Brooke,
   subwf    ms_counter,W
   btfsc STATUS,Z  <-bit test file skip when clear.
   banksel PORTB ; NEED banksel here or bcf PORTB,7
 ; bcf    PORTB,7    ; heart beat off PC4 Testing


I find the code as follows. When the ms_counter result
is non zero it skips banksel and bcf. But when it is
zero it executes banksel. Now the thing is bcf and
banksel has to go hand in hand. Select the correct
bank for PORTB before using PORTB. I cannot explain
why your LCD gets corrupted since I can't see all the
code and schematics.

Regards,
John
   

--- Brooke Clarke <brooke@pacific.net> wrote:

> Hi:
> 
> My clock program uses an ISR triggered from Timer2
> once every 1.0 ms.  
> In the ISR there is code that turns on an LED at the
> top of the second 
> and turns it off after 10 ms.  I'd like to use the
> LED for debugging and 
> so commented out the two lines that control the LED,
> but after doing 
> that the LCD display turned to garbage.  It turns
> out that placing a 
> "banksel PORTB" at the end of the ISR fixes the
> problem when the LED 
> code is commented out.  But I don't understand why? 
> 
> Have Fun,
> 
> Brooke Clarke
> 
> Here is the end of the ISR code:
> 
> End_ISR   
>   movlw .10    ; 10 ms on (1 ms is too short)
>   subwf    ms_counter,W
>   btfsc STATUS,Z
>   banksel PORTB ; NEED banksel here or bcf PORTB,7
> WHY-O-WHY
> ; bcf    PORTB,7    ; heart beat off PC4 Testing
> Switches <<<needed 
> inorder to work,
> ; commenting out the bsf and bcf lines causes the
> clock to
> ; have garbled display.  Why?
> ; --- pop pclath, status & w ---
>   movf  PCLATH_TEMP,W
>   movwf  PCLATH
>   swapf    STATUS_TEMP,W
>   movwf STATUS
>   swapf W_TEMP,F
>   swapf    W_TEMP,W
>   retfie    ; turns on GIE
> ;================== End of ISR
> ===========================
> 
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