I have a possibly odd question. I would like to try to use the MK20 TSI module to perform very small capacitance measurements. This of course implies some sort of calibration. While playing with the module I noticed something odd: The resulting count did not scale linearly with the reference oscillator current as I would expect.
Let me back up for a second. I set up the module and attach an external 100 pF capacitor. When the prescaler and the measurement counter are set to one each and both currents are set to 2 uA this gives 100 counts, since the reference cap is 1 pF. I would expect that increasing the reference current would increase the counts in the same way. This is however not the case. Going to 4, 6, and 8 uA gives an almost linear increase, but above that the count pretty much stays the same.
Am I missing something fundamental here? Is this behavior within the expected tolerances and I am demanding something to unusual? Or should I suspect that my controller's TSI module is faulty? I would greatly appreciate any feedback and will gladly provide more information if needed.
Thank you very much and best regards,