Clocks!

Clocks are giving me headaches atm. But first, the good news.

I took my Namco Classics Vol 1 PCB into work today and measured the clock to the YGV608. I didn't get the result I expected, but it actually makes more sense. The MAME source suggests the base clock to the YGV608 is the 49.152MHz oscillator and bases all its calculations off that, however it's actually the nearby 25.326MHz oscillator that the clock is derived from. That clock is halved to give a 12.663MHz clock signal; I measured 12.658MHz.

If you plug that clock into the YGV608 register settings that NCV1 programs at start-up, you get a 288 pixel active display area (with 32-pixel borders either side) and 224 lines with a 1-pixel border top and bottom. 224x288 is a very common arcade resolution. The horizontal frequency is ~15.7kHz, and the vertical ~60Hz. So no doubt the clock is correct.

Next step was to redo the clocks in the design and see what happens. It took quite a few iterations due to unrelated technical issues with the PLLs and a few required changes to the CRT timing logic I had programmed into the design, but eventually I had a display with the correct resolution!

Screen shot taken from MiSTer

Now the bad news. There are two (presumably) unrelated issues.

1. I'm getting what I can only assume is SDRAM bus lock-up issues. I thought it was related to PLL clocks but it seems to depend more on the build than what changes I make to the design. In this case the design loads but displays a series of stripes on the screen and the OSD doesn't respond.
2. Whilst the display itself - in terms of timing - is stable, the tiles exhibit severe ghosting and sparklies. These tiles are retrieved from SDRAM, so it may be related to the first issue, but I don't think so. It's also rock-solid in simulation, but there's no SDRAM in simulation and the video generation is done at a different level with no DDR3 frame buffer.

The 1st issue is the most annoying, as I have no idea whether a build is going to run or not. The 2nd I can deal with, as the pattern generator code is only a quick hack, and I need to look at it further under simulation.

It's not supposed to be this hard, especially when you're basing it on a working design...