In modern embedded systems, communication between the microcontroller and external devices is essential. The AVR microcontrollers include a powerful and flexible hardware module called the USART (Universal Synchronous and Asynchronous Receiver and Transmitter), which enables both serial data transmission and reception. USART allows the microcontroller to communicate efficiently with other controllers, sensors, computers, and communication modules like GSM, GPS, or Bluetooth. It supports both synchronous mode, where clock and data are transmitted together, and asynchronous mode, where only data is transmitted without an external clock. The AVR USART is capable of full-duplex operation, meaning data can be sent and received simultaneously. Understanding its working principle and the registers involved is very important for programming efficient and reliable serial communication. In this post, we will discuss in detail the internal working of the USART module, its operation, and the main SFRs (Special Function Registers) used for configuring and controlling serial data transfer in AVR.
Overview of USART Operation
The USART module in AVR can operate in two modes — synchronous and asynchronous. In asynchronous mode, data is transferred without a clock signal. Each data frame consists of a start bit, data bits, an optional parity bit, and one or two stop bits. In synchronous mode, a separate clock signal is used for data synchronization between transmitter and receiver. The USART consists of two main sections — the transmitter and the receiver. The transmitter converts parallel data from the CPU into serial data and sends it out through the TXD pin. The receiver, on the other hand, accepts serial data through the RXD pin and converts it back into parallel data for the CPU. These two processes can happen simultaneously, making the communication full-duplex.
The diagram above shows the basic flow of data in the USART. The CPU sends data to the transmit shift register, which shifts the bits serially to the TXD pin. Simultaneously, the receive shift register accepts serial bits from RXD and assembles them into a complete data byte, which is then made available to the CPU.
Key SFRs Used in USART
The AVR USART is controlled using a few important Special Function Registers. These registers are used to configure the baud rate, select data format, enable transmitter or receiver, and check transmission status. The main USART registers include:
| Register Name | Function |
|---|---|
| UBRRL and UBRRH | These two registers together form the 16-bit baud rate register. The baud rate determines how fast the serial data is transmitted or received. |
| UCSRA (USART Control and Status Register A) | Contains status flags such as transmit complete, data register empty, and receive complete. It also indicates errors like frame or parity errors. |
| UCSRB (USART Control and Status Register B) | Used to enable or disable transmitter and receiver, and control data bits size. It also includes interrupt enable bits for TX and RX operations. |
| UCSRC (USART Control and Status Register C) | Controls the data frame format — number of data bits, parity mode, stop bits, and operation mode (synchronous/asynchronous). |
| UDR (USART Data Register) | This register holds the data to be transmitted. For receiving, it contains the data received from the RX line. |
Baud Rate Generation
Baud rate defines the communication speed between devices. It is derived from the system clock and the value written in the UBRR registers. The baud rate formula for asynchronous mode is:
For example, if FCPU = 16 MHz and UBRR = 103, the baud rate will be approximately 9600 bps. The same registers also control double-speed operation when the U2X bit in UCSRA is set.
Conclusion
The USART module in AVR microcontrollers is a highly versatile communication interface. By configuring the correct baud rate and control registers, it enables smooth and reliable serial data exchange with external devices. Understanding its SFRs is crucial for programming efficient data transmission and reception routines in embedded projects. Whether you are interfacing a PC terminal, Bluetooth module, or another microcontroller, mastering the USART operation is one of the key steps in embedded system programming.
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