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I have seen this exact same circuit on the net several times, just redraw=
n. =20
In some versions it shows it connected to PortB.  I suspect someone redre=
w=20
it with PortA but didn't add a pullup.  I have looked at this circuit ove=
r=20
and over and just don't see a wiring flaw.  Sometimes it is staring you i=
n=20
the face though and you just can't see it.  I'll have another look at it.
Thanks to all,
FJ


From: Rolf <learr@rogers.com>

I had considered this flaw in the schematic as well.

My assumption is that there was an internal pull-up on A1. The only
thing that could make sense.

Rolf


Russell McMahon wrote:
 > I'll copy this from below as its crucial
 >
 > *** The turnoff of A0 must wait until after the button is released ***
 > or the still-pressed button will retrigger the circuit.
 >
 > _______________________
 >
 > There's a fundamental problem with the circuit diagram AND there's
 > almost certainly a problem with your circuit implementation.
 > Adding a pullup on A1 makes the circuit operation defined and this may
 > have been specified elsewhere.
 >
 > All Off
 >
 > PNP =3D Q1
 > NPN =3D Q2
 > Upper diode =3D D1
 > Lower diode =3D D2
 >
 > Button =3D ground diode junction on so PNP on so power on.
 > PIC pits A0 high so NPN on so PNP held on and all is well.
 >
 > Now, A1 cannot either affect or see anything useful.
 > It is behind two opposed diodes so it cannot drive Q2_c hi or low
 >
 > If A1 is input and Q2 is off then Q2_c is high but D2 blocks the Q2_C
 > high so A1 see's a floating switch.
 > Pressing button changes A1 from float in to gnd in but this is not
 > overly useful at this stage.
 >
 > With Q2 on =3D power on then Q2C ~~=3D 0 volt so D1 cathode and D2 cat=
hode
 > are at float.
 > Again, closing button takes A1 from float to ground.
 > A pullup on A1 allows the button press to be seen and then the
 > processor can turn power off using A0.
 > *** The turnoff of A0 must wait until after the button is released ***
 > or the still-pressed button will retrigger the circuit.
 >
 > _____________
 >
 > With button open and no PIC then A1 *MUST* float. If it has a 10k
 > pullup it must pullup. If it doesn't the cct is not as shown.
 >
 > A P channel FET will work well as long as gate drive voltage is not
 > too high or too low.
 >
 >
 >         Russell
 >
 >
 >

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