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TMP451: ALERT/THERM2 goes LOW immediately after power up

Oba
Genius 17295 points
Part Number: TMP451
Other Parts Discussed in Thread: DXP,

 

My customer is facing an issue on TMP451.

Now they don’t use remote sensor. Then DXP and DXN are connected together and also connected to GND as the datasheet(SBOS686A) mentions in P11 and P29 (Is this configuration, connecting DXP, DXN and GND, really OK?)

The issue they are facing is that ALERT/THERM2 goes LOW immediately after power up without any register configuration.

It mean all register value are default value.

In this case, why does ALERT/THERM2 go LOW?

 

 

Regards,

Oba

  • 0
    TI__Guru**** 244400 points
    Hello Oba,

    The default behavior of the configuration register is ALERT function is enabled. Hence by tying DXP and DXN to GND, the sample output from the remote sense may be causing the behavior. This is not a valid case for the device and I would suggest using a remote diode to actually test the function.
  • 0 in reply to Amit Ashara
    TI__Genius 17295 points

    Hello Amit,

     

    The datasheet says that

    “When not using the remote sensor with the TMP451 device, the D+ and D– inputs must be connected together to prevent meaningless fault warnings.” in P11 and “Tie the D+ pin to GND if the remote channel is not used and only the local temperature is measured.” in P29.

     

    So I don’t think that not using remote sensing is not valid case.

     

    The datasheet also says that “Short-circuit conditions return a value of –64°C.” in P11

    I think it means remote temp register value is all 0 in this case.

    Remote High and Low are also all 0 because any registers are not changed yet.

    So I think ALERT should not go low in this condition.

    But It goes low. Why does it go low?

    Regards,

    Oba

  • 0 in reply to Oba
    TI__Guru**** 244400 points
    Hello Oba,

    Agreed. Going through the datasheet it does not seem that the ALERT/THERM pin should go low at power up. I would need the following data points to help the customer debug

    1. Scope snapshot of the Power Up and ALERT/THERM2 pin going low. What I need to say is what is the delay from power up to the pin going low?
    2. Once the pin goes low, can they do a read of all the registers to see what the values are?
    3. Is this happening on all boards or only a small number of boards in a sample set?
  • 0 in reply to Amit Ashara
    TI__Genius 17295 points

    Hello Amit,

    I got TMP451EVM and tested it.

    Then I confirmed the same issue. When reading the status register, I found RLOW=1.
    All other registers are default value.

    Could you please also test with TMP451EVM and please explain why RLOW=1?

    Also could you tell me how to solve this issue?

    The blow is waveform at power up. CH1(Yellow) = VDD, CH2(Blue)=ALERT/THERM2

    Regards,

    Oba

  • 0 in reply to Oba
    TI__Guru**** 244400 points
    Hello Oba,

    Let me check the behavior. However as per my last post can you please provide the following information

    >> Once the pin goes low, can they do a read of all the registers to see what the values are?
  • 0 in reply to Oba
    TI__Guru**** 244400 points
    Hello Oba,

    I have reproduced the issue on the TMP451EVM. After debugging it with the team, that is expected. The reason is as follows

    When the D+ and D- are connected together, the device reads the code for -64 °C (which in standard mode is 0x000) and then compares the value with the remote limit register which is 0x000 as well. This results in a comparison flag for RTLOW being set. Now w.r.t the behavior, at power on, the device starts a conversion and the results for the conversion are available after ~34ms. Hence with RTLOW being set, the ALERT pin becomes 0 after 34 ms of power on.
    The Alert function is enabled by default and hence cannot be masked out unless

    1. The MCU programs the same before the 34 ms conversion time
    2. The actual remote diode/transistor is used instead of shorting it if unused.

    I would like to hear your thoughts as well. At this point if they are not using the remote sensor, then we need to move the customer to a non-remote sensing device. Please advise.
  • 0 in reply to Amit Ashara
    TI__Genius 17295 points

    Hello Amit,

    Thanks for your reply. The datasheet says RLOW is set when the value is BELOW the limit value like the below figure.

    But actually it is set when the value is less than or equal to the limit ?

    And sorry, I can't understand the below your suggestion. What does it mean?

    1. The MCU programs the same before the 34 ms conversion time

    Regards,
    Oba

     

  • 0 in reply to Oba
    TI__Guru**** 244400 points
    Hello Oba,

    Yes, I checked with the team. The condition is less than or equal to.

    The suggestion is to not use the Alert function. This can be done by the MCU if it disables the Alert function in the configuration register before the first conversion is complete.
  • 0 in reply to Amit Ashara
    TI__Genius 17295 points
    Hi Amit,

    OK, I understand...

    Regards,
    Oba