Sunday, March 21, 2010

Counter is Counting...

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:

  1. The buffers on the clock/control board can drive 4+ chips on an external board without any clock jitter or slew.
  2. 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.
I still have a lot of work to do on the program counter, though:
  1. Wire up the 7-segment displays and driver/decoder so that I have an address display that is always available.
  2. Wire up the ADDR LOAD pins and socket
  3. Wire up the EEPROMs and verify that 24-bit program data can be read successfully.
  4. Install two sets of buffers/line drivers for the program data
Frankly I'm not sure if I'm going to have enough real estate on the board for all this. But we'll see...stay tuned!

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