This is a multi-part message in MIME format.

------=_NextPart_000_010A_01C30DC0.84FD98C0
Content-Type: text/plain;
        charset="iso-8859-1"
Content-Transfer-Encoding: 7bit

> 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)

Here's an attempt to meet your full specification.
I haven't tried it and it's not fully designed yet but I think a little
thought and/or playing would make a reliable working circuit from it.

There are too many requirements on the single pin to allow a simple digital
0/1 full rail signal level to be used.
Roman Black recognises this in his SCR design and "cheats" by making the
turnoff function independent of the pin - in his cct the PIC reduces its
current consumption below the SCR hold on current. This is a clever and
legitimate solution but will not work in cases where there is other
circuitry drawing current in excess of the SCR holding current.

My solution also uses a "trick" - but using the PIC pin to do it.
Transistor TR2 is held on by R3+R4 BUT the voltage at the junction of R3 &
R4 is a LOGICAL low level which the PIC can read. When the button is
operated after startup the PIC pin sees a logical high.

To turn the circuit off the PIC (or other cpu) takes R3/R4 junction to
ground. Proper design may wish to see a small series resistor to avoid
possible damage from the 100uF capacitor discharging into the pin.
The true low level turns off TR2 and initiates shutdown. Somewhere along the
way the PIC will lose the ability to maintain an active low output and C1/R3
must have a long enough time constant to hold the transistor off after this.

The time that the voltage into the regulator (on capacitor C7) stays
actively high will be an important factor in turn off.
Suitable care and thought would definitely turn up a working combination but
odds are that as shown the component values are wrong.

If the regulator has a substantial output capacitor but a smaller input cap,
the PIC may be designed to stay alive after TR3 is turned off. This would
ensure the circuit is well shut down before the PIC expires.

PIC "death throes" need to be carefully thought through - as is always the
case in this sort of circuit.

Z1 stops PIC pin being taken illegally high.
R5 provides turn off for TR2 in absence of other drive sources.
R3/R4 divider, which holds TR2 latched on, must provide low enough output to
place a logical 0 on PIC pin, while R7/R4 divider provides a logical high.
R7 sized to provide enough level for a logical 1 - higher voltages will be
clamped by Z1. D3 left over from a prior circuit - does no harm but Z1
probably sufficeth at 4v7.
C1 value should be as low as reasonably possible consistent with turn off
timing requirements. R4 can be made much larger (depending on transistor
beta, load current supported  etc) - allowing scaling up of other resistors
allowing smaller C1



        Russell McMahon




--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics



------=_NextPart_000_010A_01C30DC0.84FD98C0
Content-Type: image/gif;
        name="Psw1pin.gif"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
        filename="Psw1pin.gif"

