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PICList Thread
'Connecting iButton to PIC'
1998\09\21@182059 by N Steenkamp

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Hi,

Thanks for all the help on my previous RS485 question.  I'll let the list
know how things turn out.

I have another little thing I would like to run by the list:
I am using the Dallas iButton Serial Code buttons (DS1990A) to allow
users to identify themselves.  I have recepticals on my units and the
users simply press their buttons on the receptical.
Now, the obvious way of connecting the receptical to the PIC is to
connect it directly with a 5K pull-up resistor, as is suggested by Dallas.
But, is it a good idea to bring a pin of your microcontroller so directly
out of the enclosure.  What about static discharges onto the receptical,
for instance?  Would isolation by transistors by of any benefit?  I was
thinking of something like:

           +5V
           |
           R 5K
           R
           |--------+------------- Dallas Button
PIC Tx     |/c       |
-----------|         |
       npn|\e       |
           |        |
           GND      |
                    R
                    R 56K
            +5V     |
            |       |
            R       |
PIC Rx       R 10K   |
-------------|       |
           c\|      |
             |------|
           e/| npn
           |
           GND

This has the disadavantage of splitting up the transmit and receive
lines, thus needing two instead of one pin (and it inverts the Ts and Rx
signals, but that is not a problem), but at least the PIC pin is not
directly brought out into the big bad world ;-)

Comments?

Niki

1998\09\21@184556 by David Reinagel

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> I have another little thing I would like to run by the list:
> I am using the Dallas iButton Serial Code buttons (DS1990A) to allow
> users to identify themselves.  I have recepticals on my units and the
> users simply press their buttons on the receptical.
> Now, the obvious way of connecting the receptical to the PIC is to
> connect it directly with a 5K pull-up resistor, as is suggested by Dallas.
> But, is it a good idea to bring a pin of your microcontroller so directly
> out of the enclosure.  What about static discharges onto the receptical,
> for instance?  Would isolation by transistors by of any benefit?  I was
> thinking of something like:

How about trying this circuit instead:



                 +Vcc     +Vcc
                   |       |
                   |       R
                   _       R 5K
           1N914   A       R
                   |       |
PIC pin   100 ohm   |       |    100 ohm
------------RRR-----+---+---+------RRR------ Dallas Button
                       |
                       |
                       _
                       A  1N914
                       |
                       |
                      GND


This now only uses 1 PIC pin, and gives you protection for that
pin.  The 100 ohm resistors limit the current through the
protection diodes and to the PIC's internal protection diodes.
Also, the pair of diodes can be gotten in a single SOT23 surface
mount device, like the BAV99 from Zetex and Vishay/Liteon (both
in DigiKey cat.).

Dave Reinagel
Cisco Systems, Inc.

1998\09\23@152944 by Kestas Biliunas

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David Reinagel wrote:

{Quote hidden}

We are using similar circuit:

           +Vcc
            |
            R
            R 5K
            R
            |
PIC pin  R1  |            R2
--------RRR--+------------+--RRR--- Dallas Button
                         |
                         |
                         _
                         A  BZW06-5V8
                         |
                         |
                        GND



R1 = R2 < 390 ohm (usualy 100 ohm).
Works several years in hundreds units.
Electrostatic discharge sometimes damages iButton, but PIC - never.

Kestas
Kaunas, Lithuania

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