SPI CS Decoder - Block AA


        There are several SPI serial devices used in this design plus the expansion port (Block Z).  The MPC555 has a limited number of I/O lines that can be dedicated to a SPI Chip Select function.  So as we expanded our SPI  devices we needed a means to also expand the CS capabilities.  The 74HC154 4 to 16 line decoder fit the bill.  Because of the state the I/O lines of the MBC555 come up and way we needed to qualify an output we loose two of the outputs.  No great loss as there are plenty of spares.  It also seems like the SPI interface has no standard convention on the on what edge will be used to clock data into a device.  You will see that we grouped the rising edge and falling edge devices together.
    Spare output pins are brought to proto pads that can be wired to spare pins on the rear DB-37 connector if needed (Block ZZ).