First, What Is It

• GPIO (General-Purpose Input/Output): pins that can be read and written — output high/low to light an LED, or sample an external level to detect a button press. It is the most basic "switch and eye" of an MCU.
• Read vs write: writing a pin controls the output level (like flipping a switch); reading samples the external level (like checking the switch state).
• Why start with GPIO: it covers output, input, pull-up/pull-down, and interrupts — the four fundamentals that every other peripheral builds on.

Breaking Down the Principle

1. Internal Structure: Output Driver and Input Sampler

Each GPIO pin internally contains an "output driver + input sampler + optional pull resistors". Configuring modes switches these internal circuits:

2. Output Modes: Push-Pull and Open-Drain

Mode	Structure	Features	Typical use
Push-pull	Drives high and low actively	Strong drive, fast	LEDs, buzzers
Open-drain	Can only pull low; high level needs an external pull-up	Different voltages, wired-AND	I2C, shared buses, level shifting

The open-drain "wired-AND" property: when several open-drain outputs are wired together, the bus is low if any of them drives low — the physical basis for I2C multi-device sharing.

3. Input Modes: Floating / Pull-Up / Pull-Down

• Floating: the level is undefined when the pin is not driven — easily disturbed, generally avoided.
• Pull-up: a resistor pulls the default level high; a button to ground pulls it low when pressed — the most common button wiring.
• Pull-down: the default level is low; a button to VCC pulls it high when pressed.

Whether "pressed" is high or low depends on where the other end of the button is connected and the pin's default level — wiring it backwards gives "pressed without touching" or "pressed but no response".

4. Alternate Function (Pin Mux)

GPIO is not only GPIO: UART TX/RX, SPI clocks, PWM outputs, etc., all occupy pins. Each pin can be muxed to multiple peripheral functions, chosen via pin-function configuration (in the SDK, commonly interfaces like GLB_GPIO_Func_Init). When you want to move a signal to another pin, first check which functions that pin supports.

5. Levels and Drive Strength

• High/low are relative: in a 3.3 V system, "1" ≈ 3.3 V and "0" ≈ 0 V, but the threshold to read as "1" is usually lower than 3.3 V (e.g., ≥2.0 V).
• Drive strength limits the load: lighting an LED needs a few milliamps; driving a relay or motor directly requires a transistor or driver chip, otherwise the pin can be damaged.

How the SDK Implements It

• Related pages: GPIO Output (LED), GPIO Input (Button), EXTI Interrupt

In the LED example, the SDK configures a GPIO output as "get device → configure mode → write level":

struct bflb_device_s *gpio = bflb_device_get_by_name("gpio");
/* Configure IO12 as GPIO output (Ai-M61-32S-Kit onboard RGB red LED, active high) */
bflb_gpio_init(gpio, GPIO_PIN_12, GPIO_OUTPUT);
bflb_gpio_set(gpio, GPIO_PIN_12, 1);   /* Output high, LED on */

Button input first configures the pin as input with pull-up, then reads the level (or uses an interrupt):

bflb_gpio_init(gpio, GPIO_PIN_2, GPIO_INPUT);      /* BOOT button commonly on IO2 */
bflb_gpio_pullup(gpio, GPIO_PIN_2, true);         /* Default high, low when pressed */
bool pressed = bflb_gpio_read(gpio, GPIO_PIN_2);  /* false means pressed */

Common Exam & Interview Questions

Difference between push-pull and open-drain? When to use open-drain?

Push-pull actively drives high and low with strong drive; open-drain can only pull low and needs an external pull-up for high. Use open-drain for I2C, shared buses, and cross-voltage level shifting.

Why add a pull-up resistor to a button?

The pull-up keeps the pin high by default; pressing the button to ground makes it low, so "released = 1, pressed = 0" is well defined and the pin is never floating.

What is pin muxing (alternate function)?

A pin can switch between GPIO and multiple peripheral functions. Because pins are limited, muxing lets UART, SPI, PWM, etc., share pins; configuration selects the current function.

Can GPIO drive a relay or motor directly?

No. Relays/motors need more current — use a transistor, MOSFET, or a driver chip (add a flyback diode for inductive loads), otherwise the pin can be damaged or the system may reset.

Software debounce vs hardware debounce?

Software: confirm the level after a few milliseconds of delay or sample periodically; hardware: parallel capacitor or RC filter on the button. Both prevent mechanical bounce from causing multiple triggers.

Have questions?

For any other questions, visit the unified Q&A and discussion board: Ai-Thinker Discussions