The Universal Synchronous and Asynchronous serial Receiver and Transmitter (USART) is a highly flexible serial communication device.

The USART can also be used in Master SPI mode. The Power Reduction USART bit in the Power Reduction Register (PRR.PRUSARTn) must be written to '0' in order to enable USARTn. USART 0 and 1 are in PRR0.