The sync sheet | Ghosts'n Goblins

Draft. This sheet was read from the original 1985 drawing and cross-checked against MAME, but it has not yet been walked pin by pin against a physical board. The logic described here is what the drawing shows.

▸This sheet is part ofHow a frame is drawn

The subsystem followed end to end, across every sheet that builds it.

What this sheet is for

This is the metronome. A 12MHz crystal at XL is divided down to produce the pixel clock, the CPU clock, the sound clock, and the horizontal and vertical counters that define where the beam is at any instant.

Nothing here is game logic. Everything here is the reason the rest of the machine agrees with itself.

One crystal, every clock

The 74LS161 counters at 1K, 1L, 2K and 2L form the chains. Horizontal counts 384 dots, vertical counts 262 lines, and 6MHz divided by 384 by 262 is 59.64Hz.

That is precisely Commando's raster. The video timing was clearly considered solved and was carried over wholesale, even though almost everything else about the machine changed.

Everything in the machine is the same crystal divided down, which is why the sound chips stay in step with the picture. 384 dots by 262 lines at 6MHz gives 59.64 frames a second, the identical raster to Commando.

Sending the counters, not copying them

The individual counter bits — H1, H2, H4 up to H256, and V1 through V128 — are buffered by 1J and sent across the band cable to the video board.

It would have been possible to put a second counter chain on the video board and start them together. Capcom did not, and the reason is that two chains can drift or start a cycle apart, and a one-pixel disagreement between the boards would be visible forever. One counter, distributed, cannot disagree with itself.

Every chip on this sheet

Click any of them to open the schematic with that chip selected in both panels.

← open the schematic for this sheet