On Mon, 25 Mar 2002 11:15:30 +0000, Dave Dilatush wrote:

>Bob Barr wrote...
>
>>For some reason, the ICD self-test fails indicating a bad MCLR
>>voltage. It's indicating that my Vdd is 6 volts and Vpp is 17 volts
>>even though they measure at 5.02 and 12.98 volts respectively. As long
>>as I know that it's only the software's readings that are off (and as
>>long as the ICD's working), I'm not going to concern myself about
>>this.
>
>You should be able to bring these readings back into line by
>adjusting the ratio of the R1/R2 voltage divider if getting an
>LED with the right forward voltage isn't convenient.  Just
>substitute a pot for either one of these resistors and adjust it
>until ICD reports values that are close to what you're seeing on
>the DMM; then measure the pot and plunk in a fixed resistor of
>the same value.
>
>I haven't done this, but looking at the circuit it appears you
>should be able to do that.
>

Dave (and Allen too),

Thanks for the suggestions. I'll look into them. It sounds like it
will be easy enough to get the software readings into line with
reality.

It surprises me that a designer would use an LED's forward voltage as
an ADC reference voltage.=20

I work a bit on simple hardware designs (more at a 'hack-at-it' tech
level, rather than a professional level). Fortunately, I have access
to a real design engineer to keep me out of any serious trouble. :=3D)
I'm certainly open to correction but I really don't like this
implementation.

Since the designer specified the LED to be used, he obviously knew its
Vf spec but using it in this way just doesn't sound like a good idea
to me.=20

The only specs I've seen for an LED's Vf are the maximum and 'typical'
values. Is the Vf among the same type of LEDs from different batches
really consistent enough to trust as a reference voltage? How about
differences among different LED manufacturers for the same type of
LED?

Please feel free to flame away if I'm missing something brain-dead
obvious.


Regards, Bob

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