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'Capacitive PIC ??'
1995\05\22@041515 by Nick Howard

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Hi
Whilst testing my 64 based project I noticed that a finger brought up to
the quartz window caused the output leds connected (via 2003 darlintons)
to ports C and D to behave oddly. This is not caused by the oft mentioned
optical effect as the window was covered. Neither is it a decoupling
problem as each pair of supply pins has 100n across them. The supply
comes from a 78L05 and the timing is courtesy of a three terminal (4MHz)
ceramic resonator.

1) Has anyone else come across this effect.
2) Will the effect go away when I use the OTP versions.

BTW I have tried two windowed chips, both show the 'capacitive effect'.

Thanks in anticipation of any forthcoming help

Nick

1995\05\22@171150 by Przemek Klosowski

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  Hi
  Whilst testing my 64 based project I noticed that a finger brought up to
  the quartz window caused the output leds connected (via 2003 darlintons)
  to ports C and D to behave oddly. This is not caused by the oft mentioned
  optical effect as the window was covered. Neither is it a decoupling
  problem as each pair of supply pins has 100n across them. The supply
  comes from a 78L05 and the timing is courtesy of a three terminal (4MHz)
  ceramic resonator.

I don't know if this is relevant, but recently I ran into this:
I have been building a PIC programmer, and I used a 7812 with
a voltage divisor  to boost the voltage slightly (to 14 V).
      _____            The voltage measured with a voltmeter
_______|    |________   looked fine, but when I looked at it
      |    |   |       with the scope, it was 14VDC plus a 10MHz
      --+---   <       oscillation with 1 Vpp amplitude. Kind of
        |      >       shook my confidence in 78xx parts. I suggest
        |      <       you look carefully for a parasitic oscillation
        |      >       (of course hooking scope probes could influence
        +------|       what you are looking for---welcome to the
               >       wonderful world of analog).
               <
               >       BTW, I ended up using LM317---works like a charm.
----------------+-----

               greetings

                       przemek klosowski (spam_OUTprzemekTakeThisOuTspamrrdstrad.nist.gov)
                       Reactor Division (bldg. 235), E111
                       National Institute of Standards and Technology
                       Gaithersburg, MD 20899,      USA

                       (301) 975 6249

1995\05\22@233248 by Paul Greenwood

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{Quote hidden}

This would be caused because your ground was moving - right?

Probably some capacitance on the input line (on the 7812 side of your
voltage divider) would have solved the problem I think.

I took the liberty to put it in the picture.

Of course, I could be wrong.....

--

           -- Paul Greenwood --  (pablospamKILLspamaustin.ibm.com)

What the large print giveth, the small print taketh away.

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