I've reached one (of many) important milestones this evening: the program counter is now counting! It's surprising how much work it takes just to arrange four 4-bit counters and extract the resulting 16-bit address. But it's working! I've verified that the counter is able to count successfully from 0x0000 to 0xFFFF (65,536 for you decimal readers) without any missed bits or anything.
I went ahead and wired it up so that I have a 2x10-pin header that matches with my logic analyzer's termination adapters. That way it's simple to bring the ADDR signal into the analyzer as a trigger.
This accomplishment is important for a couple of reasons, because it proves a number of ideas that I will be using going forward:
- The buffers on the clock/control board can drive 4+ chips on an external board without any clock jitter or slew.
- I can pull 5V and GND through the 10-pin ribbon cableon the clock/control board without causing voltage sags or any latent current on GND.
- Wire up the 7-segment displays and driver/decoder so that I have an address display that is always available.
- Wire up the ADDR LOAD pins and socket
- Wire up the EEPROMs and verify that 24-bit program data can be read successfully.
- Install two sets of buffers/line drivers for the program data