Pic Dude <PICLIST@mitvma.mit.edu> wrote:

> 1.544 MHz external oscillator .... (which I connected to
> OSC1/CLKIN).  OSC2/CLKOUT is now open.
>
> Pre-scaler is now 1:8 and ISR routine divides a few times:  first by
> 193, then by 25, then by 10, to give me 1-sec pulses.  (There's a
> reason for those divisors in case you're wondering -- I can get 0.1
> sec pulses I need for something else).
>
> However, it's running MUCH slower than that -- taking a few minutes to
> generate a 1-sec pulse!

Neil:

Lemme guess... It's taking exactly 4 minutes and 16 seconds per "1-
second" pulse, right?

I think you're misunderstanding how the TMR0 interrupt works.  The
TMR0 register is incremented by the prescaler, but the TMR0 interrupt
doesn't fire until the TMR0 register overflows from 255 to 0... So
your interrupts are actually happening at 1/256 the rate that you
thought.

Does that explain the behavior you're seeing?

-Andy

=== Andrew Warren -- aiw@cypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
===
=== Opinions expressed above do not
=== necessarily represent those of
=== Cypress Semiconductor Corporation

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