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'External D/A'
2000\01\17@152519 by David E. Olson

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I've got a project where I'm doing some porta readings and portb settings
and I also have to monitor a threshold from a hall sensor (gear tooth). From
what I've researched, the code necessary to process the sensor data and
monitor the threshold point would seriously effect my current processing.
The hall sensor is not linear so I'd have to D/A it and wait for my
threshold. Considering that I don't want to D/A in my existing code can I
use one of Maxim's chips to do the D/A and then have my PIC code poll the
D/A chip/circuit? I think one of Maxim's can be serially accessed however,
just pushing a port high when my threshold has been met would be easier.

I'm using an F84 now and I'd like to keep it in the 18pin range.

-DO

2000\01\22@185700 by Mark Willis

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Using an op amp as a comparator would do this pretty readily, just use
series resistor to a back-to-back diode pair to clip the signal if your
Op-Amp is running off a supply other than Vcc/Ground.  20-turn pot or
something to set your other input of the op-amp as the threshold, and
you should be set.

Alternatively, set the threshold with an RC low-pass filter off the
input, perhaps with a zener to pull the cap up most of the way towards
the set point, to make startup quicker <G>  (A PIC pin could do this
"precharge", alternately.)  Any time constant large enough to be over a
90 or so degree phase delay "will do", especially if the op amp's set at
'infinite' gain =)

I Am Not A Linear Expert, this's better than no answer to your question
though...

 Mark

David E. Olson wrote:
{Quote hidden}

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2000\01\22@202241 by David Olson

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Thanks Mark.

This gives me something to think about. Since I'm not really doing any
"serious" D/A here - just looking to make a logic high or low depending on a
threshold, I'd rather keep it simple.

-DO

{Quote hidden}

2000\01\22@204803 by Mark Willis

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The other thought, of course, is to use a Schmidt Trigger input of the
PIC, and use the Op Amp to level shift/amplify the incoming signal to
"feed" the schmidt trigger input properly, and there are a gazillion
other good ways to do this <G>

 Mark

David Olson wrote:
>
> Thanks Mark.
>
> This gives me something to think about. Since I'm not really doing any
> "serious" D/A here - just looking to make a logic high or low depending on a
> threshold, I'd rather keep it simple.
>
> -DO
> <snipped>

--
I re-ship for small US & overseas businesses, world-wide.
(For private individuals at cost; ask.)

2000\01\22@205846 by David Olson

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I thought of this earlier but I was missing the Op Amp piece. If the output
of the hall sensor was a linear increase in voltage, then a Schmidt trigger
would work nicely. However, my understanding (before I scope it) is that the
output of a gear tooth hall sensor was of varying frequency.

I still need to scope the output of the hall sensor to see that's going on
when I need the event to trigger. It is a 12v sensor so I'm going to have to
do some "conditioning" anyway.

-DO

> The other thought, of course, is to use a Schmidt Trigger input of the
> PIC, and use the Op Amp to level shift/amplify the incoming signal to
> "feed" the schmidt trigger input properly, and there are a gazillion
> other good ways to do this <G>
>
>   Mark

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