Thanks Bob!  Very helpful.  I have worried of the buttons changing state while this was executing.  What I'm gathering is that the use of temporary registers or just doing a movlw PORTA and reading from W is important in reading i/o ports.

Regards,
Bob

Bob Ammerman <rammerman@VERIZON.NET> wrote ..
> ----- Original Message -----
> From: "Bob Japundza" <piclist@MAIL.CSCAPE.NET>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Friday, June 18, 2004 1:01 PM
> Subject: [PIC]: anding/masking
>
> ; -- Here is a version that only fetches PORTA once, ensuring a consistient
> view
> ; -- of the button state
>
> movf    PORTA,W
> movwf button_status        ; Bits are in RA2 and RA3
> btfsc    button_status,2     ; Is bit two set?
> xorlw   b'00001000'        ; Yes: flip bit 3
> andlw  b'00001000'         ; Were the two bits the same?
> skpnz
> goto    stopmotor             ; Yes: stop it
> btfsc    button_status,3     ; Was up button on?
> goto    motorup
> goto    motordown
>
> ; -- If you don't mind reading PORTA multiple times then...
>
>     movf    PORTA,W
>     btfsc    PORTA,2
>     xorlw   b'00001000'
>     andlw  b'00001000'
>     skpnz
>     goto    stopmotor
>     btfsc    PORTA,3
>     goto    motorup
>     goto    motordown
>
> ; -- here is a version that only reads port A once and doesn't use any
> ; -- temporaries
>
> movf    PORTA,W
> andlw   b'00001100'        ; Are they both zero?
> skpnz
> goto     stopmotor            ; Yes
> xorlw   b'00001100'        ; Are they both zero?
> skpnz
> goto     stopmotor
> andlw   b'00000100'        ; Is the up button down?
> skpz
> goto     upmotor
> goto     downmotor
>
> Bob Ammerman
> RAm Systems
>
>
>
> > Hi All,
> >
> > I was wondering if my code below can be made to be a bit better.  It
> works
> fine as is, but just seems to be a bit clunky to me.  In a nutshell what
> I'm
> doing here is checking two buttons on port A, turning off/on the motor
> and
> if one button is pressed, that dictates motor direction.  I'm wondering
> if
> there's a way without using the temporary register "button_status" to check
> the the two bits I'm looking at.
> >
> > movlw b'00001100'
> > andwf PORTA,W                           ;mask all but the RA2,3 bits
> > movwf button_status
> > movlw b'00001100'
> > xorwf button_status,w
> > btfsc STATUS,Z                          ;are both buttons pressed?
> > goto stopmotor                          ;yes, stop the motor
> > movlw b'00001100'
> > andwf button_status,w
> > btfsc STATUS,Z                          ;are both buttons not pressed?
> > goto stopmotor                          ;yes, stop the motor
> > btfsc PORTA,2                           ;check if up button is on
> > goto motorup
> > btfsc PORTA,3                           ;check if down button is on
> > goto motordown
> >
> > Regards,
> > Bob
> >
> > --
> > http://www.piclist.com#nomail Going offline? Don't AutoReply us!
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> >
>
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