Clearing interrupt flag

[tubos]

I understand when program flow is inside an interrupt routine
it is necessary to clear the appropriate interrupt flag.

Now i have 2 questions about this:
-Is it important if you do this in the beginning or the end of ISR ?
-Could someone explain what happens if you dont clear it?

Heres an example that clears the flag in the end.

     INT_SERV:
     INCF COUNTER, F

     BTFSS COUNTER, 0    ; light the appropriate LEDs
     BCF PORTB, 4
     BTFSC COUNTER, 0
     BSF PORTB, 4
     BTFSS COUNTER, 1
     BCF PORTB, 5
     BTFSC COUNTER, 1
     BSF PORTB, 5
     BTFSS COUNTER, 2
     ...... other stuff     
     BCF INTCON, INTF    ; clear the appropriate flag
     RETFIE              ; this also set global interrupt enable

Here's another one ex (microchip) that does it in the beginning

RTCisr 
  BANKSEL TMR1H
  BSF TMR1H, 7 ; Preload for 1 sec overflow
  BCF PIR1, TMR1IF ; Clear interrupt flag
  INCF secs, F ; Increment seconds
  MOVF secs, w
  SUBLW .60
  BTFSS STATUS, Z ; 60 seconds elapsed?
  RETURN ; No, done
  CLRF seconds ; Clear seconds
  INCF mins, f ; Increment minutes
  MOVF mins, w
  ... etc
  RETURN ; Done

 

[Mosaic]

By clearing the Int. flag you 're-enable' the interrupt vector at 0x0004( 16F). If u don't clear the flag the interrupt won't redirect your code execution after the first time.


If there's a chance that an interrupt can happen while still in the ISR...clear the flag at the end of the ISR. Otherwise it's your choice.

 

[Mike - K8LH]

By clearing the Int. flag you 're-enable' the interrupt vector at 0x0004( 16F). If u don't clear the flag the interrupt won't redirect your code execution after the first time.

I'm not sure that's worded very well. May I supplement it?

If you don't clear the interrupt flag you'll go right back into the ISR after the return-from-interrupt instruction because the mcu will interpret the interrupt flag as a pending interrupt.