brucebeaty wrote: > From: "Artie Jones" >> If I use an RC circuit (1 meg ohm and 100 uF) for timing and a >> comparator with built in reference voltage (2V), am I am I violating >> the maximum input specs if I yank the power away from the >> chip while the IN- comparator input is still at 2V? I wouldn't worry about it at power down. The capacitor will discharge thru the protection diode into the Vdd supply. During that time the PIC doesn't need to operate, so this should be OK unless your Vdd supply is forced to 0 so fast that the 100uF discharge current becomes significant. Let's say your Vdd supply takes 100mS to drop from 1.5V to 0V. 1.5V * 100uF / 100mS = 1.5mA. That's probably OK, but if something is forcing the supply down quicker than that, then you may have a problem. The real problem however is on power up. If current is flowing thru the protection diode while Vdd is coming up, the whole chip could SCR. A common solution is to put a schottkey diode from the pin to Vdd so that the pin voltage can't exceed the schottkey forward drop above Vdd, which is less than the forward drop of the silicon protection diode. Unfortunately schottkey diodes leak, and with a 1Mohm source impedence this solution is probably not possible. You may need to step back and redesign the circuit to deal with this issue properly. > If you were to discharge a 100 mfd cap through one it would > probably destroy it, Yeah, Vdd would have to drop *very* slowly for that. Fortunately he's using a cap 1000 time smaller. ******************************************************************** Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products (978) 742-9014. Gold level PIC consultants since 2000. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist