Hi,
I have just started working with PIC's.
I wanted to get a 16F877 to generate a square wave on PORTD by toggling it
periodically.
I decided to use timer 0 to generate an interrupt periodically.
At the end of the message I list the short program.

The problem:
When I run the program I do get a square wave ( oscilloscope ) but for some
reason it isn't perfect.
Every few interrupts I get some kind of a glitch which slightly reduces the
pulse width
as if the timer decided to interrupt the processor earlier.

Things I checked:
Made sure that WDT is off.
I get a perfect square wave with a plain loop. So I assume my crystal is
stable.
Run out of ideas....

I would appreciate it if somebody could help me out.
Thanks

Ioannis--


 list p=16f877

 ; Include file, change directory if needed
 include "p16f877.inc"

intcon equ 0x0b
 ; Start at the reset vector
 org 0x000
 nop
 goto Start

 org 0x004   ;interupt service rutine
 goto isr

Start
 banksel PORTD
 clrf PORTD  ;Clear PORTD

 banksel OPTION_REG
 movlw B'10001000' ;prescaler assigned to WDT
 movwf OPTION_REG
 clrf TRISD  ;PORTD all outputs

 banksel TMR0
 movlw 0
 movwf TMR0

 banksel intcon
 bcf  intcon,2
 bsf  intcon,7
 bsf  intcon,5     ;gloabal interupt enabled

circle    goto circle

isr         bcf intcon,2 ;clear TMR0 interupt flag
             comf PORTD,f
             RETFIE


 end