Thanks, The 16C621 Absolute Maximum Ratings says "Voltage on any pin WRT Vss : -0.6V to VDD+0.6V". I think there is some confusion as to what the DC Characteristics "Input Low Voltage" and "Input High Voltage" mean. I believe that these specifications do not dictate limits to what can be applied to the pins. These specifications dictate what Microchip guarantees will be the thresholds of the input logic circuit. An input voltage will produce a zero input guaranteed if the input voltage is VSS. But the threshold could be as high as 0.8V maximum (ttl buffer). Anyone care to differ? Jim Hartmann Alan Pearce @MITVMA.MIT.EDU> on 10/11/1999 10:38:22 AM Please respond to pic microcontroller discussion list Sent by: pic microcontroller discussion list To: PICLIST@MITVMA.MIT.EDU cc: Subject: Re: PICs: Good Design More on (ab)use of PIC protection diodes & bad design >Would you kindly point me to a page and line where it says that a negative >voltage WRT Vss on a pin violates the operating conditions? I do not have the data sheet for the 16C622 which I believe you are using, but I looked up the sheet for the 16C6x (DS30234D) and page 183 lists "Maximum ratings any pin to Vss as -0.3V to Vdd+0.3V, i.e less than a diode voltage drop outside the supply rails. This is listed the same through subsequent pages for the various models of this PIC. Page 186 gives "Standard Operating Conditions" i.e. the conditions it would expect to operate under as Parameter D030 - Input Low Voltage Min Vss - not (Vss-0.3V) The next parameter is "Input High Voltage" Max Vdd - not Vdd+0.3V. the following pages list the same for the other models of this family. I also checked the data sheet I have for 16F87x (DS30292A) and this gives the same set of voltages for the same parameters. I repeat that "Maximum ratings" are the limits that will not destroy the device, but are not "standard operating conditions". All micros, digital and analogue ICs, transistors diodes and practically any component I can think of is specified this way. An application note which shows a resistor being used off the mains to drive a pin does not necessarily give licence to use a chip like that. A collegue I worked with always said "It is not what is in the circuit on the application note, but what they left out that is important", and he is right. The device used in the app note may have happened to work because an adjacent pin wasn't used, or the chip was a preproduction one that had a different mask pattern, and the circuit designer was lucky and got away with it. It may even be that the circuit he used had the same problem you do, but the circuit that got printed did not show the mods used to get around the problem.