It's been a while so I thought I'd bring everyone up to date on how things are going. The first two boards being built by the BETA Test Team were failing to boot-up. After a lot of inspecting there didn't appear to be anything wrong with the PCB assembly. However there was one difference between my working XEL V1.0 board and the ones that the BETA Tester's had built, and that was the fact that they were using the original Candle U1MB's and mine had a Lotharek version. At first whatever differences between these two U1MB versions we could find didn't seem to be an issue. But thanks for AA member Van's due diligence the problem was discovered, and that came down to Candle's board needing A13 on the OSROM header and the Lotharek board picked this up solely from the MMU header. In the V1.1 production design A13 was intended to appear on both headers, but this wasn't so on the V1.0 prototype boards which I had borked by putting the wrong signal to it. Anyway suffice it to say with a quick trace cut and a jumper to put A13 on the OSROM header, everything started to work
Another unrelated problem that was discovered had to do with the 556 dual timer Reset/Power Control circuit. We had already modded this somewhat from what was originally done, but apparently my mods were only good for certain 'brands' of 556 chips (not good ). Essentially what we needed to see, was for the /RST line to be held low on initial power-up, so that the system would have a clean start-up when power had stabilized. So some alternative solutions were tried...
Photo by Van
Example of analog approach to a sustained reset on power-up (R46 and C51 form an RC time delay circuit, with DCHG from the Power-Control timer re-initializing it when powered OFF)...
And this did work quite well, and we also discovered at the same time that there was another possible good side effect. By lengthening the reset even more, thus allowing the PS2 keyboard to fully boot before the XEL booted, it was possible to have things like holding down the START, or OPTION, or HELP keys be recognized and acted upon when the XEL did boot-up. So just like on a real A8, Basic could be disabled, a cassette could be booted from, and the U1MB Setup Menu could be directly entered upon start-up. So we gave that a try and sure enough it worked! Then after thinking about it a bit more, it struck me that the built-in TK-II keyboard chip could handle this all by itself, thus eliminating the reset timer all together. So a revised TK-II firmware build was done and tested yesterday incorporating a sustained reset upon first time power-up, and I'm happy to report this test was successful. Now I just need to tweak around with how long reset really needs to be sustained to allow time for all the various PS2 keyboards to boot-up. At any rate, this TK-II sustained reset feature should also work on a normal A8 installation, and in most cases not require any hardware modifications (however the XEGS will be an exception).
There has been a lot of work behind the scenes by six very dedicated and busy individuals as they one by one get their XEL boards up and running. And a lot of good stuff has come out of this synergy that just wouldn't have been possible on my own, and I am extremely thankful to all of their efforts and ideas in this process.
By early next week I suspect that all six BETA boards will be up and running, and then the real testing will begin. Meanwhile I've been busy implementing changes into the schematics and PCB layout to keep up to date with the problems that have been discovered and fixed, as well as a incorporating a few additional features into the design (nothing crazy). My plan is to have a 2nd small run of boards made strictly for test purposes in another 2-3 weeks. This will hopefully be the final production design. If all goes well when testing that version (V1.1), the design files and gerbers will be released. So possibly we are looking at late June or early July for that to happen.