Reliable data exchange between microcontrollers is a core requirement in embedded systems. UART (Universal Asynchronous Receiver/Transmitter) is one of the simplest and most widely used serial communication protocols for point-to-point communication.

This article outlines a practical approach to implement UART communication between two STM32 microcontrollers using a structured data frame and interrupt-based reception.

System Overview

Two STM32 boards are connected via UART:

• TX (Master) → RX (Slave)
• RX (Master) ← TX (Slave)
• Common GND

Baud rate and frame format must match on both sides (e.g., 115200, 8N1).

Data Frame Design

To avoid parsing errors, use a structured frame:

<stx> ID,VAL1,VAL2,CHK <etx></etx> </stx>

Example:

$01,123,456*5A#

Where:

• $ = start (STX)
• 01 = device ID
• 123,456 = payload
• *5A = checksum (optional)

• = end (ETX)

A clear delimiter-based frame prevents partial reads and simplifies parsing.

Transmit (TX) Implementation

On the master:

1. Format payload into string
2. Append delimiters and checksum
3. Send via UART

Pseudocode:

sprintf(txBuf, "$%02d,%d,%d*%02X#", id, v1, v2, checksum);
HAL_UART_Transmit(&huart1, (uint8_t*)txBuf, strlen(txBuf), 100);

Receive (RX) Using Interrupt

Use interrupt (or DMA) to avoid blocking:

• Enable HAL_UART_Receive_IT (or DMA)
• Buffer incoming bytes
• Detect end marker #
• Parse complete frame

Parsing Strategy

After a full frame is received:

1. Validate start/end markers
2. Verify checksum (if used)
3. Split CSV payload
4. Convert to numeric values

Example (concept):

• Find $ and #
• Extract substring
• sscanf or tokenization

This ensures deterministic and error-tolerant parsing.

Timing and Reliability

Key considerations:

• Match baud rate on both devices
• Use hardware UART (not bit-bang)
• Keep frames short to reduce latency
• Add timeout/reset for incomplete frames
• Consider DMA for high throughput

Error Handling

Improve robustness with:

• Checksum (XOR or CRC)
• Frame timeout (discard incomplete data)
• Re-sync on next $ if corruption occurs

Practical Applications

• Sensor node → controller communication
• HMI ↔ controller data exchange
• Multi-board modular systems
• Industrial gateways

UART provides a simple and effective method for microcontroller communication. By combining a structured data frame, interrupt-based reception, and basic error handling, STM32 systems can achieve reliable and maintainable serial communication.

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