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

[Bruce Borer]

>From extended-basic-unravelled page A-11 address $FF98 bit 3 indicate the
refresh rate 0 for NTSC, 1 for PAL.
is this what you are looking for?

0769 FF98 BIT0 LPR0 LINES PER ROW (SEE TABLE BELOW)
0770           BIT1 LPR1 LINES PER ROW (SEE TABLE BELOW)
0771           BIT2 LPR2 LINES PER ROW (SEE TABLE BELOW)
0772           *BIT3 H50 1 = 50 Hz VERTICAL REFRESH*
0773           BIT4 MOCH 1 = MONOCHROME (ON COMPOSITE)
0774           BIT5 BPI 1 = BURST PHASE INVERTED
0775           BIT6
0776           BIT7 BP 0 = ALPHA, 1 = BIT PLANE

Bruce P. Borer
brucep.borer at gmail.com

On Thu, Mar 20, 2025 at 7:05 PM RETRO Innovations via Coco <
coco at maltedmedia.com> wrote:

> On 3/20/2025 4:20 PM, Allen Huffman via Coco wrote:
> >> On Mar 20, 2025, at 4:17 PM, Juan Castro via Coco <coco at maltedmedia.com>
> wrote:
> >>
> >> The timer interrupt hits 60 timer per second in CoCos made in the
> Americas
> >> and a couple of other places, and 50 Hz in Europe and elsewhere. This
> means
> >> if I want to count seconds with BASIC's TIMER function, I need to be
> aware
> >> which kind of CoCo I have, 50Hz or 60Hz. Is there a way to find out
> which
> >> frequency I have by software?
> > Considering this was one of the questions apps would ask (like Nick
> Marentes’ games), I think not :(
> >
> >> I just thought of a horrid solution: count how many times a loop ran
> >> between two interrupts, and apply some tolerance. Of course, I'd have to
> >> set CPU frequency to baseline 0.89 MHz beforehand. Get typical values
> and
> >> apply some tolerance. Do I really need to do that?
> > That might indeed be the way to go. It would only take a 1/50th of a
> second, if you could time it that fast ;-)
> >
> >               — A
> >
> >
> >
> My VDG is rusty, but can you IRQ per scanline?  If so, can't you count
> the number of lines in a frame, or do the lines outside the screen not
> register?
>
> Jim
>
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