On Sat, Jun 01, 2002 at 06:56:34PM -0400, Michael A. Powers wrote:
> Hi All,
>
> I'm using the PIC 16F877's ADC unit to determine voltages, and I want a
> Vref+ of 24V.  Is this possible?

Nope.

>  The datasheet seems to say no, but I'm not sure.

Be sure. The answer is no.

>  The maximum value of Vref+ is stated as AVdd + 0.3, but I'm not sure
> what "AVdd" is.  What is meaning of the "A" in AVdd?

Not sure. But it doesn't matter. Here why:

PIC I/O pins have clamping diodes to Vdd and Vss. This includes the pins that
are used for A/D. The diodes will shunt excess voltage to their corresponding
rail. So it doesn't matter what AVdd represents.

>  Then again, the spec
> also says that Vref+ - Vref- can be no more than Vdd + 0.3.  Correct me if I
> am wrong, but it seems that I cannot use a 24V input direct to the ADC.

Right.

>  I suppose I'll have to use a voltage divider to get the input's max to
> something within specifications, but is there a better way?

Nope. It's the only way. Voltage divide into the range, then measure.

BAJ

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