Like a UART (Universal Asynchronous Receiver/Transmitter), a USART provides the computer with the interface necessary for communication with modems and other serial devices. However, unlike a UART, a USART offers the option of synchronous mode. In program-to-program communication, the synchronous mode requires that each end of an exchange respond in turn without initiating a new communication. Asynchronous operation means that a process operates independently of other processes.
Practical differences between synchronous mode (which is possible only with a USART) and asynchronous mode (which is possible with either a UART or a USART) can be outlined as follows:
- Synchronous mode requires both data and a clock. Asynchronous mode requires only data.
- In synchronous mode, the data is transmitted at a fixed rate. In asynchronous mode, the data does not have to be transmitted at a fixed rate.
- Synchronous data is normally transmitted in the form of blocks, while asynchronous data is normally transmitted one byte at a time.
- Synchronous mode allows for a higher DTR (data transfer rate) than asynchronous mode does, if all other factors are held constant.