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 in the Control B register (USARTn.CTRLB) to CLK2X.
When enabled, the baud rate for a given asynchronous baud rate setting shown in Table 1 will be doubled. 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 Asynchronous Data Reception for more details.