Some points in response to no particular post but the question in general:- 1. The internal protection diodes have always been given only an "absolute maximum" rating. That means that when you pass *any* current through these diodes at all you are taking the part outside of it's reccomended operating conditions - chip not guaranteed to function normally. Keeping within the absolute maximum rating under these conditions guarantees only that the chip will not be destroyed and will function normally again afterward. Typically this value used to be +/-10mA, although interestingly I just noticed on the data sheet for a PIC16F886 it is now +/-20mA (both input clamp current and output clamp current). 2. In real world applications where input protection is required against over/under voltage you must consider what current, if any, will flow in the PIC's input protection diodes. If you pass *any* current through these diodes you must realise that you are immediately outside the bounds of what the data sheet describes and you are on your own. It is a grey area. Having said all that people can and do use these diodes, but, I repeat, results are not guaranteed. As a rule of thumb it *might* be reasonable to design such that under worse case operating conditions the current would be no more than say 10% of the absolute maximum rating. Then you would want to test this to give yourself some kind of confidence the PIC in question behaved acceptably under these conditions (no latch up, program glitches, etc). 3. The absolute maximum specification of (Quote from PICF886 data sheet) says "Voltage on all other pins with respect to VSS ....-0.3V to (VDD + 0.3V)". This is worth worrying about. It used to be -0.6V to Vdd + 0.6V on some if not all earlier PIC's. It is much, much, harder to design for +/-0.3V than 0.6V. I don't know why the apparent change of this spec. 4. The BAT54S is a nice little series connected dual schottky diode that is great for voltage clamping. It's small and cheap. But to prevent any current going into the PIC input at the all external protection diode must not exceed a Vf of 0.3V. For the BAT54S that means no more than about 0.5mA @ 25 deg C. That's not much to play with given it's a ~200mA part. 5. Other devices like MOV's, VDR's, TVS's, all have relatively large slope on the clamping voltage so not much use for this kind of precision clamping. Good for use closer to the real world signal but still need more protection before the PIC input. 6. A zener diode can limit +ve voltage pretty well, but -ve is a problem at -0.6V. 7. External protection circuits are inevitable, they do take up PCB real estate, add to parts and assembly costs, etc. There are many tricks and compromises but no simple circuit that works that in every scenario. 8. It would be very, very, nice if the PIC internal protection diodes were fully characterised and useable. If they have no spec how can we use them within spec? As has been mentioned the CMOS 4000B series chip have nice specifications on these diodes and that makes the chips a delight to design with. Need to interface 5V logic output to 3V logic input? Fine - add a series resistor. External protection circuitry for PIC inputs would be far, far easier to design if there was a useable current handling ability of the input protection diodes. 9. In reality, for now at least, input protection circuits will continue to be designed that inevitably result in some +ve/-ve current flowing through the PIC input protection diodes under certain conditions. The goal of the designer must be keep this current as low as practicable. -- Brent Brown, Electronic Design Solutions 16 English Street, St Andrews, Hamilton 3200, New Zealand Ph: +64 7 849 0069 Fax: +64 7 849 0071 Cell: 027 433 4069 eMail: brent.brown@clear.net.nz -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist