ZX81 16K RAM Pack

I tried adding WiFi to the firmware, but there seems to be an issue with the current implementation in the Pico C++ API. DMA and PIO are both used - the PIO usage has been mitigated by only using one PIO engine for the RAM pack. However, the use of DMA seems to stall the RAM pack usage and cause the ZX81 to crash due to bad timing.

From looking at the API, it may be necessary to change the code to push and pull data from the WiFi PIO in CPU code rather than using DMA. Performance is not the most important thing when dealing with small amounts of data when for example, loading a game to the RAM pack.

I've added to GitHub page the gerbers for a prototype PCB which looks like this:

N.B. This does not include the change for High Res. It's just a demo PCB, so caution is advised at present.

In response to a comment I am suggesting a modification to include the RFSH signal for reads that will allow high res games to be played. I think the following is what's needed:

          ------
/WR     o| 7400 |o      5V
/WR     o|      |o      /RD
|--     o|      |o      /RFSH
|--     o|      |o      --|
A14     o|      |o      --|
/RAM_WR o|      |o      A14
GND     o|      |o      /RAM_RD
          ------

 Replacing one of the /RD inputs with /RFSH signal should allow reads for either /RD or /RFSH which is needed by some high resolution implementations.