UARTs stands for Universal Asynchronous Receiver/Transmitter. It is a device or module in an integrated circuit that converts data from a synchronous serial or parallel interface to an asynchronous serial interface for use in signaling standards like RS232. The synchronous bus is attached to a central processing units (CPUs) system, and the UARTs receives or transmits data to and from this local bus and buffers this inside the UARTs. The buffers contain the data that is either received from the asynchronous bus or to be transmitted onto the asynchronous bus. To facilitate data transfer, most UARTs have FIFO buffers on receive and transmit paths. Multiple UARTs in a single package are referred to as a DUARTs.

 

A UARTs is said to be universal because it is a very popular method to connect and is available in most digital systems, performing a variety of communications interface roles. It is asynchronous because it does not require a digital clock signal to be encoded or decoded. Data words are framed by start and stop bits of configurable characteristics. These are detected by the receiver to synchronize the bit stream and recover the data. Data words are sent as bits that are encoded as signal levels. The connected UARTs must be configured to be operating at the same data rate. This is referred to as the baud (Bd) rate and represents the overall bit rate of communication including the framing overheads.

 

A UARTs will have an internal clock generator that runs at a much higher frequency than the bit period. The input stream is sampled by this clock to determine the framing bits, and the 1 or 0 state of the transmission in the middle of each bit period. When a word has been sent or received, an interrupt signal is usually sent to the microprocessor to indicate this has occurred. Alternatively a software driver will periodically monitor the levels of data in the FIFOs and transfer data between the UARTs and CPU system based upon the FIFO buffer states. This is called polling.

 

UARTs can detect and signal overrun, underrun, parity and framing errors to the CPU. They can also detect breaks in communication where the data stream stops. UARTs are used for slower serial communication interfaces, like slow control data communication between subsystems or packet data to and from packet modems. Typical rates are 1200, 2400, 4800, 9600, 19.2k, 38.4k and 153.6k Baud, but other rates are also supported.