Changing 18F WDT postscaler on-the-fly

[Bruce]

Someone asked about this recently on the Microchip forum. It's handy to
know so I figured I would post the answer here also.

    ; Changing WDT postscaler settings on-the-fly

    PROCESSOR 18F2620
    #include p18f2620.inc
    
    CONFIG OSC=HS,WDT=ON,WDTPS=128,LVP=OFF
   
    CBLOCK  0x000
      WDT_POST        ; value for new WDT postscaler
      LOOPS           ; loop var
    ENDC
	
    org    0x0000
    bra    Main
	
    org    0x0020
	
Main                    ; starts with WDT postscaler set to 1:128
    clrf    PORTB
    clrf    TRISB

    movlw   0x0A        ; toggle LED on/off 5 times
    movwf   LOOPS       ; at ~0.5 second intervals
Next
    sleep               ; WDT postscaler = 1:128
    nop                 ; so we sleep here for 128 * ~4mS
    btg     LATB,0      ; toggle LED on RB0
    decfsz  LOOPS,F     ; decrement loop count
    bra     Next        ; repeat until loops = 0
	
    ; point to CONFIG2H reg at 300003h
    movlw   0x30
    movwf   TBLPTRU
    movlw   0x00
    movwf   TBLPTRH
    movlw   0x03
    movwf   TBLPTRL

    movlw   B'00001011'     ; value for 1:32 WDT postscaler
    movwf   WDT_POST
	
    movlw   B'11000100'     ; enable write to config area
    movwf   EECON1

    movff   WDT_POST,TABLAT ; Write new postscaler value
    tblwt   *               ; to table latch
    clrwdt                  ; kick the dog before the final write op

    movlw   0x55            ; Unlock
    movwf   EECON2
    movlw   0xAA
    movwf   EECON2
    bsf     EECON1, WR      ; Start the write
    nop
    bcf     EECON1, WREN    ; Lock up when we're done

    movlw   0x0A
    movwf   LOOPS
	
Finish                      ; toggles LED faster showing WDT change
    sleep                   ; at ~0.128 seconds
    nop                     ; WDT postscaler now = 1:32 so
    btg     LATB,0          ; LED toggles at ~32 * 4mS
    decfsz  LOOPS,F
    bra     Finish
	
Done
    clrwdt
    bsf     LATB,1          ; show we're done
    bra     Done
	
    end

[Darrel Taylor]

Hi Bruce,

I was looking at this code when it struck me that it looked very familiar.
A quick slap on the forehead and "oh Yeah", now I remember.

Run-Time Config
http://www.picbasic.co.uk/forum/showthread.php?t=4093

[Bruce]

Hey Darrel,

Never saw that one before. Could have saved myself a little time...;o}

Would be nice if Mchip could stuff this somewhere in a data sheet. It's really
handy. I was fiddling with AN851 when I noticed the CFGS bit in EECON1.

[Darrel Taylor]

Hey Bruce,

AN851

Never saw that one before. Could have saved myself a little time... writing the Run-Time Config.

P.S. There's a nice autobaud routine in there too. Cool, needed that.<br>

[Bruce]

Nice job on the macros too. Thanks for the link.