At 11:58 PM 5/7/03 +0200, Stef wrote:
>hi All,
>
>I wondered if it was possible to create a DAC with just 1 ADC pin.
>The reason is that I have just a few pins and pwm is too slow.

Robert Rolf suggested a method to me many, many years ago: 100R from pin to
1u0 cap +.  Other side of cap is Gnd.  Output voltage comes from cap
+.  This is a high impedance output - buffer as required.

Read the voltage on the pin and compare to the desired value, then set the
pin as output (either HI or LO, as required) for a very short period to
'nudge' the voltage in right direction.  The voltage on the cap will track
the desired value.

The requires frequent reading of the ADC - stick the routine in your
background task.

It is possible to make this reasonably fast: make the output pulse width
wider as you get farther away from the desired value.  A simple subtract
and some right shifts to approximate the required delay time is all that is
needed.

I've been told this works quite well.

dwayne

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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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