Ray Gallant wrote:
>
> Hello,
>      I trying to write a program which will set RA3 high for exactly one
> second then set the pin low. The crystal used for the PIC is 4Mhz.
> Therefore it will take 1 us per instruction.  I'm using MPLAB to write the
> program.  Any help would be great.
>                          Thanks
>                                Ray
Hi!

You can set the delay by adjusting the value of PRESET and the 17.

You can calculate this as follows:

Total delay = loop*[prescaler*(256*preset)+delay+6]

Loop = Period/65536 (round up to integer)

Prescaler = period/loop/256 ((round UPwards to 256,128,64,etc)

Preset = 256-[period/loop/prescaler] round UP to integer<256

delay = [period/loop] - [prescaler*(256-preset)] - 6

====================================================
PRESET  EQU     D'61'           ;Delay loop counter (1 msec delay)

MOVLW   PRESET                  ;Get preset value ready
        MOVWF   TMR0                    ;Write to timer
        MOVLW   4                       ;Set up 4 * 50 msec loop
        MOVWF   CNT4
        CLRF    UDBIT

;Wait for Timer roll Over Routine

LOOP1   BCF     INTCON,T0IF

TEST    BTFSS   INTCON,T0IF
        GOTO    TEST

        MOVLW   D'17'         ;SET TO 17
        CALL    VARDLY
        NOP
        MOVLW   PRESET
        MOVWF   TMR0
        DECFSZ  CNT4,f
        GOTO    LOOP1
=================================================

>>>>>rest of program<<<<<<

VARDLY looks like this:
==================================================
;DELAY LOOP SUBROUTINE: 1 milli-second (993 Cycles)
;CORRUPT W,COUNTD. STACK:1 LEVEL

        COUNTD  EQU     2F      ;AT END OF USER RAM

DELAY   MOVLW   D'247'          ;FIXED DELAY ENTRY POINT
VARDLY  MOVWF   COUNTD          ;VARIABLE DELAY ENTRY POINT
LOOPD   NOP                     ;(DELAY=5+4*PRESET IN W)
        DECFSZ  COUNTD,f
        GOTO    LOOPD
        RETURN                  ;EXIT POINT
==========================================================

Have lots & lots of fun!
--
eric van es
vanes@ilink.nis.za
cape town, south-africa
Looking for HOLIDAY ACCOMODATION? http://www.nis.za/~vanes/