Watch out for leakage, too. I cannot use Tantalum caps on any of my designs because the leakage (1-5 uA) severely impacts battery life on a device that draws an average of 32-65 uA total. Digikey now carries large value (to 10uF at 6.3V) caps in 1206 surface mount ceramic (X7R and X5R)--be sure to avoid the Z5U temperature tolerance devices like the plague! Most of the data sheets on the Z5U imply +/-20%--but its really +20 -80%--that -80% is a killer! Kelly At 08:42 PM 1/29/00 -0500, you wrote: >Does anyone have experience with using a 1 Farad or so supercap as backup >power for a PIC design? 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. 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.) Do I have to worry about the >supercap discharging through the output of the voltage regulator? > >It seems like any reliable design might have to have a diode drop or two >between the regulator and the PIC, but I'm concerned that this might cause >other problems. The circuit will also include some TTL chips, powered >separately so that the supercap won't try to maintain them: having the >PIC's power supply be at a different voltage might cause logic level >problems. > Jason Harper > > William K. Borsum, P.E. -- OEM Dataloggers and Instrumentation Systems & San Diego, California, USA