Jason Harper wrote: > Does anyone have experience with using a 1 Farad or so supercap as > backup power for a PIC design? Nope, I note the other chap's unfavourable experience though. > I'm building a clock, and would like to have a few minutes of reserve > power to handle brief power outages, and to allow the clock to be > moved to a different outlet without losing the time setting. You'll need a power fail circuit to tell the PIC to shut off all outputs - pull down to zero is probably best as it avoids "floating" pins. > I assume I can't just connect the supercap across the 5V power > supply, as this would result in the /MCLR rise time being too slow to > reliably reset the PIC. (Or would it? There seems to be an inherent > limit to the charging current a supercap will accept.) So it appears. If you put a 47 ohm resistor (no diodes, at the low currents in question, the resistor will drop far fewer millivolts while powering the PIC) in series with the supercap, directly across the (bypassed) PIC supply, you guarantee no more than 106 mA draw anyway. But you'll want a brownout circuit too. > Do I have to worry about the supercap discharging through the output > of the voltage regulator? Yes. Not because the reference draws significant current at any voltage below 5V, but because the regulator may try to conduct current back to the input side and those other devices connected there. Careful examination of specs may find a suitable regulator, I can't recall offhand. > It seems like any reliable design might have to have a diode drop or > two between the regulator and the PIC, One diode in series with regulator output (in which case, can be the same regulator as powers the LED drivers anyway) can be balanced by a diode in series with the reference leg, or just use an LM317. > but I'm concerned that this might cause other problems. The circuit > will also include some TTL chips, You do mean HCMOS? > having the PIC's power supply be at a different voltage might cause > logic level problems. Half a volt shouldn't matter. But think in terms of an LM317 set to 5.6V with a 1N4004 from output to each part of the circuit. -- Cheers, Paul B.