Effect: This command loads into the mega168’s SPDR (SPI data register) the number of unoccupied bytes in the UART send buffer. Sending a second byte (either the NULL command or a junk data byte) over the SPI once the value is loaded will transfer it to the mega644’s SPDR. It is recommended you do not begin transmission of this second byte until 3 us after the transmission of the command byte has completed. This is to make sure the mega168 has time to load the SPDR. If the buffer is empty this command will return 32. If the buffer is full this call will return 0.
Values sent: none
command byte = 222