[Coco] Timer interrupt frequency (50Hz vs 60 Hz)

[David Broman]

If you'd like to go the route of counting hsyncs, here's some sample code you can start from. It counts scan lines so it can change the display mode at a known point in the middle of the frame. It could probably be easily adapted to simply count total scan lines between vsyncs.

https://cocotownretro.wordpress.com/2023/12/25/impossibly-colorful/

-----Original Message-----
From: Coco <coco-bounces at maltedmedia.com> On Behalf Of Alex Evans via Coco
Sent: Thursday, March 20, 2025 6:19 PM
To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Cc: Alex Evans <varmfskii at gmail.com>
Subject: Re: [Coco] Timer interrupt frequency (50Hz vs 60 Hz)

You could use something like the sync instruction with interrupts masked.

On Thu, Mar 20, 2025, 21:17 RETRO Innovations via Coco <coco at maltedmedia.com>
wrote:

> On 3/20/2025 7:51 PM, Alex Evans via Coco wrote:
> > The trouble I see is that both hsync and hsync are IRQ though you 
> > can
> check
> > the pia to see which one fired.
> Apologies, I thought one was IRQ and the other FIRQ.  But yes, you 
> could check PIA.
> > There is also the issue that servicing an IRQ is not really fast 
> > because all of the registers are preserved.
>
> Well, my thought is that this code would only run once for at most 
> 1/25 of a second (worst case situation is that fsync irq is enabled 
> immediately following an FSYNC IRQ, so it would run one frame without 
> counting, and then the next it would count.  But, the below code can 
> be optimized if needed.
>
> My main is whether it would give different values on NTSC versus PAL.  
> I assume so, since there would be 262 or 263 hsyncs in NTSC, but 
> 312/313 in PAL.
>
> >
> > On Thu, Mar 20, 2025, 20:33 RETRO Innovations via Coco <
> coco at maltedmedia.com>
> > wrote:
> >
> >> On 3/20/2025 7:07 PM, Alex Evans via Coco wrote:
> >>> Yes, you can interrupt (IRQ) on hsync on any coco.
> >> So if you write:
> >>
> >> prep:
> >>
> >> //put 0 in NUM
> >>
> >> // put 0 in START
> >>
> >> //enable fsync
> >>
> >> // wait for START to equal 2
> >>
> >> // use NUM value to calc PAL or NTSC
> >>
> >>
> >> fsync_IRQ
> >>
> >> // increment START
> >>
> >> // if START = 1
> >>
> >> // zero out NUM, enable hsync, and return
> >>
> >> // else disable fsync and hsync and return
> >>
> >> hsync_IRQ:
> >>
> >> // increment NUM
> >>
> >> // return
> >>
> >> Would the above give you different numbers in NUM you could use to 
> >> determine PAL/NTSC?
> >>
> >> Jim
> >>
> >> --
> >> RETRO Innovations, Contemporary Gear for Classic Systems 
> >> www.go4retro.com store.go4retro.com
> >>
> >>
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