I never got the original of this message for some reason.

> >16F877 chip @ 4 Mhz
> >
> >i'm considering using the timers in a project .. what i'd like is an
> >interrupt every .250 seconds
> >
> >i'm already using timer2 for PWM
> >i'd *like* to use timer1 for a counter
> >
> >somebody check my math for me please :
> >
> >timer0 runs at 1 Mhz (Fosc/4) ..
> >
> >most prescale you can get is 256 ..
> >
> >8 bit counter ..
> >
> >
> >working all this out, the *slowest* i can get timer0 to run (on my
> >calculator) is .065536 seconds
> >
> >is this right ?

No.  Timer 1 has a 3 bit prescaler (divide by 1, 2, 4, or 8), but is a 16
bit counter.  You want an interrupt every 250,000 instructions.  With a
prescaler of 4, you want an interrupt every 62,500 timer 1 increments, which
is within its 16 bit range.  You can use the second CCP to automatically
provide the period value, or you can interrupt on timer 1 overflow and then
adjust timer 1 in the interrupt routine to "skip over" the extra 65536 -
62500 = 3036 increments.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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