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

[Juan Castro]

I've printed PEEK(&HFF98) AND 8 in both PAL and NTSC and the bit seems to
be set in both.

Juan Castro
Enviado do meu Olivetti Programma 101
http://retropolis.com.br


Em qui., 20 de mar. de 2025 às 20:41, Bruce Borer via Coco <
coco at maltedmedia.com> escreveu:

> 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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