25.3.3.2.4 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.