Effect: This command loads into the mega168’s SPDR (SPI data register) the number of occupied bytes in the UART read 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 full this call will return 32. If the buffer is empty this call will return 0.
Values sent: none
command byte = 223