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enter image description hereI have looked at the spec sheets of this chip and it states that MAX Voltage on any pin with respect to VSS (except VDD, MCLR. and RA4) = -0.3V to (VDD + 0.3V). They also state that MAX Input clamp current, IIK (VI < 0 or VI > VDD) ± 20 mA.

Where I work our design actually has us bringing 20VDC to 30VDC to RA5 at < 1mA. Our boards work fine for the most part, we do have issues where there are weird offsets being written to the microcontrollers, I nee to figure out why. I am wondering if this 20VDC - 30VDC (at low current) to a pin would cause this. I've contacted MicroChip with this question and have had no luck.

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  • \$\begingroup\$ Without knowing a whole lot more about your circuit, and/or at least the fault symptoms you're seeing. I doubt we can help you. Have you done an experiment where you stop applying out-of-spec voltages to the part and see if the problem goes away? \$\endgroup\$ – The Photon Mar 4 '14 at 17:36
  • \$\begingroup\$ The errors we're seeing is information being written to the chip. I would compare it to values being written via In-Circuit Serial Programming on it own. \$\endgroup\$ – becjasl Mar 4 '14 at 17:58
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    \$\begingroup\$ Where are you getting the numbers for the current applied to the two inputs? I see 20 V / 47 kOhms = 0.4 mA, and 30 V / 2 MegOhms = 15 uA. What situation causes 2 mA or 6.7 mA to be delivered to the micro in this circuit? \$\endgroup\$ – The Photon Mar 4 '14 at 18:03
  • \$\begingroup\$ You know what, you're right. I'm not sure where I got those values. I did just verify with my meter also. Sorry for the misinformation. but regardless, 20VDC / 30VDC at .4mA or 15uA, is that an acceptable design, in your opinions. \$\endgroup\$ – becjasl Mar 4 '14 at 18:21
  • \$\begingroup\$ Just a note, you don't need to add a signature to the bottom of your posts. Your user info is added automatically. If you'd like to be known by "Jason" instead of "becjasl," just change your username accordingly. \$\endgroup\$ – JYelton Mar 4 '14 at 21:21
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The clamp current rating is a guarantee that the chip will not be damaged by latchup. It is not a guarantee of proper operation. You should not exceed the rails by more than a few hundred mV in normal operation.

In particular, operation of ADCs and analog functions can easily be affected. If that is what you mean by offset (voltage rather than address offset) it could easily be your problem. What happens is that some of the clamp current spills over into other inputs and causes a current to flow into unrelated input pins.

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You shouldn't use the input protection circuits as the normal means of limiting input voltage on a pin. They are there only as an emergency protective device.

I haven't looked at the PIC datasheet, but from other IC specs, I think that the MAX values you mention will be found in a table headed "Absolute Maximum Ratings" or words to that effect.

Atmel's Absolute Maximum Ratings table includes the following note:

"Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or other conditions beyond those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability"

I would use a voltage divider and a 5 volt zener diode to limit the voltage applied to the IC to the recommended values.

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