Double-Speed Operation

The double-speed operation allows for higher baud rates under asynchronous operation with lower peripheral clock frequencies. This operation mode is enabled by writing the RXMODE bit field in the Control B (USARTn.CTRLB) register to 0x01.

When enabled, the baud rate for a given asynchronous baud rate setting will be doubled, as shown in the equations in The Fractional Baud Rate Generator. In this mode, the receiver will use half the number of samples (reduced from 16 to 8) for data sampling and clock recovery. This requires a more accurate baud rate setting and peripheral clock. See Error Tolerance for more details.