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Definition of "Clamp Current" spec please

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Electrojim
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2019/11/11 12:50:01 (permalink)
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Definition of "Clamp Current" spec please

I'm planning to use a 16F18855 in my very first PIC project, and have a question involving the I/O pins.  I'm assuming that, like most CMOS parts, there's an internal diode from these pins pointing toward Vdd and one pointing to the pin from Vss (ground).  Is the spec reference to 'Clamp Current' what one of these pins can safely absorb when a voltage outside the PIC's supply rails is applied from an impedance-limited source?  In particular, if one is using a higher voltage logic in other circuitry, and applies it to a PIC input pin through a high value resistor, will this compromise the PIC?  In my circuit, I have an op-amp output that can go a +9V rail, and maybe down to -1V under some circumstances.  Would a 10k resistor to the PIC pin protect the input?  I know that it would work with a 4000-series CMOS part, but using something like a hex inverter to cap voltages at 0 and +5V would be overkill if the PIC does the same thing.  Thanks!
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    mbrowning
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    Re: Definition of "Clamp Current" spec please 2019/11/11 13:08:25 (permalink)
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    Clamp current is an absolute max value above which damage will occur. The absolute max on all IO pins (but MCLR) is 0.3V outside the rails which will prevent any diode current.
     
    Although in many applications you can get away with it, operationally you should avoid any protection diode current. You risk raising Vdd above specification, and any current through the protection diodes will tend to result in poor accuracy of the ADC and other analog circuits.
     
    It is true that for old PICs the internal protection diodes were often used as you describe in various apnotes, but for modern process geometries this is not recommended.

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    ric
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    Re: Definition of "Clamp Current" spec please 2019/11/11 14:42:20 (permalink)
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    This
    mbrowning
    ...  any current through the protection diodes will tend to result in poor accuracy of the ADC and other analog circuits.

    Bottom line, if you are using the ADC, do not allow ANY current to flow through the protection diodes!
     
    A rail to rail opamp powered from Vdd and Vss would protect your PIC.
    You would have to determine if that would still work correctly in your analog circuit.
     

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