STM32 GPIO Configuration — Push-Pull, Open-Drain, Speed, Pull-up/Pull-down

GPIO Input/Output/AF/Analog mode comparison

4.1 The Four GPIO Modes

Every GPIO pin is configured in one of four modes:

Mode	MODER Value	Description	Example Use
Input	00	Read external signals	Buttons, motor error pins, interrupt inputs
Output	01	Drive signals out	LEDs, motor Enable, Motor Stop
Alternate Function	10	Connect pin to a peripheral	UART TX/RX, SPI, CAN, PWM
Analog	11	Analog input/output	ADC input (torque sensor, angle sensor), DAC

4.2 Output Type: Push-Pull vs Open-Drain

Push-Pull and Open-Drain output comparison

When in Output or AF mode, you choose the output type:

Push-Pull (PP):

VDD ─── [P-FET] ─┬── pin output
                  │
GND ─── [N-FET] ─┘

Output HIGH → P-FET ON, N-FET OFF → drives VDD (3.3V)
Output LOW  → P-FET OFF, N-FET ON → drives GND (0V)

• Actively drives both HIGH and LOW
• Use Push-Pull in most cases (LEDs, SPI, UART TX, PWM, etc.)

Open-Drain (OD):

         ┌── external pull-up resistor ── VDD (or 5V!)
pin ─────┤
         └── [N-FET] ── GND

Output LOW  → N-FET ON → drives GND
Output HIGH → N-FET OFF → pulled up to VDD by external resistor

• Only actively drives LOW; HIGH relies on an external pull-up
• Required for I2C (SDA/SCL)
• Level shifting: use when a 3.3V MCU communicates with 5V devices

4.3 Pull Configuration

Setting	Effect	When to Use
No Pull	No pull-up or pull-down	AF mode (peripheral controls the line), when external pull-up/down is present
Pull-Up	Internal ~40kΩ resistor to VDD	Button inputs (active-low), UART RX idle state
Pull-Down	Internal ~40kΩ resistor to GND	Prevent floating pins, when a default-LOW state is needed

4.4 Output Speed

Speed	Max Frequency	When to Use
Low	~12 MHz	GPIO toggling (LEDs), low-speed signals
Medium	~60 MHz	UART, I2C
High	~85 MHz	SPI, SDMMC
Very High	~100 MHz	High-speed SPI, FMC

⚠️ Rule: Always choose the minimum speed required. Higher speeds increase EMI (electromagnetic interference) and current consumption.

• LED, Enable pins → Low
• CAN, UART → Medium
• SPI → High or Very High

4.5 GPIO Configuration with the HAL Library

Structure of the code auto-generated by CubeMX:

/* Core/Src/main.c — inside MX_GPIO_Init() */

static void MX_GPIO_Init(void)
{
    GPIO_InitTypeDef GPIO_InitStruct = {0};

    /* Enable GPIO port clocks */
    __HAL_RCC_GPIOA_CLK_ENABLE();
    __HAL_RCC_GPIOB_CLK_ENABLE();
    __HAL_RCC_GPIOC_CLK_ENABLE();
    __HAL_RCC_GPIOD_CLK_ENABLE();
    __HAL_RCC_GPIOE_CLK_ENABLE();

    /* === Example 1: LED output (PB0) === */
    HAL_GPIO_WritePin(GPIOB, GPIO_PIN_0, GPIO_PIN_RESET);  // initial state LOW
    GPIO_InitStruct.Pin   = GPIO_PIN_0;
    GPIO_InitStruct.Mode  = GPIO_MODE_OUTPUT_PP;    // Output Push-Pull
    GPIO_InitStruct.Pull  = GPIO_NOPULL;            // no pull-up/pull-down
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;    // low speed (it's an LED)
    HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);

    /* === Example 2: Button input + interrupt (PC13) === */
    GPIO_InitStruct.Pin   = GPIO_PIN_13;
    GPIO_InitStruct.Mode  = GPIO_MODE_IT_FALLING;   // falling edge interrupt
    GPIO_InitStruct.Pull  = GPIO_PULLUP;             // internal pull-up
    HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

    /* Enable EXTI interrupt */
    HAL_NVIC_SetPriority(EXTI15_10_IRQn, 5, 0);
    HAL_NVIC_EnableIRQ(EXTI15_10_IRQn);

    /* === Example 3: Motor Enable pin (PD3) === */
    HAL_GPIO_WritePin(GPIOD, GPIO_PIN_3, GPIO_PIN_RESET);
    GPIO_InitStruct.Pin   = GPIO_PIN_3;
    GPIO_InitStruct.Mode  = GPIO_MODE_OUTPUT_PP;
    GPIO_InitStruct.Pull  = GPIO_PULLDOWN;           // default OFF (safe state)
    GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
    HAL_GPIO_Init(GPIOD, &GPIO_InitStruct);
}

4.6 GPIO Control Functions

/* Set pin HIGH */
HAL_GPIO_WritePin(GPIOB, GPIO_PIN_0, GPIO_PIN_SET);

/* Set pin LOW */
HAL_GPIO_WritePin(GPIOB, GPIO_PIN_0, GPIO_PIN_RESET);

/* Toggle pin */
HAL_GPIO_TogglePin(GPIOB, GPIO_PIN_0);

/* Read pin */
GPIO_PinState state = HAL_GPIO_ReadPin(GPIOC, GPIO_PIN_13);
if (state == GPIO_PIN_SET) {
    // pin is HIGH
}

4.7 Exercise: Blinking an LED (First Sanity Check)

The first test to run after assembling the board:

/* Core/Src/main.c */

/* USER CODE BEGIN Includes */
/* USER CODE END Includes */

int main(void)
{
    HAL_Init();
    SystemClock_Config();
    MX_GPIO_Init();

    /* USER CODE BEGIN 2 */
    // (additional init code)
    /* USER CODE END 2 */

    while (1)
    {
        /* USER CODE BEGIN 3 */
        HAL_GPIO_TogglePin(GPIOB, GPIO_PIN_0);  // toggle LED
        HAL_Delay(500);                           // wait 500ms
        /* USER CODE END 3 */
    }
}

If the LED blinks every 0.5 seconds, you have confirmed:

1. Power is good (VDD 3.3V)
2. Clock is good (HSE → PLL → SYSCLK)
3. GPIO is working (output functional)
4. HAL library is working (HAL_Delay works = SysTick is running)
5. Flash programming succeeded (code is executing)

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