Before I do that, however, I thought I'd start the Scratch Pad Array board and see if that shows any problems with my inter-board connector pinout. The Scratch Pad is very similar to the Instruction Pointer board, with a few exceptions:
- The array is 8x8 (64 bits) instead of 12x4 (48 bits).
- There is one 3-bit counter rather than two 2-bit counters.
- There is less decode logic.
Originally I'd thought the array would fit better rotated 90 degrees clockwise, but looking at it this way I've pretty much decided to orient it as shown above.
Surprisingly, this accounts for 64% of the components allocated to this board. I'm not sure yet where the counter will end up, but if need be it'll squeeze into the space below the array by rearranging the stages horizontally rather than vertically. Based on how the layout of the IP board went I doubt I'll have too much trouble fitting in the remaining 36%.
Now I'm debating whether to hold off on ordering the IP board until I finish the SP board, and submitting them both as one order. Depending on whether PCB-Pool will let me submit two boards a one order under their 20% off the first order with their sooper-dooper Eagle ordering button, this could save me $75 in fab costs and $25 in shipping. On the other hand, if I've made a serious design or layout error, I'll end up with two useless boards instead of one. Decisions, decisions...