Datasheet

Bit 0 – TXB8: Transmit Data Bit 8
TXB8 is the ninth data bit in the character to be transmitted when operating with serial frames with nine
data bits. Must be written before writing the low bits to UDRn.
This bit is reserved in Master SPI Mode (MSPIM).
Atmel ATmega644A [DATASHEET]
Atmel-42716C-ATmega644A_Datasheet_Complete-10/2016
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