Hi!

I use a PIC 16F84 with a R/C Circuit connected to a port to measure the
resistor in it. (will be variable in future)

I use the formular R=t / (C * ln (U0 / UC))

where R = resitance in Ohms
C = capacity in Farad
U0 = Voltage on full charged Capacitor
UC = Tereshold

With the configuration of R=1k and C=1u it works fine but with R=3k and C=1u
the result (time) is to high.

Here're some results i get:

calculated: 1k the dischage time until the thereshold is reached sould be
1,370 ms, and with a 3 k it sould be 4,112 ms. (U0 = 5.12 Volt, UC = 1.3
Volt)
Measured with scope i become the following results: with the 1k resistor
1,389 ms (more or less perfect) and with the 3k around 5,324 ms.

Does anyone know a correction method to fix this or a better/other formular?
Or a other reason which explains the vary of the measurement?

TIA

cu, Doc ...

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