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CC2540 Battery Monitor Logic Issue

James Coleman
Intellectual 355 points

Hello,  I am using the following circuit to measure the battery voltage using the ADC of the CC2540.  The battery voltage range that I am measuring is between 3.6V to 6.0V.   I am using an npn transistor and MOSFET to control when I measure the voltage via the voltage divider.  The resistor values of the voltage divider are 64.9K and 15K.  For a 6.0V input voltage I expect the input voltage to the ADC to be approximately 1.126volts which it is.  When we enable the ADC to make a measurement the voltage input to the ADC increases to 1.980 volts.  Any idea why we are seeing this behavior in our circuit.  A PDF of schematic with battery monitor circuit is also attached.

Thanks,

James

Sheet 3.pdf

  • 0
    Prodigy 170 points
    I wonder if you are enabling the ADC correctly OR if somehow you are keeping the internal pull up of the GPIO active. My thinking is the internal pull up is active .

    Your input to the chip reduces to a 1.12V source with a 12k impedance in series. The internal pull up of the GPIO is ~22k and is supplied by VDD which you note to be 3.6V in the schematics. So we have 3.6V followed by a 22k pull up resistor, followed by a 12k input, followed by a 1.12V source. Based on that the expectation would be to see just below 2V appearing between the 22k and the 12k (the input of the chip). Do you have APCFG configured correctly, check the users guide?

    Regards,
    GoBK
  • 0 in reply to GoBK
    Intellectual 355 points

    GoBK,

    Thanks for the information.  We will take a look and let you know what we find.


    James

  • 0 in reply to James Coleman
    Intellectual 355 points

    GoBK,

    Our issue is fixed.  We did have the pull-up enabled on the GPIO that we are using for the ADC input.  We made the revision to the code and all is well.

    Thanks again for your help.

    James