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

[William Astle]

FF98 is write only. You cannot read it to find out which mode is set.

On March 20, 2025 5:52:48 p.m. MDT, Juan Castro via Coco <coco at maltedmedia.com> wrote:
>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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>> >
>> >
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