UART Communication Between Two Arduino Boards

UART (Universal Asynchronous Receiver/Transmitter) is one of the simplest and most widely used serial communication protocols in embedded systems. In this project, I connected two Arduino Nano boards to exchange sensor data reliably.

Hardware Setup

Arduino A (TX)  →  Arduino B (RX)
Arduino A (RX)  ←  Arduino B (TX)
GND  →  GND

No level shifting needed — both boards run at 5V logic.

Protocol Design

Raw UART has no notion of message boundaries. I used a simple frame format:

[0xAA] [LEN] [DATA...] [CRC]

0xAA — start byte
LEN — payload length
DATA — the actual payload
CRC — simple XOR checksum

Implementation

#define START_BYTE 0xAA

uint8_t send_frame(uint8_t *data, uint8_t len) {
    uint8_t crc = 0;
    Serial.write(START_BYTE);
    Serial.write(len);
    for (uint8_t i = 0; i < len; i++) {
        Serial.write(data[i]);
        crc ^= data[i];
    }
    Serial.write(crc);
}

On the receiving end, I implemented a simple state machine:

enum { WAIT_START, WAIT_LEN, WAIT_DATA, WAIT_CRC } state = WAIT_START;

void process_byte(uint8_t b) {
    switch (state) {
        case WAIT_START:
            if (b == START_BYTE) state = WAIT_LEN;
            break;
        case WAIT_LEN:
            expected = b; idx = 0; state = WAIT_DATA;
            break;
        case WAIT_DATA:
            buf[idx++] = b;
            if (idx == expected) state = WAIT_CRC;
            break;
        case WAIT_CRC:
            // verify checksum and reset
            state = WAIT_START;
            break;
    }
}

Key Takeaways

Always use a frame protocol — raw byte streams are error-prone
A CRC or checksum catches corrupted data
State machine parsing is robust and simple
At 115200 baud, a Nano can handle several thousand frames per second without missing data