At 07:28 PM 3/31/97 -0300, Mauricio Scaff wrote:
>I'm using a PIC 16f84. Right now, I am  having the following problem:
>When I turn off the power supply, the voltage in the power pins of the PIC
>starts to fall slowly as expected, because I have a 470uF cap. across the
>power pins(I need to keep the pic working for  at least 10 secs without
>power) . While the voltage is dropping, if I turn back the power on, when
>the voltage across the power pins is about 1.2V .. 1.5V , the PIc doesn't
>seem to work. Even a hardware reset  doesn't seem to solve the problem, so
>I must power down the circuit, short circuit the cap. and power it up
>again.
>
>Does anyone, have any clues about why this happens and how I can fix it.
>
>TIA
>
>Mauricio
>

You need a brown out circuit. That will give the PIC a hard reset when
the voltage falls below a certain point. There should be one in the
data book you have. It consists of a PNP transistor holding the
reset line high (or pulling it low) with the base either on a
resistive divider or on a zener diode. I use this circuit on
a PIC based product I sell.

Alternately, you can use the WDT to implement a software brownout.
There is also a ap note on this (but it slips my mind at the moment).

Regards,

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| Jeff King      Aero Data Systems |
| jeff@mich.com  P.O. Box 510895   |
| (810)471-1787  Livonia, MI 48151 |
|F(810)471-0279  United States     |
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