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Introduction to USART

1. USART Definition

USART (Universal Synchronous/Asynchronous Receiver/Transmitter) is a communication protocol used to transfer data between electronic devices, such as microcontrollers, sensors, and other components. This protocol is highly flexible as it supports two main modes:

  • Synchronous
  • Asynchronous

2. Operating Modes

A. Asynchronous Mode

In this mode, the USART module transmits data without an external clock signal. Synchronization is achieved using data frames consisting of:

  • Start Bit: Indicates the beginning of transmission.
  • Data Bits: Contains the primary information (typically 5-9 bits).
  • Parity Bit (Optional): Used for error detection (Even, Odd, or None).
  • Stop Bit: Indicates the end of transmission.

Characteristics: Suitable for long-distance communication or between devices that do not share the same clock.

B. Synchronous Mode

Uses a clock signal to synchronize data transfer between the transmitter and the receiver.

  • Requires the same clock configuration on both devices.
  • Allows for faster and more reliable data transfer compared to asynchronous mode.

Characteristics: Ideal for multimedia applications or high-speed bulk data transfers.

3. Configuration and Baud Rate

To use USART, several parameters must be defined:

  1. Baud Rate: The speed of data transmission (bits per second).
  2. Data Format: The number of data bits, parity, and stop bits.
  3. Interrupt: Enables notifications when data is finished being sent or received.

Baud Rate Calculation Formula (UBRR)

The UBRR (USART Baud Rate Register) value is calculated based on the CPU clock frequency (F_CPU) and the desired Baud Rate:

UBRR = (F_CPU / (16 * BAUD)) - 1

Calculation Example:
If F_CPU = 16 MHz and the target BAUD = 9600 bps:

  1. UBRR = (16,000,000 / (16 * 9600)) - 1
  2. UBRR = 104.16 - 1
  3. UBRR ≈ 103 (Hex: 0x67)
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