Date:    Thu, 29 May 1997 07:08:52 EST
From:    Glen Benson <benson@BENSONASSOC.COM>
Subject: New To PIC What am I doing wrong?

Hi,

  I am new to PIC, cant get a handle on asm fast enough. So I bought a
basic compiler. I can write all sorts of neat things I dont need to do,
But
I cant get the following code to work in less than about 2 seconds. I
need
to take an ADC reading stuff it in to a varible and send it to the PC
via
lpt port. For my test I use qbasic and turn on Pin 7 of the PIC with the
(out lpt,1 statement) then
I read the level of pin 6 on the pick to see if hi or low then I shift
the
value in B0 , and do it 7 more times. I have to put 2 second delays in
the
qbasic code
to make it work. Thats .5 hz (its got to go faster than that). Please if
you see my blunder, can you show me? :)


' a qbasic program turns on PIN 7, then the qbasic prog. reads
' the value on (lpt pin10) connected to (PIC on PIN 6) and repeats
' 8 times toget the whole byte. The qbasic prog has to have
' 2 second delays to get the right data. How could this
' be written better? It cant get worse.



Symbol  DDIR = Dir6           ' Shift data pin direction is Dir1
Symbol  DPIN = Pin6           ' Shift data pin is 6
Symbol  CPIN = 1              ' Shift clock pin is 1
        input 7               ' Make pin 7 an input

START:  B0=65                 ' value to shift out

loop: pause 1                 'didnt seem to work without this
      if pin7=1 then getbit   ' pin 1 from lpt just went high
      goto loop

getbit: DDIR = 1             ' set the direction of the pin
        DPIN = bit0          ' get the bit send it out to pin 10 on lpt
        B0=B0/2              ' shift the value in B0 over
        goto loop


Thanks
Glen Benson

------------------------------
Glen,

Try changing your code to this.

loop:
      if pin7=1 then getbit      ' pin 1 from lpt just went high
      goto loop

getbit: DDIR = 1                  ' set the direction of the pin
        DPIN = bit0                ' get the bit send it out to pin 10
on lpt
        B0=B0/2                    ' shift the value in B0 over
        if pin7=0 then loop       ' wait for Pin 7 to go low before
looking for new bit

Essentially you are implementing a synchronous data transfer, with the
PC providing the clock. It looks like your original code would sense the
first clock pulse and then shift out a bunch of bits before the second
clock pulse. The extra IF should keep you in sync. If you want to speed
it up a bit more, send byte wide or nibble wide data for every PIN 7
clock pulse. You have up to 9 input pins on a standard parallel port. So
the limit is the number of pins available on your PIC.

Let me know how it goes,
Paul Kolesnikoff
pkolesni@ball.com