If you designed this timer system, in 6 months, would you really be able to explain how it worked? :)
These were really hard to get because the design (and hence schematic) is a huge mess (I don't even know if the designers of the game knew what half of this crap did) and the timings came from a PROM. In other words, figuring this out by hand is really hard (I foolishly made an attempt). I ended up writing a little C program to figure it out for me which was a lot easier. The above graph is the results of my C program.
UPDATE: Warren got a capture on real hardware and it matches my emulated version perfectly! :)
A few observations:
- The main 6809E CPU runs at 1 MHz. This was what I assumed for a long time but now I've "proven" it for sure.
- The 4 MHz signal does not have a smooth rhythm. I had also assumed this since it gets derived from the 6 MZ clock. Had they derived it from the 12 MHz clock, they could've got the rhythm smooth. I assume that despite the unusual cadence that it still performs its function just fine (controlling the blitters).
- Plugging in the laserdisc video signal will throw these timers off temporarily. The game has a (pretty neat) mechanism to sync up to the laserdisc's video. Once in sync, I assume the timers will resume normal operation (not confirmed).
- Despite Q2 and CAS' being slightly different on the schematic, they appear to be identical in practice (at least as long as the laserdisc video input is not plugged in)
- Same thing with Q7 and 4 MHz. Q7 is connected to the 4 MHz flip-flop's clear pin, but this seems to have no effect (either that or I made a mistake).
- LATCH seems intended to begin after E and end before E.
- RAS' and CAS' are used to keep the video DRAM chips from losing their state. What a horrid thing to have to deal with from a designer's perspective :)