piclist 2001\04\26\115656a >
Thread: Pin impedance
face BY : David VanHorn email (remove spam text)

At 11:54 AM 4/26/01 +0300, you wrote:
>What's your limiting resistor when discharging ?

Min value was 1k.
With the cap charged to 5V, that would give a max of 5mA

>  It should be fairly large
>(current under 1mA). The curve is not linear with time but you already
>know that.

Yes, but time constant = R*C. Even though it's non-linear, 2R crosses the
same threshold in twice the time.

>The intersect point with the threshold voltage will not change
>linearly with R, because of the shape of the curve. The RC timeconstant
>values are only valid for 10%-90% standard step measurements.

??? Never seen that expressed in a formula

>If you want
>a really linear input wire a NPN as a Miller integrator with your
>(smaller) cap and use that with your present circuit. I don't think that
>$0.01 for a BC547B will bankrupt your project. ;-) You can use RBPU for
>pullup. I'd recommend the double slope approach using a Schmitt Trigger
>input pin...

Not a bad idea, but the boards are already done.
This was working, but now using the MPLAB ice, it's broken.
I didn't expect such variability in the how the pins act.

Dave's Engineering Page: http://www.dvanhorn.org
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