with pull-up or pull-down capability
  • Alternate function : push-pull or open-drain with pull-up or pull-down capability.

GPIO Design

Each GPIO pin of the microcontroller consists of three key components:

  1. Enable Line
  • Configures the pin mode: input, output, alternate function, analog, etc.
  • Controls the activation of internal pull-up resistors
  • Enables/disables input/output buffers and logic enable
  1. Output Buffer
  • It is a combination of two transistors (PMOS + NMOS)
  • In Push-Pull mode both can work
  • In Open-Drain only NMOS is active
  • Allows to output logic levels to an external pin

When the signal 0 comes out, control goes through the NMOS transistor and no logical one is supplied to the pin.

When the signal 1 comes out, control goes through the PMOS transistor and a logical one is supplied to the pin.

  1. Input Buffer
  • Monitors the signal level on the pin
  • Can be enabled or disabled - for example to disable reading during output
  • Used to read logic level or generate interrupts

When a 0 signal is received on the pin, control passes through the NMOS transistor and goes to GND

When a 1 signal is received on the pin, control passes through the PMOS transistor and is powered from +VCC

⚙️ GPIO Modes

  • By default all GPIOs on most microcontrollers are configured in Input mode.
  • In this mode they are not connected to either Vcc or GND - the pin is in the High Impedance (Hi-Z) state.
  • This means that the GPIO is not "involved" in the circuit's operation, but may be subject to noise and flow currents, especially if the input is left "floating".
ModeDescriptionExample
InputReading signal from outsideButton, sensor
Input Pull-UpInput with pull-up to Vcc (internal resistor)Button to GND
Input Pull-DownInput with pull-up to GNDButton to Vcc
Output Push-PullNormal output (0 or 1)LED, relay
Output Open-DrainOnly "0", "1" — via external resistorI²C, external bus
AnalogADC/DAC is used on the outputTemperature sensor, audio
Alternate FunctionUsed by peripherals (SPI, PWM, UART)SPI, I²C, PWM

If we wanted to use some Pin as an input, we would use either .into_pull_up_input(), .into_pull_down_input() to tell the chip to expect the system voltage (3.3 volts) to be present or absent, and react if that changes.

// RP2040 (Rust embedded-hal, hi-Z)
    _pin_28.into_pull_down_input(); // Set GPIO 28 as input with pull-down
    _pin_28.into_floating_input(); // Set GPIO 28 as floating input
    _pin_28.into_pull_up_input(); // Set GPIO 28 as input with pull-up

    _pin_28.into_pull_up_disabled(); // Disable pull-up on GPIO 28
    _pin_28.into_pull_down_disabled(); // Disable pull-down on GPIO 28

    _pin_28.into_push_pull_output(); // Set GPIO 28 as push-pull output

Output

Push-Pull

  • GPIO uses two transistors: PMOS (to supply "1") and NMOS (to supply "0").
  • In Push-Pull mode, the pin can actively:
    • Supply the line with logical 1 (Vcc) via PMOS
    • Supply logical 0 (GND) via NMOS
  • This is the standard mode for driving LEDs, relays and other loads.

Open-Drain

  • In this mode, GPIO can only short the pin to ground (0) via NMOS.
  • Internal PMOS is missing or disabled → you cannot apply "1" directly.
  • When GPIO is driven to logic "1" → transistor is off and pin becomes floating.
  • To make the line to be logic "1", a pull-up resistor is used:
    • Internal (enabled by software if available)
    • External (usually 4.7k–10k Ohm to Vcc)

Useful when When need a common bus with many devices connected to it. (All participants can only "pull down" to avoid conflict)