Brad Mitchell <bmitchel@KODAK.COM> wrote:

> I'm taking something I wrote in Parallax 's assembler, and now just
> starting to learn the Micro chip MPASM , I find something a bit
> confusing.
>
>         1. Port direction control
>         2. port reading/writing.. ie: every interface to the world..
>
> The problem arises because Parallex in their manual states that
> this is the instruction.. MOV #literal !RA for example sets the
> direction for the register, but in the Micro chip version, they are
> using the TRST command that is, according to the Micor chip test,
> not to be supported in the future.

Brad:

You didn't say which PIC you're using, so I'll assume that it's one
of the 16C5x family.

Port Direction control on the '5x is accomplished via the TRIS
instruction, the syntax of which is "TRIS port", where "port" is
PORTA, PORTB, or (on the C55 or C57) PORTC.

TRIS uses the contents of the W-register to select the direction of
the port's I/O pins:  0's are outputs and 1's are inputs.  Here's an
example:

        MOVLW   00001111B               ;Make RB4-7 outputs, RB0-3 inputs.
        TRIS    PORTB

To read the I/O pins, just use any instruction that reads from the
port's register.  Here are two examples:

        MOVF    PORTB,W                 ;Copy the contents of PORTB to W.
        BTFSS   PORTB,3                 ;Skip the next instruction if RB3 is
 high.

To write to the I/O pins, use any instruction that writes to the
port's register.  Here are another two examples:

        MOVWF   PORTB                   ;Copy the contents of the W-reg to
 PORTB.
        BSF             PORTB,3                 ;Pull RB3 high.
        CLRF    PORTB                   ;Pull all the PORTB outputs low.

Use of the TRIS command in 16Cxx (NOT 16C5x) parts is, as you
mentioned, discouraged by Microchip in their data books.  However,
they have yet to produce a 16Cxx part that doesn't include the TRIS
instruction.

If you're using the 16C5x parts, on the other hand, you can ignore
the warning; the TRIS command is the only way to set the port
direction on the 16C5x.

-Andy

P.S.  You can probably learn a lot by comparing the Parallax
"instructions" to the PIC opcodes to which Parallax's assembler
translates them.  Last time I looked, Parallax included a translation
table in their PASM documentation.

Andrew Warren - fastfwd@ix.netcom.com
Fast Forward Engineering, Vista, California
http://www.geopages.com/SiliconValley/2499