STM32 UART Polling vs Interrupts vs DMA - LED control example

The STM32F401RE Nucleo-64’s UART offers three methods—polling, interrupts, and DMA—to handle serial communication, each with trade-offs. The previous stm32 programming tutorial used LED control via a MAX232 and DB9 port from a PC. In brief, polling method continuously checks the UART status, making it simple but CPU-intensive, ideal for basic LED toggling with minimal code. This was illustrated in the tutorial STM32 UART LED Control with interrupt. Interrupts trigger on events like data received or sent, freeing the CPU for other tasks while efficiently managing LED on/off commands, as seen in interrupt-driven transmission. This was illustrated in the tutorial STM32 UART Interrupts LED Control. DMA (Direct Memory Access) offloads data transfer to hardware, enabling high-speed, non-blocking communication for complex applications, such as streaming LED patterns, with minimal CPU intervention. This tutorial explores these approaches using the LED control example to highlight their practical differences.

UART Implementation Methods Comparison

1. UART Alone (Polling Mode)

// Basic UART transmission
HAL_UART_Transmit(&huart2, data, size, HAL_MAX_DELAY);
HAL_UART_Receive(&huart2, data, size, HAL_MAX_DELAY);

Characteristics:
- CPU actively waits for transmission/reception
- Blocks until operation completes
- Simplest to implement
- Best for small data transfers
- High CPU usage

2. UART + Interrupts

// Interrupt-based transmission
HAL_UART_Transmit_IT(&huart2, data, size);
HAL_UART_Receive_IT(&huart2, data, size);

// Callback when complete
void HAL_UART_TxCpltCallback(UART_HandleTypeDef *huart)
{
    uart_tx_complete = 1;
}

Characteristics:
- CPU can do other tasks while waiting
- Uses interrupt handler
- Moderate CPU usage
- Good for medium-sized transfers
- Requires callback handling

3. UART + DMA

// DMA-based transmission
HAL_UART_Transmit_DMA(&huart2, data, size);
HAL_UART_Receive_DMA(&huart2, data, size);

// DMA completion callback
void HAL_UART_RxCpltCallback(UART_HandleTypeDef *huart)
{
    // Process received data
    HAL_UART_Receive_DMA(&huart2, data, size); // Restart reception
}

Characteristics:
- Direct memory access without CPU
- Lowest CPU usage
- Best for large data transfers
- Most complex to setup
- Requires DMA configuration
- Can handle continuous data streams

Performance Comparison

Choose based on:
1. Data transfer size
2. CPU availability needs
3. Code complexity tolerance
4. Response time requirements