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Reputation: 780

How to move a variable's value into another variable in assembly

I am trying to learn assembly programming with MPLAB X and a PIC18F1320 microcontroller. I have been following the MPASM User's Guide (http://ww1.microchip.com/downloads/en/DeviceDoc/33014J.pdf) and have gotten an LED to blink from the RB0 pin on my microcontroller. I wrote a program to make an LED blink once every 512 ticks. I am having trouble figuring out how to change the delay from 512 to a variable quantity so I can change it somewhere else in the code. Ideally, the line

        movf    0xFF,count1

would be replaced with 

        count1=delay1

where delay1 is the variable set to 0x20 earlier in the code.

Here is the code:

#include "p18F1320.inc"

 CONFIG  OSC = INTIO1          ; Oscillator Selection bits (Internal RC oscillator, CLKO function on RA6 and port function on RA7)

 cblock 0x20    ;start of data section
 count1         ;delay variable
 delay1         ;length of delay
 endc

        org     00          ;\
        movwf   PORTB       ; |
        movlw   0x00        ; |
        movwf   TRISB       ; |--Start program and configure I/O pins
        movlw   0x00        ; |
        movwf   ADCON1      ; |
        movlw   b'00000110' ;/

        movwf   delay1      ; Set the variable delay1=0x20
        movlw   0x20        ;/

loop    call BLINKONCE      ; Blink loop
        goto loop           ;/

BLINKONCE                   ;\
        bsf     PORTB,4     ; |
        call    DELAY       ; |--makes I/O pin RB4 turn on and off once with a delay in between
        bcf     PORTB,4     ; |
        call    DELAY       ;/
DELAY
        movf    0xFF,count1 ;I want to be able to set count1=delay1 right here

loop2   decfsz count1, 1    ; Delay loop with length=count1
        goto loop2          ;/

        return  ; end program
        end     ;/

THANKS!

Upvotes: 3

Views: 3568

Answers (1)

Jon
Jon

Reputation: 506

On the PIC18 device you can use the movff instruction to do what you are after - copy a value between two registers. The movf instruction only allows you to copy the value from a register into the working register.

Also the ordering of your movlw and movwf instructions at the start of the program are back to front. You call movlw to load a constant value out of program memory into the working register, then movwf to copy that value out of the working register into data memory.

This site has an online PIC simulator and tutorials that explains how this all works in more detail:

http://www.microcontrollerjs.com

Upvotes: 1

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