Mark Newland wrote:

> And how do we get this counter to equal .250 seconds?  Have it overflow 3 times
> and you only have .196608 seconds.  Loop it 4 times and you have .262144
> seconds.  You would need count about 3.8147 times.

Yes, so whats the problem?
You CAN make the timer do a DIFFERENT last value for the 4th time thru!

> If you look for an exact
> timer count AFTER you have loop 3 times, you may get closer but still not get
> exactly .250 seconds every time.  Picking a frequency other than 4 MHz (such as
> 3.2768 MHz) will make it easier to loop an exact number of times to get an exact
> time of delay.
>
> Examples:
> 3.2768 MHz with prescaler of 32 = .010 sec interupts. 25 loops = .250 seconds
> 3.2768 MHz with prescaler of 16 = .005 sec interupts. 50 loops = .250 seconds
> 4.0960 MHz with prescaler of  4 = .001 sec interrupts. 250 loops = .250 seconds
>
> In fact, watch crystals use a frequency of 32.768 kHz for the reason of timing.
>
> Mark Bishop wrote:
>
> > Just have a counter in the interrupt routine that won't act until you get to
> > .250
> >
> > -----Original Message-----
> > From: David Dunn [mailto:ddunn@TCAINTERNET.COM]
> > Sent: Monday, June 11, 2001 10:56 AM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: timers
> >
> > 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 ?
> >
> > thanks,
> >
> > david dunn
> >
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  |  __O    Thomas C. Sefranek  tcs@cmcorp.com
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