The information in the wiki is based on these 2 posts:
1. viewtopic.php?p=116691#p116691
...falling edge of A12 instead of the rising edge, so it occurs 4 PPU cycles later compared to MMC3
My interpretation could be totally wrong, but this seems to indicate emulator experimentation relative to understanding of MMC3, rather than hardware evaluation of some kind....in my emulator and the game is working correctly now
2. viewtopic.php?p=242427#p242427
This is based on oscilloscope readings. In that post, prior to the oscilloscope test, the same theory was made about falling edge. However, the oscilloscope shows, from what I can tell, rising edge.Acclaim mapper counts every A12 pulse instead of waiting for long phase in low state. Counter clocking happens once per 8 A12 cycles at first cycle. Writing to $C001 resets pulse counter.
Now, the emulator I'm working on is perfectly cycle-edge accurate according to every test I can find, and this is what I've found:
1. Implementing the MC-ACC falling edge behaviour fails, in all cases
2. Implementing rising edge succeeds, IF
2a. 8-clocks-divider is reset immediately on writes to $C001
2b. IRQ counter is clocked on first divider clock at A12 (rising) edge as mentioned in the oscilloscope findings (though not shown in the posted images), not the 8th. Divider is reset on 8th A12 (rising) edge, of course.
This gives correct behaviour in Letterland, both on the goofy sequence getting into the truck, as well as the correct number of scanlines in the status bar during game play with no jumps at any time. It also matches the 2 glitchy far right hand side scanline at the top of the status bar as which is also seen in the recording from a front-loader here:
https://www.youtube.com/watch?v=qHQK1jpwY_I
This also correctly runs Crash Dummies and any other MC-ACC I've tried.
I suspect the wiki information/conclusion on falling edges is incorrect, and it also misses the details about $C001 and clocking on the first of 8 rather than the last from the divider.
A lot of emulators still get this wrong - usually with the scanline, or at least in Mesen's case I see the pre-level hop-in-the-truck sequence is faulty.
Do we have any actual evidence of a decapping or reading the signals aside from the oscilliscope tests in the link above to confirm? Can we/should we update the wiki?