66. UART Programming

UART (Universal Asynchronous Receiver/Transmitter) is a full-duplex, asynchronous serial communication protocol that uses two wires, Tx and Rx.

Note: RTS/CTS ( Request To Send / Clear To Send) hardware data flow control lines. Those are Optional.

Short-distance communication  <10–50 cm, when using TTL voltage levels.

Limitations
     1. Point-to-point communication only (not multi-master/slave like I2C/SPI)
     2. Slower speed than SPI/I2C.

UART is commonly used for debugging with serial monitors, communicating with Bluetooth/Wi-Fi modules, GPS receivers, sensors, etc.

• Baud rate is the number of bits transmitted per second.

For example, the baud rate is  4800, 9600, 19200, 57600, and 115200, etc.

• The clock signal is not present for synchronization, so the Tx/Rx sides must use the same baud rate for data transfer.
• Timing formula

1 bit transmission duration = 1 / Baud rate 

E.g., 9600 baud, then  104.167 μs required to transmit 1 bit, and 9600 bits transmitted per second.

UART is asynchronous(No clock is used), so both devices must use the same baud rate and data format for correct data transmission.

Frame Structure 

Frame Description

The RS-232  is used for long-distance serial communication based on UART between two controllers.

• Parity Bit: Checks if the number of 1’s in the data is even/odd.
  • Even Parity: The Total number of 1s in the data is even.
  • Odd Parity: The Total number of 1s is odd.
  • No Parity: No error-checking bit is sent.
• Framing Error: If the stop bit is not received correctly
• Overrun Error: If a buffer is full and new data arrives

As an important communication protocol, it is present in all controllers, i.e., AVR, PIC, and ARM.

Understanding of UART considering the AVR (ATmega328P)controller.

Generally following registers are used for UART communication.

Fosc = System clock Frequency, e.g., 16MHz in Arduino UNO

UART Data Transmission Driver

1. Set Baud Rate: Configure the baud rate for communication.
2. Configure Frame Format: Set data bits, parity, and stop bits.
3. Enable Transmission: Enable the UART transmission to start sending data.
4. Wait for the Data Register to Be Empty
5. Write Data to UART data buffer: Write the data byte to the UART data buffer for transmission.
6. Wait for Transmission to Complete.
7. Optional: Disable Transmitter.

UART Data Reception Driver

1. Enable Receiver: Enable the UART receiver to start receiving data.
2. Wait for Data to Be Received: Wait until data is available in the UART data register.
3. Read Data from UDR: Read the received data byte from the UART data register.
4. Check for Errors: Ensure no framing, parity, or overrun errors occurred during reception.
5. Process Received Data
6. Optional: Disable Receiver

Note:
1.The hardware serial library has a 64-byte buffer for data transmission/reception.
2.The serial library uses an interrupt to transmit data.

Send character 'A' from one Arduino UNO to another via UART; on receiving 'A', the second UNO toggles an LED.

Hardware Connection Diagram

Arduino UNO 1 Code

void setup() {
  Serial.begin(9600); // Initialize UART at 9600 baud rate
}
void loop() {
  Serial.write('A');  // Send a character
  delay(1000);        // Send after every 1 second
}

Arduino UNO 2 Code

void setup() {
  Serial.begin(9600); // Initialize UART at 9600 baud rate
  pinMode(LED_BUILTIN, OUTPUT);
}

void loop() {
  if (Serial.available() > 0) {
    char receivedChar = Serial.read(); //Read the received Char
    if('A' == receivedChar)
    // Toggle LED when data is received
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
   
  }
}