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LM83: Local Temperature is read back Zero while Remote Temperature Sensing is fine

Francis_
Prodigy 40 points
Part Number: LM83

Hello, I have a problem getting a meaningful *local* temperature reading from LM83.  It is read back zero most of the time while the *remote* temperature sensing always works.  I checked I2C transactions on oscilloscope and was not able to catch any problems.  The interesting observation is that the read back value is not zero all the time.  Sometimes, it is 1 or 2 or 3 but it seems to be always in small values.  Any comments will be appreciated.  Thanks.

  • 0
    TI__Mastermind 31590 points

    Hi Francis,

    Could you please provide me the scope photo just for local temperature reading only?

    Aaron

  • 0 in reply to Aaron Heng
    Prodigy 40 points

    Hi, Aaron,

    The good one shows 26 degrees Celsius.  Please see attached Read_Local_Value__Good_26.png.

    The bad one shows 4 degrees Celsius.  Please see attached Read_Local_Value__Bad_4.png.

    The bad one shows 0 degrees Celsius.  Please see attached Read_Local_Value__Bad_0.png.

    They are all read back from the same register of the same LM83 chip.

    Thanks

    Francis

    Good 26 degrees Celsius

    Bad 4 degrees Celsius

    Bad 0 degrees Celsius

  • 0 in reply to Francis_
    TI__Mastermind 31590 points
    Hi Francis,

    Thank you for provding the scope photos. How many device attach to the same bus? According to your scope photo, the slave ID address that you sent to the device isn't specified in the datasheet. It looks like you are sending a 0x30, but the device only works with the following addresses 0x18, 0x19, 0x1A, 0x29, 0x2A, 0x2B, 0x4C, 0x4D, 0x4E in 7-bit. Could it be you were reading a different device that coincident connected to the same communication line?

    Could you please provide the schematic?

    Aaron
  • 0 in reply to Aaron Heng
    Prodigy 40 points

    Hi, Aaron,

    Please refer to the attached annotated image with all the bits labeled.  Note that the address that I used is 0x18.  Sorry about the poor resolution of the scope plot.

    Note that:

    A - ACK

    W - Write bit

    R - Read bit

    N - NACK

    And the correct temperature of 26 deg. C. is returned in this case.

  • 0 in reply to Francis_
    TI__Mastermind 31590 points

    Hi Francis,

    Thank you for decoding the waveform for me. You haven't answer me the number of devices on the bus. On the bad temperature reading, there are also glitches in the middle of the data read. It shoud not be there. If you take a closer look at the low side of the glitches, there is something happening. It looks like a bit higher than the rest of waveform.
     
    Aaron

  • 0 in reply to Aaron Heng
    Prodigy 40 points
    Hi, Aaron,


    You are correct that there is a suspicious glitch in the middle of the data readout. This gives me a direction to look deeper into this problem. I am still looking into it but I am quite confident that LM83 is not the root-cause of the problem.


    Thanks for your support, Aaron.


    Best regards,


    Francis