Re: [hatari-devel] video freq set to 60 Hz instead of 50 Hz for Falcon VGA

[Thomas Huth <th.huth@xxxxxxxxx>]

Am Wed, 28 Feb 2024 09:17:42 +0100
schrieb Nicolas Pomarède <npomarede@xxxxxxxxxxxx>:

> Le 28/02/2024 à 09:07, Miro Kropáček a écrit :
> > On Wed, 28 Feb 2024 at 09:03, Nicolas Pomarède <npomarede@xxxxxxxxxxxx 
> > <mailto:npomarede@xxxxxxxxxxxx>> wrote:
> > 
> >     but then, out of curiosity, why do TOS and Emutos set ff820a when
> >     configuring videl video mode ?
> > 
> >     Or maybe ff820a is taken into account in "st compatible mode" when
> >     writing the resolution to ff8260 ?
> > 
> > Frankly, I don't know. I wasn't even aware of 820a until this thread. 
> > :-) My guess would be they wanted to keep it for backward compatibility, 
> > perhaps mainly for the ST modes as you have said.  
> 
> If you look at the thread "Sync byte on Falcon" here (in sept and nov 
> 2023), Roger made some tests on his falcon and the content of ff820a is 
> modified depending on the video mode.
> 
> so indeed, it's not very clear what it's used for :(

I don't have my Falcon anywhere a usable state (and I'm even not sure
whether I still have a suitable VGA monitor somewhere in the basement), but
my personal guess is that ff820a is completely without function on the
Falcon, it's just likely that they implemented a flip-flop here so that it
keeps the value that has been written to it. That way TOS could set the bit
anyway to provide information for programs that only read the register to
know whether they are running in 60 Hz or 50 Hz. But that's just my guess -
if someone has a Falcon up and running with a monitor that can handle both
refresh frequencies, it would be cool to know whether writing to that
register makes any difference at all with the screen refresh rate.

 Thomas