I am an absolute novice to assembly programming on PIC16F877A(or, any μC for that matter!). I am using microchip's ICD2 to debug this first 'hello-pic-world' program:

include <p16f877.inc>

BCF STATUS, RP0 ;
BCF STATUS, RP1 ; Bank0
CLRF PORTA ; Initialize PORTA by
; clearing output
; data latches
BSF STATUS, RP0 ; Select Bank 1
MOVLW 0x00 ; Configure all pins
MOVWF ADCON1 ; as digital outputs
MOVLW 0x00 ; Value used to
; initialize data
; direction
MOVWF TRISA ;


MOVLW 01h
MOVWF PORTA

START

INCF PORTA,1

GOTO START

END

I am trying to actually see how PORTA behaves by stepping over each line of the code(and that's why the attempt to increment PORTA values one-by-one). I have connected the PORTA lines(pins 2-5 of PIC16F877A-I/P) to 4 LEDs. On the debugger, everything looked fine yesterday but today, PORTA seems to be too stubborn to change!

Can anyone rescue me please?

 

Hi, try this .

LIST p=16F877a ;tell assembler what chip we are using
include <p16f877a.inc>
;Put your configuration bits here
cblock 0x20 ;start of general purpose registers
x
endc
org 0x0000 ;org sets the origin
;this is where the program starts running
bsf STATUS, RP0 ;select bank 1
MOVLW 0x06 ; Configure all pins
MOVWF ADCON1 ; To digital
movlw 0x00
movwf TRISA
bcf STATUS, RP0 ;select bank 0
movwf x
START
incf x
movfw x
movwf PORTA

GOTO START

END

 

Hello Alberto and Elal,

Thank you very much for your support and time.

Indeed, I tried configuring the PORT as just digital output(by making ADCON1 = 0; CMCON = %00000011) but it still didn't work. I might now have to perhaps try with ADCON = 7 as well. I try doing that. I also tried using PORTB as this doesn't seem to be shared for common purposes but in vain.

As a matter of fact, when I use C-code(compiled by Hitech's picc std compiler), things work pretty fine, I mean, the LEDs are dancing to my tunes! But again, I see that the port-values aren't updated in the SFR view of MPLAB though I use it in the ICD2 debug mode! This is quite weird to me but I don't know what's going on! The LEDs driven by the port glow but the SFR for the port shows '0'.

@Alberto: Thank you very much for such a detailed walk through the basics - my salute to your patience(I see you are a senior member to this forum).

@Elal: Thank you very much for the modifications and the corresponding comments. I haven't tried your code yet - shall update as soon as I do it...

 

Did you check your configuration bits?
Testing code with a simulator is not alawys the same then the real thing.
Set OSC to HS if you are using a crystal oscillator with more then 8MHZ
Clear WDT, PUT, BODEN, LVP, CPD, WRT_ENABLE and CP.
It will look like this __config 0x3F3A ,insert this code in the third line of your code like this.

LIST p=16F877 ;tell assembler what chip we are using
include <p16f877.inc>
__config 0x3F3A ; configuration bits
cblock 0x20 ;start of general purpose registers

NB If you working with MPLAB assembler you need to put two underscore charaters before config. __config



Here is the complete code


LIST p=16F877 ;tell assembler what chip we are using
include <p16f877.inc>
__config 0x3F3A
cblock 0x20 ;start of general purpose registers
x
endc
org 0x0000 ;org sets the origin
;this is where the program starts running
bsf STATUS, RP0 ;select bank 1
MOVLW 0x06 ; Configure all pins
MOVWF ADCON1 ; To digital
movlw 0x00
movwf TRISA
bcf STATUS, RP0 ;select bank 0
movwf x
START
incf PORTA
GOTO START
END


With this code and depending on the speed of your oscillator you gone see
the LEDs allways on, you need to slow down the process, if you want to see the LEDs incrementing you need to insert a delay routine before
incf PORTA .