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Double Speed Operation (U2X)
The transfer rate can be doubled by setting the U2X bit in UCSRA. Setting this bit only
has effect for the asynchronous operation. Set this bit to zero when using synchronous
operation.
Setting this bit will reduce the divisor of the baud rate divider from 16 to 8, effectively
doubling the transfer rate for asynchronous communication. Note however that the
receiver will in this case only use half the number of samples (reduced from 16 to 8 ) for
data sampling and clock recovery, and therefore a more accurate baud rate setting and
system clock are required when this mode is used. For the Transmitter, there are no
downsides.