Setting up Timer Module

[Bruce]

Here are a few ways to do this without using a slower osc.

For a PIC12F1822 using a compare interrupt.

@  __config _CONFIG1, _FOSC_INTOSC & _MCLRE_OFF & _CLKOUTEN_OFF
@  __config _CONFIG2, _PLLEN_OFF & _LVP_OFF
 
DEFINE INTHAND HzOut
 
LED VAR PORTA.0
 
TRISA = 0
ANSELA = 0           ' All digital
 
CCP1CON = %00001011  ' Compare mode with auto reset of Timer1
CCPR1H = $06         ' Load for match every 1,538uS
CCPR1L = $02         ' 1,538uS x 2 = 3.076mS. 1/3.076mS = ~325.09 Hz
OSCCON = %01101000   ' 4MHz internal osc
T1CON = %00000001    ' Enable Timer1, 1:1 rescale
PIR1.2 = 0           ' Clear int flag before enabling interrupt
PIE1 = %00000100     ' Enable CCP1 interrupt
INTCON = %11000000   ' Global & peripheral ints enabled
 
Main:
  TOGGLE LED         ; Do whatever here. You have ~1,538 instruction
  PAUSE 1000         ; cycles before each interrupt
  GOTO Main
 
ASM
HzOut                ; Eats-up 5 instruction cycles
  MOVLW B'00000100'  ; Load WREG with %00000100 for XOR op below
  XORWF PORTA,F      ; Toggle PORTA,2 on each 1/2 cycle
  BCF   PIR1,CCP1IF  ; Clear CCP1 interrupt flag
  RETFIE             ; Go get some more
ENDASM
 
  END

And one for the 12F683 using Timer2/PR2 match interrupt;

@ __config _INTRC_OSC_NOCLKOUT & _WDT_OFF & _MCLRE_OFF & _CP_OFF
 
DEFINE INTHAND HzOut
 
wsave VAR BYTE $70   SYSTEM
ssave VAR BYTE BANK0 SYSTEM
psave VAR BYTE BANK0 SYSTEM
 
LED VAR GPIO.0
 
TRISIO = 0
ANSEL = 0            ' All digital
CMCON0 = 7
OSCCON = %01100000   ' 4MHz internal osc
T2CON = %00111100    ' Enable Timer2, 1:1 prescale / 1:8 postscale
PIR1.1 = 0           ' Clear TMR2 int flag before enabling interrupt
PIE1 = %00000010     ' Enable TMR2 interrupt
INTCON = %11000000   ' Global & peripheral ints enabled
PR2 = 191
 
Main:
  TOGGLE LED         ; Do whatever here.
  PAUSE 1000         ; 
  GOTO Main
 
ASM
HzOut                
  movwf wsave        ; Save W
  swapf STATUS,W     ; Swap STATUS to W (swap avoids changing STATUS)
  clrf  STATUS       ; Clear STATUS
  movwf ssave        ; Save swapped STATUS
  movf  PCLATH,W     ; Move PCLATH to W
  movwf psave        ; Save PCLATH
 
  MOVLW B'00000100'  ; Load WREG with %00000100 for XOR op below
  XORWF GPIO,F       ; Toggle GPIO,2 on each 1/2 cycle
 
  BCF   PIR1,TMR2IF  ; Clear TMR2 interrupt flag
 
  movf  psave,W      ; Retrieve PCLATH value
  movwf PCLATH       ; Restore it to PCLATH
  swapf ssave,W      ; Retrieve the swapped STATUS value (swap to avoid changing STATUS)
  movwf STATUS       ; Restore it to STATUS
  swapf wsave,F      ; Swap the stored W value
  swapf wsave,W      ; Restore it to W (swap to avoid changing STATUS)
 
  retfie             ; Go get some more
ENDASM
 
  END

Interrupts are a LOT faster & easier with newer PIC types with auto context save/restore.