25.3.3.2.4 Double-Speed Operation

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 bits in the Control B (USARTn.CTRLB) register to 0x01.

When enabled, the baud rate for a given asynchronous baud rate setting will be doubled. This is shown in the equations in 25.3.2.2.1 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 25.3.3.2.3 Error Tolerance for more details.