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TMP75: The First Sample Triggers the Alert PIN and is then Cleared

Part Number: TMP75
Other Parts Discussed in Thread: , TMP1075

We are using four TMP75AIDGKT  temperature sensors on our current design which have their Alert signals (pin 3) connected to a single wire net. We seem to be experiencing an occasional erroneous ALERT signal for the very first temperature sample after power up.

I’ve measured this occurrence to approximately 1 in 10000 power ups on average whilst the board is at room temperature and no warmer than 35 Celsius. This can  be as many as 30,000 cycles and as low as a few hundred power cycles in any given test run, it seems to be a very random event.

The TMP75 is starting with its default values and so it would require the measured temperature to exceed 80C for the Alert pin to become active. No I2C reads/writes or MCU activity to the device take place until after this Alert Event has occurred.

I’ve attached a plot of the captured Alert Signal (Blue and Red Traces) to this thread below.

Would it be possible for the ADC inside this device to pass on a bad reading on rare occasions (due to environmental noise) and cause the Alert to be active for the 1st sample after power up?

Thanks

  • Hello Ian,

    If the issue is indeed due to an incorrect reading at power up, can you please try the following two things

    1. When the failure occurs, can the temperature data be read back to see what the value is which caused the Alert thresholds to be crossed and if it is the same value or changes on every failure? Also can you confirm the reset value of the configuration register when the failure occurs?
    2. What is the power supply ramp up time and is there a 0.1uF deCap on the device? Does the failure change with change of the ramp time?
  • Hi Ian,

    We're not aware of an issue like this with TMP75. You might consider trying a more modern drop-in replacement such as TMP75B or TMP1075 to see if they better meet your needs.

    Thanks,
    Ren
  • Hi Amit,

    Thank you for your reply.

    1) The readings (MS Byte) I get back from the 4 sensors after I see a power up alert are: 0x1F, 0x1D, 0x1F and 0x7F (31C, 29C , 31C and 127C)

    One of the sensors is getting a full scale reading after power up causing the Alert to be triggered and this is consistent. The Config Register should be in the default state but I haven't captured this yet.

    2) The power supply ramp up time is 2ms from 0V to 3.3V and there is a 0.1uF decoupling capacitor on the supply pins of each device with 0.6mm pad to pad spacing.

    I do see some supply noise when the rail reaches ~2.6V as another supply rail is turned on. The 3.3V drops by 130mV for 100us.

    The below table shows the I2C reads from the temperature register. I am also changing the config to Interrupt Mode (TM=1) to clear the Alert pin signal so a software workaround is possible.

    Thanks

    Ian

  • Hello Ian,

    Thanks for the data. I would think it is the 0x7F which should be triggering the Alert. If there is any other value that is triggering the Alert, then I would suggest running the experiment with a clean monotonic supply ramp instead of the glitch at power
  • Hi Ren,
    Given that we may be experiencing an issue due to power supply noise on ramp up, which device do you recommend as being the best for PSU noise rejection (TMP75B or TMP1075)?
    Also is the TMP1075 a lower price in volume?
    We are running the device from 3.3V Rail
    Thanks
    Ian
  • Hi Ian,

    I recommend TMP1075. Yes, our products are typically discounted in sufficient volume.

    Thanks,
    Ren
  • Hello Ian,

    Is there any further update on the same?
  • Hi Amit,

    Yes we're now issuing an I2C general call reset which clears the alert condition and then setting the Fault Queue to 6.

    This has resolved our issue with the TMP75.

    We're looking to use the TMP1075 for future designs

    Thanks for your help

    Ian