Thanks for your comment, I have ditched the timer, I ran out of space, I
am trying to build the board single sided with through hole components.
Also after some thinking about it the fixed pulses aren't what I need
any way...

I have gone through your math and I think I understand it. After reading
the data sheet, it would appear that when I start the timer, I need to
write '2' into the timer as it won't increment for 2 cycles after being
set.

Is there anything bad going to happen when I switch interrupts on and
off, and switch the edge for my interrupt/RB0 (I'd like to determine
when the trigger is released) I know there are some things that can
cause a device reset...

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jinx
> Sent: Saturday, January 17, 2004 8:13 PM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Help with first time circuit
>
>
> > The 555 is for giving 60Hz without any complicated divisors. I had
> > thought of doing it in software, but I chose to go the other way.
>
> To get an "exact but average" 60Hz with a 20MHz isn't too
> difficult but there are easier ways
>
> 60Hz = 83333(recurring)us period = (83333+83333+83334)/3,
> which could be done using a cycle of 3 timing loops
>
> However, if you change to a hex value crystal something close
> to your 20MHz (18.432MHz or 19.6608MHz) it becomes quite simple
>
> (18432000/4) / 256 = 18000 TMR0 IRQs / sec = 150 per 1/2 cycle
>
> Similarly 19.6608 -> 160 IRQs / 1/2 cycle
>
> Allow TMR0 to free-run and have an interrupt to bump a RAM counter
>
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