PIC 16F872 oscillator at exactly 1000 hertz

• KILLspamphil at KILLspamadvancedarchitecture.org shares this code:

  
   
  ;Title 1khurts.asm
  ;a 1.00kHz square wave generator.
  ;program runs a xtal oscillator PIC16f872 to exactly 1000 hertz.
  ;comments: phil@advancedarchitecture.org
  
         include "P16f872.inc"  ; use definition file for 16F872
  
  ; configure the chip for a crystal oscillator, with code protection and watchdog timer off.
  
  __CONFIG _CP_OFF & _XT_OSC &_WDT_OFF
  
  ; define RAM =, give RAM registers names.
  
         CBLOCK 0x20   ; RAM starts at address 20h
  d1		
         ENDC
  ;
         org 0x0000      ; start address = 0000h
  
  ; INITIALISE PORTS
  ; binary used to see individual pin level
  
         movlw b'00000000'       ; all port pins = low
         movwf PORTA
         movlw b'00000000'
         movwf PORTB
         movlw b'00000000'
         movwf PORTC
  
         bsf STATUS,RP0  ; set RAM Page 1 for TRIS registers
  
  ; initialize ports
  ; set all pins to outputs, and analog pins to digital.
  
         movlw b'00000000'       ; all IO pins = outputs
         movwf TRISA
         movlw b'00000000'
         movwf TRISB
         movlw b'00000000'
         movwf TRISC
  
         movlw b'00000110'       ; all analog pins = digital
         movwf ADCON1
  
         bcf STATUS,RP0  ; back to RAM page 0
  
  ; LED FLASH LOOP
  
  Loop    bsf PORTC,4     ; RC4 = high
  	nop		; these two nop's make each half of cycle the same delay.
  	nop
          call Delay
  	
  	bcf PORTC,4	; RC4 = low
          call Delay
          goto Loop
  
  ; trial 1kHz delay subroutine
  ; delay is period/2
  ; 1kHz = 1000/s => period = .001 = 1ms
  ; 1ms/2 = 0.5 ms = .0005s
  ; 0.0005x1Mips = 500 instruction cycles  (clock is 4MHz).
  ; 500 - 5 (overhead of calling delay, and looping) = 495
  
  Delay	movlw	0xA4	;A4 = 164, see below, Delay is 2 instruction cycles so 495 - 2 = 493.
  	movwf	d1	
  Count	decfsz	d1	;493 required by Count, Count is 3 instructions each loop + 1 for skip, 492, w/o skip  492/3=164=A4
  	goto Count
  
  	return
  
  	END
  

  .