Moar Video Memory

Brad and uBee on MSPP ganged up on me. The Microbee Premium is capable of running a full 32K of PCG RAM, if you unsolder the two 0.6" wide 6264s it came with and substitute 4 0.3" wide ones. They reckoned that FreeBee Fremium should be able to do that too. uBee even has code that displays pictures that totally needs it.

I tried to fob them off, saying there wasn't room on the board for the decode logic for the extra address line. And besides, bigger RAMs are all 0.6" wide. So Brad totally shamed me by designing a daughter board that does it, using a single 74AHC00 for decode and a 0.3" wide 64K x 8 cache RAM. Okay, I can take a hint! We need to grow the video RAM chip to 32 pins, which is straightforward, but the hard bit is we need to squeeze in an extra chip (a 74AHC00) to decode the A15 line for the RAM.

So the key to squeezing this in is realising that inner layer pads and outer layer pads don't need to be the same size and shape. Easy solderability favours generous elliptical pads on the outer layers, but the inner layer pads can be minimum size and round. TBy using a 50 thou inner layer pad, and 10 thou tracks and spaces, we can squeeze two inner-layer traces between pins. This resulted in a week or so of advanced maze game, adding in the extra chip for decode and doing a huge shuffle to get it to route. I didn't stop there, I kept going to reduce the length of many lines, and get stuff out from under the keyboard. This way, if I want to cut the keyboard off, I can.

Importantly, this will still work with a 28 pin 32K RAM. Just insert it so the ground pin is in the right place and it'll be happy, just like a stock Premium!

After a little more finessing, I've got a board that I can chop in half to seperate the keyboard. Now if the keyboard is done in two layers (still four for the main computer), then it gets quite a bit cheaper to make, and as an added bonus you can position the keyboard however you like.