R0lGODlh7gEeAYAAAAAAAP///yH5BAAAAAAALAAAAADuAR4BAAL/jI+py+0Po5y02ouz3rz7D4bi
SJbmiabqyrbuC8fyTNf2jef6zvf+DwwKh8Si8YhMKpfMpvMJjUqn1Kr1is1qt9yu9wsOi8fksvmM
TqvX7Lb7DY/L5/S6/Y7P6/f8vv8PGCg4SFhoiAGQCICgmEjTeLBoIHkoQXmpqEBZyXe5sPkCOhnJ
aUk6ijopWnrnqTmz6srqICubOosH+froeBqwiuuL2csYnLcYm+OJbNzATJrcbCcpChyz/Cv9mX1a
rV1nyw2bWWwtDWpL/r3O3u7+Dh8vP89WS/x7z1p9H07fEi1OVwJ0/AQ2MsfDlcJz6ITd8uciFjKA
qoo9bJhk4UBt/xiJPYMII9lHi6M2UcPo64hGksYafswH8t+2kfgsmsyGUhVMIS93NSOYimbMFQCL
GqS2kQHChAG7nQi3FAvKlUOJbnuI9ebFpOKKEDS5s0O/Mep0koxa9cNBhzop4vM2MGycdF/Rpr3b
5aDevSSgohrbbq/cQ4ILGz6MOLHixYURMbZrQSjSpvMatzwDOUJmDpL/poRnOYjBz/s2a7Bmmgrq
shBSn9bkaHLXYI8V3yjKaKoKua6lRLXbe0RwQbUT3746G7VVpZg1TxgeAjrea/xIM1+OnAzCwT5t
SB9SHHEraMmzp1DuM2fEz2Kdm8LxnWd04dxp1b9GXv3s886Qx/+PzFVq6D0AnDKR3WcCa2aZx9mB
A5oGWEis0VTWWueFhVskdQkXYG5aBYShMwqK+N8GmVkYikBtudUgBSiytB+BXBHn34yUeYBiLWx1
NSBnKsaoVHhCDklkkYZFZ+SLQPbnoWAXJAlllOExB9eOar3yoY7saSilkzz9yIKSSmJlYgVjluei
jTC+p9J8M3V345UbZWlljyb+WGKCCJYw4ohLIuIgk38Kag6EGbnJYIVbchjXV7AxeNqePeQJYKVy
Iuman4vKiMR/lLapxqdpOhjloZYa4WlzodYgahaGdoqodmu0ymYuT5p6qayrPrLHq6DmStasrPZ6
q0qStqZqGrT/undMsV6BmcGy8pkR2np6+ErEmYBSm+ya2Nl6KnjQbluGtKJBuhux4apVkKbPHctp
ud1uioK5qjkLwljQpSqDu98GC2eY6prJqKCv5dsvvelyG/C/0+ALrJrk4oiftwIzLPHCza7b3nUd
R/yvvegyASYwLDoMDsQUb7UyyLvpBcu9t4TY8IUiR4FtvjB9x6+8U/gF6c1vwTtvrV7FCvBTESlo
8sh6+utGztkiLcayNwsF5Mn1jjuXyuBRTRTMCddrbZUSC/3WtV63zGVrXPc8dsw/p9Sn00qrzbFY
utHS5GITp+hd0YMmOPCoBf+l5eCHWxs4xhbzhzfBfc154zAy/0moLcpgaA35xpIv3kviQqO9A+md
B9Uw6aY3ITWwotNd299VV5GPbN6qXvhzfdXO+4d8I+y4b5QHXbHnhqMKtlVcazx3bgYr7Bhfj8vR
+rQuQ257KFa45PH0gQaZ/cN5nwu89tysvvrdTqWOI3BEo1E9+deT7T39P1eXld2abYb1HPEDAbel
Qc1+usqY7iyVPlwd72jlK6ADofe7z5Gpa+MDYPK+kMDGGRBZHMsgAyX4QbYl7YET5GCjWGInCoJw
ag0coQtLGMETLmg7KavgDwI4u+DBsHsyHNpv6PC/G17QCx7khfQW6KEV/dB/a5Nf+7oExSiKTVnR
ss9JMlc/wf+Z8FKWa5vsWEePTFnRhwX6xFHaVcTFGe1jsmnjF0kWxje2bXk8hJG+ttfEKnpmg2sE
Y2XkaJb32emOtMsjucI3nDSK8B0CItAAn2dHpilyiJBsTz8S2Tx5BDFtZTJP/xSnwAPOR0sRihcU
JqlDZpGoRXAK3w6XsMm/Veh9oAwhI+G1lEfG0I7C0CWsbOgDVHYSNHSsWRKHGTVDPil2ygQgNuIh
RSz2cVcr9NFjrMhMIZ6vlrOQpjG9eLA3xHKRSWOGMMmCS7dhEg7j/JjjYhNHYBbTlG1oJyuXaRwW
liSe1Txhb855HGDOT4vsaOQYuZlFn/VTjcL6oyHVYc8cLlT/BPvykh81CUhOvvKb8GsmOalJTIOC
j48kJegu+URLLfiyFN50GtNEyU6P3mmlq3xCS7s5z5HVbYsxFag1U3qFm+ojp8b0Uy6BKNNI0XQL
Qh3qP0dax4SatJL0AagRHRrOY0YVoQpFIp9UCE13ItGqpUsqMuvJz7PClKxl9alY65FWtb6HrW2d
KCVTGRidLTNyXp1cTzGKpHfRlSluvWc9gFoISmmKHIOta18ZCj+iVqJa7DriRpNZWLlGdqmE4BwB
x5PZrM5KsuKcUH08qz5w2bWFjtzQ/RqrzcrR87KQZWJo9TjXrcIREISsJO74CtPUmhC1oezETk/K
