-----Original Message-----
        From:   M. Adam Davis [SMTP:adavis@UBASICS.COM]
        Sent:   Wednesday, 8 March 2000 7:29
        To:     PICLIST@MITVMA.MIT.EDU
        Subject:        Re: Example

        There are only 35 instructions to learn on this PIC.  The easiest
way to learn
        how to use the PIC is to read the data sheets, and jump in.  There
are some
        books which can help you do so, and many web sites which will also
help.

        See my links to such resources at
        http://www.ubasics.com/adam/pic/

        I can help a little bit here with your questions:

        Steve Horvath wrote:
        >
        > Hello
        > I am fairly new to PIC programming, but have a fair bit of
experience form
        > school involving the Intel 8031.  I was wondering if someone could
write a
        > quick program (2 minute job) to do the fallowing on a 16f84, so
that I
        > could learn about these functions by doing them:

        First, the architecture is very different between these two mcus.
You'll find
        yourself referring to the datasheet as you program.

        > -push W onto a stack, and pop W off of the stack

        There really is no direct analogy in the PIC for your stack.  You
have no access
        to the stack in the '84.  If you want to program this way, then
you'll need to
        create a software stack (which, for most things, is inefficient.
It's just a
        different architechture, and a different way of thinking)

        > - input from port A to W when there is an interrupt

        org     4
                (Save status registers if needed here (no point in saving W
;-) )
                movf PORTA, W
                (restore status registers if needed here)
                RETFIE

        > - have a 10ms delay using the timer
        > - output to pb.0 through pb.4 after the interrupt
        > - have a 1 second timer using loops, and have the output (port B)
blink on
        > and off every second thereafter.
        >
        > oh, and could someone tell em the difference between portA and
trisA?

        Each i/o pin on the PIC can be an input, or an output.  TRISA
defines whether
        each pin in portA is input(1) or output(0).  PORTA represents the
actual status
        of PORTA.  If you are driving all pins low, but an external circuit
is driving
        them all high, then PORTA will read all high (ie, the real state,
not the state
        the PIC is trying to keep it it)

        There are other examples out there which do all of what you are
asking.  Check
        my links above, and then start asking more specific questions.

        I hope this helps!

        -Adam


        A couple of explainations of the above explainations for a beginner
-

        What is org 4 ?
        Program location 4 is the instruction that will be jumped to when an
interrupt occurs.  Normally you put a goto statement here that jumps to your
interrupt handler routine.  When you are finished handling the interrupt the
retie statement goes to the program location stored in the stack and the
program resumes from where the interrupt occurred . . .
        If you haven't turned interrupts off at the start of your handler
and stored the state of certain registers before manipulating them you will
get strange results when you turn interrupts back on and attempt to retie -
beware!

        org 0 should have a goto statement that points to the start of your
startup code.  Your program code must NOT use locations 0, 1, 2, 3 or 4 as
these are all reserved for special uses.
        It is safe to start your program from memory location 5, but I
prefer to use org 6, so my programs usually have this structure -

        org 0
                goto startup
        org 4
                goto interrupt_handler
        org 6
        startup bsf status,rp1


        The difference between portA and trisA?
        Trisa is where you set up port a.  It has (almost) the same address
as the I/O port - it is actually on the second page of memory at 0x80
higher, but you can't add 0x80 to the port a address, you have to swap to
the second page and use the same address as port a.  (Don't forget to swap
back!)

        Port a is actually 2 separate sets of circuits - 8 input circuits
and 8 output circuits.  If you set port a to output then output a 1 and then
read the port you will not see your output, you will see what was last read
in from the input circuitry.  To read the current value you have to go back
to the control page and swap the port to input and then read the input.
        Moral - try to use port pins as I or O, but don't swap them if
possible.  Set up everything in one go, then LEAVE THE I/O SETTING ALONE.


        Bye.