MC6821 Port

Lets explore the operation of the PIA chip on this board. From pld file we can see that the address range for this chip is allocated from 8800..8FFF. One test is to simply set PIA for output on one of the ports ... and then set some data to the port and see (by logic probe) that the output changes as required.  Following is a table of using the 6821 registers..  (6821 Datasheet)

So to set port A for output following steps are needed..

• • Clear out control register .. (8801=00)

  • Set all bits of DDR to "1's" ... (8800=FF)

  • Set CRA for PRA access (data register) .. (8801=04)

  • Write required data to output .. (8800=0F)  .. top 4 bits off, bottom 4 bits on.

This can be tested by using the monitor  ... and modifying the data as shown above ...and then looking at the pin out of the board for port A that 4 bits are high and 4 are low ...

As shown above after setting address 8800 to '0F" .. check pins  on the port output using a logic probe(or a voltmeter). Then setting the address to "F0" would reverse the state of those pins ... and so on ..