Re: ZX21 Project ZX81 Replacement PCB
Posted: Fri Jan 08, 2021 4:51 pm
OK, I have been working on getting the YMZ284 sound going today. Well after quite a bit of RTFM I found a few issues with my design. I only roughed it out on this first version as I was pretty sure it was not going to work as this was a new design.
Well, the first hint is that for IO decoding there is no point in looking at A0 - A2 as for every add-on it will always be 111. This is actually very cool as you only need to look at 5 address lines to fully decode the add-on addresses. It is not technically fully decoding but it will not matter. I will change that on the next issue. I can easily use the debug connectors to work around this issue. The next thing having checked is that although I connected A7 to the sound chip that won't work as it is the wrong way up. I can cut the track and use spare pins on the GAL16V8 to work around this without even adding a passive. I will have to add my first bodge wire of course.
Well, the first hint is that for IO decoding there is no point in looking at A0 - A2 as for every add-on it will always be 111. This is actually very cool as you only need to look at 5 address lines to fully decode the add-on addresses. It is not technically fully decoding but it will not matter. I will change that on the next issue. I can easily use the debug connectors to work around this issue. The next thing having checked is that although I connected A7 to the sound chip that won't work as it is the wrong way up. I can cut the track and use spare pins on the GAL16V8 to work around this without even adding a passive. I will have to add my first bodge wire of course.