4.1.1.3. GPIO Firmware for STM32F411 MCU

4.1.1.3.1. 1. What is GPIO?

In the STM32F411 microcontroller, the GPIO (General Purpose Input/Output) functionality allows the user to configure pins as either input or output. This flexibility is fundamental for interfacing the microcontroller with external devices such as LEDs, switches, sensors, and other peripherals. Each GPIO pin can be individually configured with various parameters like mode, output type, speed, and pull-up/pull-down resistors.

4.1.1.3.2. 2. GPIO Functionality and Configuration

Each GPIO pin on the STM32F411 has several modes, configurable via registers:

  • Input mode: Used for reading external signals.

  • Output mode: Used to drive external devices, like LEDs or relays.

  • Alternate Function mode: Used when a pin is designated for a specific peripheral (e.g., UART, I2C, SPI).

  • Analog mode: Used for analog input/output (e.g., ADC).

Key Configurations:

  • Pin Mode (Input/Output/Alternate/Analog): Configured in the GPIOx_MODER register.

  • Output Type (Push-pull/Open-drain): Configured in the GPIOx_OTYPER register.

  • Speed (Low/Medium/High/Very High): Configured in the GPIOx_OSPEEDR register.

  • Pull-up/Pull-down Resistors: Configured in the GPIOx_PUPDR register to avoid floating inputs.

These configurations enable users to control individual pin behavior based on the connected external device requirements.

4.1.1.3.3. 3. GPIO Output Registers: ODR and BSRR

The STM32F411 microcontroller has several registers for managing the output on GPIO pins, with the Output Data Register (ODR) and Bit Set/Reset Register (BSRR) being the primary ones.

### Output Data Register (ODR)

The GPIOx_ODR is a 16-bit register used to set or clear the output level of each GPIO pin in one operation. Writing 1 or 0 to a specific bit in this register sets the corresponding pin to HIGH or LOW, respectively.

  • Usage: Directly writing to ODR controls all pins at once. If you want to control multiple pins simultaneously, ODR is efficient.

  • Example: To turn on a specific pin (e.g., GPIOA pin 5), you could set the corresponding bit in ODR to 1. For instance, GPIOA->ODR |= (1 << 5); would set pin 5 to HIGH.

### Bit Set/Reset Register (BSRR)

The GPIOx_BSRR is a 32-bit register that allows setting and resetting individual bits atomically. This register provides two 16-bit halves:

  • Lower 16 bits (BS): Used to set pins to HIGH.

  • Upper 16 bits (BR): Used to reset pins to LOW.

Unlike ODR, the BSRR register is designed for atomic operations, meaning you can simultaneously set and reset bits without affecting other bits. This is particularly useful for avoiding unintentional changes when multiple bits need to be controlled.

  • Usage: To set a pin to HIGH, write 1 to the bit in the lower half (BS). To reset (clear) a pin, write 1 to the corresponding bit in the upper half (BR).

  • Example: - Set GPIOA pin 5: GPIOA->BSRR = (1 << 5); - Reset GPIOA pin 5: GPIOA->BSRR = (1 << (5 + 16));

4.1.1.3.4. 4. Differences Between ODR (Output Data Register) and BSRR (Bit Set/Reset Register)

The Output Data Register (ODR) and the Bit Set/Reset Register (BSRR) both control GPIO pin outputs, but they have key differences in functionality, safety, and practical applications.

Comparison of ODR and BSRR

Summary:

  • ODR is suitable for quick, multi-pin changes but requires caution when used in environments with concurrent processes, as modifying individual pins might affect others unintentionally.

  • BSRR provides more precise control with atomic operations, making it safer for use in interrupt-driven or multitasking applications where independent pin control is crucial.

4.1.1.3.5. 5. Practical Example: Toggling an LED Using ODR and BSRR

Consider a setup where an LED is connected to GPIOA pin 5. We want to toggle this LED on and off.

Example with ODR:

// Configure GPIOA pin 5 as output in MODER register
GPIOA->MODER |= (1 << (5 * 2));  // Sets PA5 as output

// Toggle LED
GPIOA->ODR ^= (1 << 5);  // XOR to toggle PA5 state

In this example, the ODR register is used to toggle the output state of pin 5, thus turning the LED on or off depending on its current state.

Example with BSRR:

// Configure GPIOA pin 5 as output in MODER register
GPIOA->MODER |= (1 << (5 * 2));  // Sets PA5 as output

// Set LED ON
GPIOA->BSRR = (1 << 5);  // Sets PA5 to HIGH

// Set LED OFF
GPIOA->BSRR = (1 << (5 + 16));  // Resets PA5 to LOW

Here, BSRR is used to set or reset the pin without affecting other bits. This approach is preferred in applications where atomicity is essential, such as in real-time systems or when dealing with multiple outputs.