James Nick Sears wrote:
> (1)
> Set PORT register before TRIS register:
>     Set PORT register:
>         TRISD = xxxxxxx1
>         PORTD = xxxxxxx1
>
>         Right here is where my question arises.  Since there is a low
> voltage connected to the pin which is configured as an input,
> then won't the PIC just sample the pin low and overwrite the
> change I just made?

No, unless you do a read then write out that value back to the port
register.

> It seems that case 1 is what most people are recommending but what
> keeps the pin from getting sampled low and overwriting the change to
> the PORT register while the pin is still an input?

Port latches don't just change magically for no reason (unless perhaps you
wave a dead fish over them during a full moon).  They only change when they
are written to.  The problem comes from instructions like BCF which reads
the entire register, changes the specified bit, then writes the entire
register back out again.  When this operation is performed on a port
register, it effectively writes the actual pin state to the output latch for
all bits except the one being explicitly modified.

The easy way to get around this during initialization is to write the whole
port register to its initial state with one write.  That will set the port
latches regardless of the pin voltages.  Then write the TRIS register as
desired.

At the risk of sounding like a broken record, this is yet another totally
avoidable issues in my PIC development environment.  The /INBIT and /OUTBIT
preprocessor directives allow you to specify the initial state for all pins.
These directives leave a bunch of assembler state lying around about how you
want the ports to wake up.  The PORT_INIT routine in the standard PORT
module then uses this state to efficiently set up all the port, TRIS, and
LAT registers.  You can also specify the optional pullups available on some
input pins.  The PORT code sets up that state too, and gives you an assembly
error if you specified a state that can't be realized by that particular
PIC.

This may sound a bit complicated, but all you have to do is write /INBIT and
/OUTBIT directives in the project include file.  The PORT module does have
some complexity, but it's all canned.  You shouldn't need to mess with it at
all.

Another advantage is that you get a level of abstraction for each I/O pin.
Various symbols and string substitution macros are automatically created so
that you can write code like

     dbankif led_reg    ;set banks for access to port with LED pin
     bcf   led_pin      ;turn on the LED

without coding the exact port and pin for the LED into the body of the
source code.


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