1YH+9bEU/zUcUGT2B6wtsaR+NV5wCSdK4v4yuizjKW2bC9xp1vYqTcukH+zhL+1WNbzqZF57m1RI
RuqvqogNw4lIm1G8mndS86VoNP8L4ADXl0uMIeKUTulECPqXs13dH4PNeM3pGJZqipqgehfM3ta6
t7Vm86FWPSzh/XVoj8STcESjtad07GdvyX2aeMTXjWdGFbZT1a3OHvk6XqIuxCA0p4Zm2+KCmhWf
81QR0D6MX7w8V8Ylns6JzWRRQhFYPb1lVZRhvONtzjjETxbsldNjJTTJlscaRvKiLhzW21rixeCj
05pMS+Of9Bewq40em6k1YAvqmbp0Zi6K/UZm3AZzzmmuM/+UIxxoNV+Oz4X2M2sTLRVCg2bIgoa0
qyQtX0Vr2tLI46g/ukxpTm9Xqo2+7ldFfWlPx1W84EV1UDHtDlBv2tUJ/u6kZ21qWp/yy1Vxn2V1
neG7lJHRoQa2LU1s6lzOkjtxNrajYzLs+lVZ1c5WQrM72sejkqlQ1cZjoKMNvZLRsNvQ5TG4X/lJ
bpN7v8jONgfLa+N1dyrP50j2cAl8UHmzLsmZVqWCiXxtfU+ZzOcOsjrpLXDMIZw29t7dgxPunYUz
3N21ylEPIW7tRBccLTRbMcYvau6GywjeVPn4qLksciZRxI0mz/i3U95mKuOv5Sdvt79p22GW0xxV
EsfFxq//u7eAY7ypfQZyq4QOcaKXGrlH3znPH76On2PX6dnqOalgzVSYt/rqH9bo0PpG9XhfLDlI
P/XNDX52NlF5R2hOekB5hPZXU9zsCLRRhoidcA8a2dZ50fpdSZRzCLfd7YGrS9nHezbhPlY/cYo7
ufW+bcyevelyZPGRv4fvtvjZyPyWPK/yh9a5VzdvcIYv3nU8Yu+CmdpIfbnoEf+ryF9W2zE6/NuV
CHXCeFnphtYnOL+e9kYNnoKLtXruWP33Y4cZyDUiNRBTLKrSnKXByH80LKWc69X/+/n8y1Pt2GJ7
0VZfuc8Cy8xV32bW21akLTwJefSL3NGHfdDbd76/fVyR/xpjvdjzdxj7xRo670d9wbd1/acD8AZh
++dJ4Ad/VAV7BhgS8ed19gdVv1dKWYdr4weBi0ZVi8UR/Nd7G7hhQcMbH5iBBCiCHDhb7hJ+vpd9
BZiCxXN/g9GCX3OC6OdgjoJkped0/NN16sdbIPhsHrN2WZY4Lecr6SVnN8h8zFKEqed+O2dQSngZ
TCiBB9dKglcOjgdpTwU1ENIlfWeFDoiDjGcPfBdSHsGDQAgbpyWEp+dyIahZwWd5bKdxULglieSG
CaiA6NInO5F7SsWHF0eB+PRGawh8vxdy08c+M6V6Z4JDIDZwYKdwhid8w0cqNZhXppcxehhNmCJg
oSiKo/9IitmUC7wmWtJFhmtGe5QIh76FJ7EYiAB3ibVYiTEIMdmjboLIfEaVPH8oEegDiMMojJq4
iWwHLZ6Ig1fYRLJGfrgoQLNofS6yh4dWKmLXhx8FjcvFdas4LGMIjtsod7dChfQnh/InjhGHSlPI
bHs2hIqXjtzohDToTD74efFoW2tFj8FEWglkjPiIjs/jgTfUj9TxawDZUETYht5oRMfyW6+IkBho
RrXXcQRpj+YDkRGpUlo4iIUYgeH4iBopj03VWM4oflwokt6GexoIH2/4Rf+Yki0jVDRmkpWGhjHZ
d0RlfFP3jhzminmIk+wEfXHYkxFUSpgYlBhYfEJXk2P/tmVJGXpT1nkHiE2A5mWceDtQGZUbCXhW
aQophJRaOY4apIg01ToOOTPsEZZiGV8Rh4cWZoqEWBPO9YO1x5Yg9Y0xppdZ9lPPxZP1d5eR1pJb
iA1HqFRT+ZKBiW23N5eYYIR6c5DwCJOKWZSflWMgEpeHcXOI6JhsSJnldlVdh4iqlk8i8paUoYqf
SUJjGXNHsktReD7PtJaqCXKp1pV3loU98RLTRptEJIatWZqt1EZu5EqT2ZvMeBs7c45cVYc7lprH
iUHyAxbLeV86aIE/5pnQWZuEJWlN6TXGqZ3ZqYLO550dFJ6+6Y7EVp7VBJ7nCZgYyXjImY02iZLu
WWsJ/9E7lVmf7GmfEsmVy0lO7fmZQsJuKOhivNef0OaSc5igXagyYhKQDeoEXik8FTSdDyihNrd4
4tmEGUpwazOb2Oih8xaZFcqf84miI9pvY8WQC6qixzhWnOd9L0o7O2kgIPmCNKpJB5aiJ6mjj5eX
P6pvKdSGKSWgQrpvwblVkvJUSDqkb7J8BnekTkp9tnNJDEqlO1qiiTIa3UWfWaprg+RxWAqmnAaW
2TilZQpL6XSPavptPPpEcemmc0qndWqnUmEbB+qad8qnfeqnf0qiU/QyWwqohWqoh4qoAjQOaZqo
jeqojwqpkSqpk0qplWqpl4qpmaqpm8qpneqpnwqqoRcqqqNKqqVqqqeKqqmqqqvKqq3qqlpZAAA7

------=_NextPart_000_010A_01C30DC0.84FD98C0--