Sunday, August 26, 2012

Scratch Pad Array board

The Instruction Pointer board is now fully routed, with only some sanity checks and the addition of some power plane bypass capacitors on the back remaining. If I find time for it this evening I could send it out for fabrication tonight.

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.
The 12x4 shape of the IP array and the amount of decode logic forced the decode logic to be spread horizontally under the array rather than to the side. This isn't a problem with the SP array which, being square, could be oriented either way. To get a visual aid to layout, I started by taking the standard board layout and drawing in an outline of the 8x8 array on the Tplace layer, which ends up as the top silkscreen. Then I added outlines for the column precharge, the read sense inverters, the 2:1 data muxes, and the row select drivers. Finally I sketched in the 3-bit refresh counter. Here's what it ended up looking like:

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...

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