Negative input

[javi.ar]

Hi everyone:
One question what would I read if a digital input i aplly -5v ?
the second one would I be able to read negative values with ADC ?

Thanks a lot, sorry for the questions but I am still learning...

[SherpaDoug]

If you REALLY pull the PIC pin down to -5V you will probably destroy the PIC. Fortunately the PIC pins have protection diodes to make this difficult. Read about "Input Protection" or "Protection Diodes" on the datasheet. Also read Microchip application note AN521 to see what can be done with these diodes.

To read negative voltages on an A/D pin you need to convert the voltage to positive. Often this is done cheaply by connecting two equal resistors (maybe 1K) in series from the negative signal to the A/D +5V supply. Then a -5V signal gives 0V to the A/D, a 0V signal gives +2.5V, and +5V gives +5V.
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[rnielsen]

Check out the electrical specifications for the part you are using. Most of them state that an input LOW can be VSS-0.3V. You can't measure a negative voltage, directly, with the ADC. If you're not sure about something, read the specification sheet first!

Ronald