Serial UART Communication Experiment

1. Serial UART Communication Overview

UART is the most commonly used inter-board communication interface, requiring only TX/RX two wires for full-duplex transmission and reception. This experiment uses ESP32's hardware UART2 (Serial2), with TX (GPIO14) and RX (GPIO5) connected with Dupont wires to form a loopback. A string is sent every 2 seconds and the data sent by itself is received, demonstrating the complete usage of Serial2.begin, write, and readBytes.

Typical Application Scenarios:

This project demonstrates UART2 loopback communication: the MCU sends a string via TX, RX receives the echoed data, achieving self-send and self-receive hardware verification.

2. Core Concepts

2.1 UART Frame Format

UART transmits data in frames (common 8N1 format = 8 data bits + no parity + 1 stop bit):

​x
IDLE  ┌──┐ ┌──┬──┬──┬──┬──┬──┬──┬──┐ ┌──┐
      │St│ │D0│D1│D2│D3│D4│D5│D6│D7│ │Sp│  IDLE
──────┘  │ │  │  │  │  │  │  │  │ │  ├──────
         └─┘  └──┴──┴──┴──┴──┴──┴──┴─┘  └──┘

      Start bit ←──── Data bits (LSB first) ────→ Stop bit

On the same bus, the sender and receiver must have identical baud rate, data bits, parity, and stop bits, otherwise garbled characters will appear.

2.2 ESP32 Hardware UART

The ESP32-S3 has 3 hardware UART controllers, supporting GPIO matrix remapping, with any pin configurable as UART function.

2.3 Key APIs

• Serial2.begin(115200, SERIAL_8N1, RXD_PIN, TXD_PIN) — Initialize UART2

• Serial2.readBytes(buf, size) — Read bytes from RX FIFO, more reliable than readString()

• Serial2.flush() — Wait for all data in TX FIFO to be sent

2.4 RX Pin Pull-up Timing Trap

pinMode(RXD_PIN, INPUT_PULLUP) must be set before Serial2.begin(). Once begin() is called, the RX pin is taken over by the hardware UART, and setting INPUT_PULLUP afterwards will be ineffective. When idle, a floating RX will cause noise data to be received.

3. Hardware Dependencies

ESP32-S3 UART Hardware Features:

Connect GPIO14 and GPIO5 with a Dupont wire to form a "self-send and self-receive" loopback.

4. Project Architecture and Flow

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setup()
  ├─ Serial.begin(115200)           // Monitor serial UART0
  ├─ pinMode(RXD_PIN, INPUT_PULLUP) // Must be before Serial2.begin
  ├─ Serial2.begin(115200, 8N1, RX, TX) // Initialize UART2
  └─ Print startup message

loop()
  ├─ Serial2.write("GOOD GOOD STUDY...\r\n")  // Send
  ├─ Serial2.flush()                          // Wait for FIFO to finish
  ├─ Serial2.setTimeout(200)
  ├─ len = Serial2.readBytes(rxBuf, 255)      // Receive loopback data
  ├─ Filter non-printable characters → Print RX content via serial
  └─ delay(2000)                              // 2 second interval

5. Source Code

Source Code Location: Source code → Arduino → 2.Serial communication → 1.UART → 1.UART.ino

5.1 Macro Definitions and Global Variables

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#define TXD_PIN 14
#define RXD_PIN 5
#define BUF_SIZE 256

uint8_t rxBuf[BUF_SIZE];

5.2 setup() — Initialize Dual Serial Ports

Configure the monitor serial port first, then initialize UART2 for loopback communication.

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void setup() {
  Serial.begin(115200);

  // Important: Set RX pull-up before Serial2.begin()
  pinMode(RXD_PIN, INPUT_PULLUP);

  Serial2.begin(115200, SERIAL_8N1, RXD_PIN, TXD_PIN);
  Serial.printf("UART Started, TX=GPIO%d, RX=GPIO%d\n", TXD_PIN, RXD_PIN);
}

pinMode(RXD_PIN, INPUT_PULLUP) must be called before Serial2.begin(). When TX/RX are shorted with Dupont wires, pull-up can be omitted, but keeping it is good defensive programming practice.

5.3 loop() — Transmit and Receive Loopback

Send a string, then read it back from the same port. Filter non-printable characters to keep the monitor readable.

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void loop() {
  Serial2.write("GOOD GOOD STUDY,DAY DAY UP!!!!!!\r\n");
  Serial2.flush();  // Wait for FIFO to finish

  Serial2.setTimeout(200);
  int len = Serial2.readBytes(rxBuf, BUF_SIZE - 1);

  if (len > 4) {
    for (int i = 0; i < len; i++) {
      if (rxBuf[i] == '\r' || rxBuf[i] == '\n') continue;
      if (rxBuf[i] < 0x20 || rxBuf[i] > 0x7E) rxBuf[i] = '.';
    }
    rxBuf[len] = '\0';
    Serial.printf("[RX] %s\n", rxBuf);
  }

  delay(2000);
}

Filtering replaces non-printable characters with .. The len > 4 threshold filters out noise.

6. Operation Flow

6.1 Compile and Flash

Arduino Flash Process: See 1. Before Development → 2. Build and Flash.

1. Short GPIO14 (TX) and GPIO5 (RX) with a Dupont wire

2. Open 1.UART.ino

6.2 Normal Phenomena

Open the Serial Monitor (115200 bps), printing every 2 seconds:

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[UART] UART Started, TX=GPIO14, RX=GPIO5
[UART] [TX] GOOD GOOD STUDY,DAY DAY UP!!!!!!
[UART] [RX] GOOD GOOD STUDY,DAY DAY UP!!!!!!

[UART] [TX] GOOD GOOD STUDY,DAY DAY UP!!!!!!
[UART] [RX] GOOD GOOD STUDY,DAY DAY UP!!!!!!

[UART] [TX] GOOD GOOD STUDY,DAY DAY UP!!!!!!
[UART] [RX] GOOD GOOD STUDY,DAY DAY UP!!!!!!

[UART] [TX] GOOD GOOD STUDY,DAY DAY UP!!!!!!
[UART] [RX] GOOD GOOD STUDY,DAY DAY UP!!!!!!

After disconnecting or removing the Dupont wire, the print becomes:

6.3 Verification Methods

7. Common Issues