All due respect, but I here lots of lore about "accepted practice" but nothing concrete. You wrote about external protection diodes as "in the spec.". Where did you see this? I've searched the data books over and over and have never seen anything. In fact on microchip's web site, AN521 "Interfacing to AC power lines" uses only a current limiting resistor to connect a PIC input directly to 110VAC power line. Where can we see some hard facts? Regards, Jim Hartmann Russell McMahon @MITVMA.MIT.EDU> on 10/09/1999 07:38:27 AM Please respond to pic microcontroller discussion list Sent by: pic microcontroller discussion list To: PICLIST@MITVMA.MIT.EDU cc: Subject: More on (ab)use of PIC protection diodes & bad design Re allowing current to flow in PIC protection diodes, various people have said- > The "specification" is an Absolute Maximum Rating. The only thing these > ratings are good for is to ensure the PIC won't burn out, i.e. suffer > permanent damage. > There is no specification for a current through the > protection diodes durring operation that won't disturb proper operation. > The closest they come to specifying that is specifying the input voltage > ratings. We'd have to assume that the maximum "normal operation" current > allowed through a protection diode is zero. /Mike is absolutely right. / /EXTERNAL clamps are needed to prevent the reverse protection /diodes being biased on, for normal operation. / /Thanks for clarifying. / /-Barry. /------------ All good stuff. MANY people over time on this list have supported the use of the PIC's internal diodes to clamp signals to an acceptable level. DON'T DO IT. The PIC is guaranteed "safe" at up to 20ma protection diode current. The PIC is NOT guaranteed to work properly with ANY protection diode current. It will often (even "usually") work "OK". It is unacceptable design practice. It is not guaranteed to work. There is no certainty what the results on the processor will be. Sometimes a whole design will fail because of it. Thjere will often be no indication of what is wrong. The "proper" way is to externally limit the voltages external ti the PIC as noted above. The easiest (not cheapest) way to do this is to use series input resistors to limit input current and to place 2 Schottky diodes (eg BAT85 or similar) from the PIC input pin to supply rails so that they conduct before the body/protection diodes. This is in spec for the PIC. The major danger here is that if the input voltage is high and the input resistor small so that the input current to the Schottky diode is higher than the PIC supply current then the PIC supply may be "pumped up" by the input current. This can be overcome by placing a suitable zener across the supply (say 5v6 for a 5v supply). Note that a standard 3 terminal regulator will NOT prevent this happening - they will just shut down and let the rail voltage rise. There are other ways of clamping input. A cheaper and not perfect but very tolerable way is to use two input resistors in series and clamp the mid pount as above with eg 1N4148 diodes. This is much cheaper than using Schottkys but still potentially allows a "whisper" of body diode current (microamps). This is also techniucally bad design but much better bad design than not doing it at all :-). Next come zener clamps or diode clamps to reference voltages just below supply and above ground but these are more complex and arguably unnecessary. regards, Russell McMahon _____________________________ >From another world - www.easttimor.com What can one man* do? Help the hungry at no cost to yourself! at http://www.thehungersite.com/ (* - or woman, child or internet enabled intelligent entity :-))