Date   : Mon, 11 Mar 2002 09:25:23 +0000
From   : Richard_Talbot-Watkins@...
Subject: Re: A couple of CRTC/Timing problems

Richard Gellman <r.gellman@...> wrote:

> Firstly, I've rennovated my vertical alignment code, so the screen
> starts at a point that can no longer change halfway down the screen
> (previously you could reprogram the CRTC in mid-frame, and this would
> cause the screen start point to move, causing judder), and this has
> almost resolved the problem but it still jiggles by 1 pixel scanline,
> which may have something to do with:

Have you tried running the little demo program I posted a little while
back?  When I tried it on BeebEm, it had real problems although you
could see that it was kind of doing the right thing.  It would probably
make fine-tuning your code a lot easier as it provides a known static
display which you can keep adjusting timings and so on until you get
the right results.  Maybe try it on a real Beeb if possible too so you
know what to expect.

> Secondly, I can't seem to get rid of that flipping "tidemark", you
> know that one scanline at the point where mode/pallette changes occur
> that always flickers... I've tried adjusting VIA timings, VSync
> delays.. nothing seems to be making it go away...

My first guess would be that old friend R3 again... the monitor vsync
should not be emulated until R3(high 4 bits) scanlines after the
vertical row counter matches R7.  If you've tried this though, then
maybe it's to do with Firetrack not having a screen with exactly 312
scanlines, but your monitor emulation assuming that it does?  Does
Revs work with a rock-steady colour/mode scanline interrupt? - I seem
to remember it being notoriously fussy with its timing, so if it works
ok, it's probably safe to assume that your VIA timing is pretty much
perfect.

Good luck!
Rich




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