Hi Glenn,

Off hand, I would think it unlikely that a PIC with an iffy power supply
voltage level would suddenly be able to get off an I2C signal to an EEPROM
which was good enough to cause a problem, but probably the safest thing to
do would be to make a brownout circuit. This could detect the voltage
dropping before it actually got low enough to cause problems, but it would
hold the PIC in reset until the power dropped too low for the PIC to
execute code at all.

Trying to bleed the caps off would,I think,be an exercise in futility since
the PIC can execute many instructions in just a few 10s of uS and it would
be hard to discharge those caps in less than a millisecond without normally
drawing way too much current from them (to discharge from 5V to 4V in one
millisecond would require a current drain of 2.2 amps).

I'm sure someone from the list could offer a tested brownout circuit. I
have not used one yet on a PIC so I don't offhand have one that I could
give to you and say "yes, this WILL work". I think there are a few examples
in mchip's datasheets or appnotes.

Good luck,

Sean

At 04:50 PM 2/6/99 -0800, you wrote:
>Hello all,
>I have a circuit (it contains a pic) which has a couple of rather large
>aluminum electrolytic capacitors (2200uF) as supply filters. the problem
>is that when the circuit is turned off, the charge takes a little while to
>dissipate (i have a power on led in series with a 1K resistor across the
>supply lines) and while its dissipating, the pic among other things is
>still funtioning. I am worred that the pic might end up in one of my
>routines which, for example writes to an I2C EEPROM, and corrupt the data
>there. so what i need is a way to disipate the capacitor reletively
>quickly after power down. I was thinking of putting a diode in paralel in
>the wrong direction (anode to gnd, cathode to vcc) but i dont think this
>will work, since the charge in the capacitor is polarized. I hope someone
>has an idea.. thanks agian
>
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>A member of the PI-100 Club:
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|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
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