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Original question:

CC2650 capacitive sensing feature

CC2640: Capacitive Sensing in Sensor Controller CC2640

Srinivas H
Intellectual 540 points

Part Number: CC2640

Hello all,

I am working on CC2640R2F launchpad sensor controller. I found one example Capacitive Touch data Logger. From the documentation provided in the help, i understood that the time difference is calculated from two rising edge outputs of comparator.  This will be helpful in detecting the touch.

My question is, Is it possible to measure the value of a fixed capacitor if i connect a fixed value capacitor instead of touch sensor. I tried connecting a 330pF capacitor to the AUX IO pin on Launchpad board. And when i perform task testing using SCS, the tdc value is a constant. And when i remove and re-connect the capacitor back to the same pin, now the constant tdc value is different from previous measured tdc value. Why is it so? (It should be the same time difference for same capacitor value right?. Please correct me if i am wrong )

Is it really calculating the time difference by charging the capacitor and measuring the charging time [using CompA, TDC and ISRC] OR any other method ?

Please help me.

Best Regards,

Srinivas

  • 0
    TI__Guru**** 317180 points

    Exactly which variable do you look at?

    The algorithm used is designed to detect a touch (change in cap) and will therefore dynamically take into account changes in the baseline value. A SLA in this will included in the Q4 SDK release. The below is from the introduction:

  • 0 in reply to TER
    Intellectual 540 points

    Hi TER,
    Thanks for the quick reply. I am looking at pTdcSmplWindowMean value and pTdcValueRaw value in Task Testing. This value is changing whenever i change the capacitance value. And every time it is giving different value for the same capacitance. it should give same time difference every time ? (will it be the same or change every time ??)

    Please help :)

    Best Regards,
    Srinivas

  • 0 in reply to Srinivas H
    Intellectual 540 points
    Hi TER,
    I debugged the problem. I figured it out why pTdcValueRaw is constant. Stop trigger wait time for TDC is very low (10s of microseconds) in the code. I increased the tdcWaitUs time bit more (1000s of microseconds) . Now I am able to detect the change in capacitance.

    Thank you for the help :)

    Best Regards,
    Srinivas