At 02:46 �� 1/5/2002, ZN wrote: ><snip> >It may be possible to use a bit of external logic to slow the 68008 down on >certain cycles and make it compatible again. I know Stuart H had to solve >that for the GC. >In fact, running the 68008 at double rate would probably make the logic >simpler, I suspect the 8301 generates it's cycles synchronously to the CPU >clock, based on the way the 68008 generates it's signals. In retrospect, it >was a wonder that an asynchronous clock for the 68008 worked at all... > >That being said: a board with a 68008 and some trivial logic to get the >correct addresses on an Aurora, would quite efficiently work as a QL with >256k or RAM - pretty much for any speed 68008. A 68008FN and extra RAM, >plus adjustment to apparent extended res screen address, would also give >the user all of the Aurora extended graphics capability. > >I have also constructed (ah, the good old days!) a 68020 add-on board. The >68020 is quite nice because it has an 8-bit bus mode. However, the timing >is different from 68008 (faster even at the same clock). I did a lot of >guesswork when doing that circuit and more logic would have been needed to >get it to work right. I wish I had the ability to do GALs at the time, it >would have made it all much simpler - as it was, I used a few 74xxx chips, >but that wasn't enough, so I could not get it to work right. The OS would >come up (at that point I had my own RAM expansion which shadowed the >internal 128k to make things faster so it did not rely on data returned >from the on-board RAM by the 8301) but the picture was garbled and >communication with the 8302 didn't work (no wonder since it's decoded by >the 8301) so there was no reaction to F1/F2. I whipped this up on a small >piece of protoboard during the week-end so it was no great loss when I >scrapped it. The whole thing was inspired by a piggyback board that was >used in the Thor20 and 21 to add a 68020 to a QL motherboard (the Thor 21 >had a 68881 FPU added too). I would probably make short work of it today >since now I have a logic analyzer and it would be very easy to see what the >8301 actually does - if I had a standard QL handy. One interesting thing >which I found out was that a 68020 used less current at 16MHz than a 68008 >at 7.5!
I volunteer mine :-) You interested? (I will be gone in two weeks and I have three QLs in Greece so I won't miss it "=_ >I also know that Branko B. had overclocked the whole thing - changed the >on-board slow RAM to fast 70ns, and clocked the 8301 and consequently the >68008 at a much higher rate by replacing the 15MHz crystal with a higher >frequency one. This increases the video synch frequency, he used what was >basically an EGA monitor. Worked just fine, too. > >Nasta That would be interesting to see... Phoebus
