When I read Mike's suggestion about a simple 8-bit shift register, I thought up
a way to use a simgle pin to control a '165. Not being good at drawing ascii
schematics, I whipped up a quick schematic and converted to Acrobat format
(about 11k):
http://www.geocities.com/SiliconValley/Haven/9004/one_pin.PDF

To load the shift register, drive the PIC pin low for a couple seconds, which
causes the cap to discharge through the 1 meg resistor. The '165 will not shift
data while the /LD pin is low, so drive the PIC pin high for a couple seconds
to charge up the 1UF cap. I don't think rise time would be a problem, as the
DIP switch states remain constant.

To shift data from a '165, you need a low-to-high transition on the CLK pin(s).
Drive the PIC pin low, wait a couple instruction cycles, then drive the PIC pin
high again. After a couple NOP's, make the PIC pin an input, and read the state
of the pin. Repeat eight times to shift all switch states. Note that a closed
switch will be read as logic high because of the transistor invertor.

The 1N4148 diode provides a discharge path for the cap when power is removed.
All resistor values probably need tweaking to optimize the circuit.

I have not tried this. Seems like it should work, though. Anyone care to
comment?

Dave Sorlien

Rigby-Jones, Michael [PAI01:4837:EXCH] wrote:

>         I've only just joined the list so this has probably been suggested,
> but if external components are ok, then why not use a simple 8-bit shift
> register and an 8 way DIL switch with ascociated pull ups?  On the PIC this
> would need one output for clock,one input for data and most likely 1 output
> to reset the shift register.  Admittedly you don't actually require 256
> values, but 8 way switches and shift registers are the most common.
>
>         Mike Rigby-Jones
>
>         >I've got a few 12C508 around right now but I dont have any 16x84's
> which is
> > >why I was just trying to use the 12C508 chip. If anyone has any ideas on
> > >implementing a way to assign the 12C508 chip an ID (1 ~ 80) through
> > >external circuitry which the chip can pick up during its program
> > >initialization stage, I would like to hear it.
> > >
> > >Thanks!
> > >
> > >Shahid
> > >
> > >
> >