16.2.2 Frame/Command Marker

The COMM_BLK allows the data that is being transferred to be marked as a command or data byte. It is expected that a software protocol transfers packets of data between the COMM_BLK blocks. To allow the receiver to correctly identify the start of a packet, the COMM_BLK block uses a 9th bit (Bit 8 of DATA_IN and DATA_OUT as shown in Figure 16-2).

When Table 16-8/Table 16-9 register is written, the 9th bit is set. When Table 16-6/Table 16-7 register is written, the 9th bit is not set.

The Table 16-4 register bit 7 gives indication to the receiver whether the next byte that will be read out of the FIFO has the 9th bit set, and therefore indicating that it is the start of a packet.

This mechanism allows the receiver to verify that no bytes have been lost and stops it from accidentally interpreting data overruns as command. The RCVOKAY and TXTOKAY status bits must be checked in the Table 16-4 register before reading and writing data or command.