I've been doing pretty well with my A/D, interrupts, timers, etc.  But one
thing that I did is very confusing to me.  I'm not sure if it is something
I'm not understanding (potentially including my test instruments too).  I'm
working with timer1 on a 16F877 at 20MHz.  I'm trying to create an interrupt
every millisecond...and I'm really really close, but not perfect....

Here's the important code fragments using CCS's compiler:

#include <16F877.H>
#device ICD=TRUE
#fuses HS,NOPROTECT,NOWDT

#define TIMERVAL -(20000000 / 4 / 1000)

int LED0;

#int_timer1 Timer1ISR()
{
    set_timer1(TIMERVAL);        // immediately reset timer
    output_bit(PIN_D0, LED0);
    LED0 = !LED0;
}


There is a lot of other code in the actual Timer1ISR routine but that
doesn't matter.

OK, so I ran all of this and put my scope on D0 and it measured 495Hz.
(Honest...I didn't even know until this morning that the scope could measure
that!).  I also have Fluke DMM with a frequency counter that also showed
495Hz so I feel pretty good that it really is 495Hz.

The question is, where is the 5Hz that I'm missing?

I did check the listing file and the compiler is putting 0xEC78
(decimal -5000) into the TMR1L/TMR1H counter.

Replacing:

#define TIMERVAL (46 - (20000000 / 4 / 1000))

...nails it at 500Hz perfectly.  But why the "46" offset?