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.  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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