The working principle of GPIO in STM32

 The GPIO (General Purpose Input/Output) pins on an STM32 microcontroller are highly flexible and are used to interact with external devices—like LEDs, buttons, sensors, or other ICs. Here's a clear explanation of the working principle of GPIO in STM32:

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Basic Working Principle of GPIO in STM32

Each GPIO pin can be configured as:

• Input (read external signal)

• Output (drive a signal high or low)

• Alternate function (used by peripherals like USART, SPI, etc.)

• Analog (e.g., for ADC inputs)

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GPIO Workflow in STM32

1. Clock Enable (RCC)

Before using any GPIO port (A, B, C, ...), you must enable its clock in the RCC (Reset and Clock Control):

c

RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;  // Enable clock for GPIOA

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2. Pin Mode Configuration

Each pin has a mode set in MODER register:

c

GPIOA->MODER &= ~(3 << (2 * 5)); // Clear mode for PA5
GPIOA->MODER |=  (1 << (2 * 5)); // Set PA5 as output

Modes:

• 00: Input

• 01: Output

• 10: Alternate Function

• 11: Analog

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3. Output Type

Set output type in OTYPER:

c

GPIOA->OTYPER &= ~(1 << 5); // Push-pull (default)

Types:

• 0: Push-pull (normal output)

• 1: Open-drain (needs pull-up resistor)

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4. Speed Configuration

In OSPEEDR, select output speed (low, medium, high, very high):

c

GPIOA->OSPEEDR |= (2 << (2 * 5)); // High speed for PA5

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5. Pull-up / Pull-down Configuration

Set pull-up/down resistors in PUPDR:

c

GPIOA->PUPDR &= ~(3 << (2 * 5)); // No pull-up/down
GPIOA->PUPDR |=  (1 << (2 * 5)); // Pull-up for PA5

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6. Write to Output Pin

Set/reset pin using ODR or BSRR:

c

GPIOA->ODR |=  (1 << 5);  // Set PA5 high
GPIOA->ODR &= ~(1 << 5);  // Set PA5 low

// OR atomically:
GPIOA->BSRR = (1 << 5);        // Set PA5
GPIOA->BSRR = (1 << (5 + 16)); // Reset PA5

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7. Read from Input Pin

Read input using the IDR register:

c

uint8_t pin_state = (GPIOA->IDR & (1 << 5)) ? 1 : 0;

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 Example: Toggle LED on PA5

c

void init_GPIOA5_output() {
    RCC->AHB1ENR |= RCC_AHB1ENR_GPIOAEN;         // Enable GPIOA
    GPIOA->MODER &= ~(3 << (2 * 5));
    GPIOA->MODER |=  (1 << (2 * 5));             // PA5 as output
    GPIOA->OTYPER &= ~(1 << 5);                  // Push-pull
    GPIOA->OSPEEDR |= (2 << (2 * 5));            // High speed
    GPIOA->PUPDR &= ~(3 << (2 * 5));             // No pull
}

void toggle_LED() {
    GPIOA->ODR ^= (1 << 5);  // Toggle PA5
}

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 Summary Table

Feature	Register
Mode selection	MODER
Output type	OTYPER
Output speed	OSPEEDR
Pull-up/down	PUPDR
Read input	IDR
Write output	ODR, BSRR