UART vs USART: Key Differences in Embedded C Programming

In embedded C, UART is a simple serial communication protocol that handles asynchronous data transfer, while USART supports both asynchronous and synchronous communication modes. USART is more versatile but slightly more complex to configure than UART.

Quick Comparison

Here is a quick side-by-side comparison of UART and USART features.

Feature	UART	USART
Communication Type	Asynchronous only	Asynchronous and Synchronous
Clock Signal	No clock needed	Clock signal required in synchronous mode
Complexity	Simpler to configure	More complex due to modes
Speed	Limited by baud rate	Can be faster with synchronous mode
Hardware Support	Basic serial ports	Advanced serial ports with clock control
Use Case	Simple serial data transfer	Flexible serial communication

Key Differences

UART (Universal Asynchronous Receiver/Transmitter) is designed for asynchronous serial communication. It sends data without a clock signal, relying on agreed baud rates between devices. This makes it simpler but limited to asynchronous mode only.

USART (Universal Synchronous/Asynchronous Receiver/Transmitter) supports both asynchronous and synchronous communication. In synchronous mode, it uses a clock signal to synchronize data transfer, allowing faster and more reliable communication over short distances.

Because USART can operate in two modes, it requires more configuration in embedded C, such as setting clock polarity and phase for synchronous mode. UART is easier to use when only asynchronous communication is needed.

Code Comparison

Example of initializing and sending a byte using UART in embedded C.

embedded_c

#include <avr/io.h>

void UART_init(unsigned int baud) {
    unsigned int ubrr = F_CPU/16/baud - 1;
    UBRR0H = (unsigned char)(ubrr >> 8);
    UBRR0L = (unsigned char)ubrr;
    UCSR0B = (1 << TXEN0); // Enable transmitter
    UCSR0C = (1 << UCSZ01) | (1 << UCSZ00); // 8-bit data
}

void UART_sendByte(unsigned char data) {
    while (!(UCSR0A & (1 << UDRE0))) {}
    UDR0 = data;
}

int main() {
    UART_init(9600);
    UART_sendByte('A');
    while(1) {}
    return 0;
}

Output

Sends character 'A' over UART serial line

USART Equivalent

Example of initializing and sending a byte using USART in asynchronous mode in embedded C.

embedded_c

#include <avr/io.h>

void USART_init(unsigned int baud) {
    unsigned int ubrr = F_CPU/16/baud - 1;
    UBRR0H = (unsigned char)(ubrr >> 8);
    UBRR0L = (unsigned char)ubrr;
    UCSR0B = (1 << TXEN0); // Enable transmitter
    UCSR0C = (1 << UCSZ01) | (1 << UCSZ00); // 8-bit data, async mode
}

void USART_sendByte(unsigned char data) {
    while (!(UCSR0A & (1 << UDRE0))) {}
    UDR0 = data;
}

int main() {
    USART_init(9600);
    USART_sendByte('A');
    while(1) {}
    return 0;
}

Output

Sends character 'A' over USART serial line in async mode

When to Use Which

Choose UART when you need simple, asynchronous serial communication with minimal configuration, such as basic sensor data or debug messages. It is ideal for low-speed, straightforward tasks.

Choose USART when you require flexibility to use synchronous communication for higher speed or more reliable data transfer, or when your hardware supports both modes and you want to optimize performance.

Key Takeaways

UART supports only asynchronous communication, making it simpler but less flexible.

USART supports both asynchronous and synchronous modes, offering more options and speed.

Use UART for basic serial tasks and USART when synchronous communication or higher speed is needed.

Both require setting baud rate and enabling transmitter in embedded C.

Configuration complexity increases with USART due to clock and mode settings.