I'd like to
1)  Turn on the device by pressing a button
2)  Later be able to have the PIC turn itself off
3)  While the PIC is on, have it be able to detect "ON" button presses again
4)  Only use one IO pin (so gnd or +5v to switch off, high-impedance input
    to detect button and stay on)

This is what I sketched out:

   .----------------------o
   |                      |
   |      ___     ___   |<
   |-----|___|-o-|___|--|
   |           |        |\
   |      .----o          |
   |      |    |          o
  ---    |o     <|   ___  |
   -    -|       |-o|___|-o---o Vcc
   |     |o     /| |
   |      |    |   |
   |      |    |   |
   '------o----o---|----------o GND
                   |
                   |
                   o
                 PIC IOx

I was thinking that the pullup on the NPN transistor would show up as
~5v on the IO pin (which can be grounded to shut off), and I could add
some diodes in to use the same momentary/NO pushbutton to also ground
the base of the NPN (harmless, since at that point power would be held
on by the button also pulling down the base of the PNP) to detect the
button.

Of course the voltage at the base/IO pin is close to zero since the
transistor is forward biased and most of the voltage is dropped in the
pullup resistor.

Can anyone suggest a way to meet my original requirements?  It just
occred to me that an N-FET instead of an NPN might work.

Thanks.

--
Ben Jackson
<ben@ben.com>
http://www.ben.com/

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