On Mon, 31 Mar 1997, Jeff King wrote:

> 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,

I had the same problem. The experience showed that WDT doesn't help! You
have to use a brown out circuit!

If you are using a 7805 to get the VCC, you will also have to place a
diode (1N4148) between input and output pins of 7805, like this:

                ---------------
                |             |
                |   7805      |
        in      ---------------         out (5V)
        ____o____/      |    \__o__________
            |           |       |
            |          ---      |
            |          gnd      |
            |                   |
            |   |  /|           |
            |   |/  |           |
            ----|\  |------------
                |  \|

This is becouse the 7805 doesn't like very much reverse tension :-)

Dan