At 18:22 20/05/97 -0400, you wrote:
>AN 513 discribes a repeatable method of doing an analouge conversion with a
>device that is digital only. The method used is based on time measurement. I
>have used this method on commercail applications but found it more reliable
>if the capacitor charging is acheaved with a variable current as opposed to
>variable voltage. Using current charging reduces dirft and increases
>accuracy. It is also possable to do the conversion with one pin (remember the
>555) but a small r should be placed in series with the pin to avoid excessive
>current when discharging the timing capacitor. The circuit used was a fixed
>resistor an ORP12 and a 470N capacitor with 100 r in series with the pin.
>this gives a repeatable time period baised on ambient light level on a pic
>54.
>
>princable of opperation
>1 pin to output
>2 pin low
>3 start counter
>4 pin to input
>5 test pin and branch if high
>6 inc counter and goto 5
>
>this gives repeatable results and is very simple
>
>Steve..........

I would add one refinement. Use another pin to switch between a known
current source (precision resistor to Vcc) and the signal. This allows
calibration and makes the results less sensitive to temperature and gate
threshold voltage.

Keith.
==========================================================
Keith Dowsett         "Variables won't; constants aren't."

E-mail: kdowsett@rpms.ac.uk
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