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TMP468: Have to reset after connecting remote diode

Bailey Riley
Prodigy 40 points
Part Number: TMP468
Other Parts Discussed in Thread: TMP461

Hello, I'm using the TMP468 to measure the temperature from 8 remote diodes.  The only problem is that I have to disconnect the diodes periodically (there's a relay in the path to switch it to a different function) and when I do that, I always get back a temperature of 256C unless I either do a Reset (write to register 0x20) or I write to the configuration register (0x30).  I have it setup to read continuously, but can you confirm that if the diode is disconnected for a while (100ms or more), it will continue to read as if it is OPEN even after it has been reconnected and is not OPEN?  From what I can tell, it reads continuously until it encounters the diode being an open circuit at which point it quits reading temperatures from that diode until the chip is reset or reconfigured.  Is that right or am I doing something wrong here?  Thanks!

  • 0
    TI__Expert 6820 points

    HI Bailey,

    Can you try to clear the OPEN fault by reading the register 0x23 and see if that resolves your issue?

    Thanks,

  • 0 in reply to David Vaseliou
    Prodigy 40 points

    I was able to confirm that reading that register did make it where I started getting valid readings but that really doesn't solve my issue.  I want the chip to convert temperatures any time the diode is connected.  I have it in continuous mode so you would think that is what it would do.  Am I missing something?  It is hurting my throughput to have to kick off the reads any time the diode is disconnected.

  • 0 in reply to Bailey Riley
    TI__Expert 6820 points

    Hi Bailey,

     

    I understand your concern.  This behavior is done to make sure the user knows that an open fault had occurred.  It also is done help to protect the system in case of unintentional open faults.

     

    Remote devices such as the TMP461 do not have this feature.

     

    Thanks,

  • 0 in reply to David Vaseliou
    Prodigy 40 points

    Okay thank you, I thought perhaps I was just not using it correctly.  Is there any chance you can add a disable option to this in a future rev?  I really need 8 remote channels so this chip is a lot better than the other one.  

  • 0 in reply to Bailey Riley
    TI__Expert 6820 points

    Hi Bailey,

    To avoid this fault behavior you could either connect the D+ channel to the D- pin instead of leaving it open, or disable the channel before opening the relay and enable the channel after reconnecting.  

    Thanks,

  • 0 in reply to David Vaseliou
    Prodigy 40 points

    Ah okay, that first option sounds like it might be the best for my situation.  I currently have 2 relays connected to the diode, 1 makes the path to the TMP468 while the other goes to a different function.  Sounds like I should be able to add a 3rd relay that operates with the TMP468 relay so that when that path is Open, the D+ will short to D- for that diode.

    Can you confirm that in this method of operation, the chip will act continuously?  So when I open the channel, D+ gets connected to D-, I do some other stuff with the diode and then reconnect it to the TMP468, disconnect D+ from D-, I'll start getting valid readings again without having to make any register read/write?

    The only thing I guess I need to watch out for is relay switching times because I really need the D+/D- relay to operate first when Opening the TMP468-to-diode relay but last when closing it.  Is there any kind of spec available on how long the D+ channel can be Open before the TMP468 will notice it (and make me have to read/write registers to get it reading again)?

  • 0 in reply to Bailey Riley
    TI__Expert 6820 points

    Bailey,

    This is correct, if the channel is not seen as opened you can continuously read the channel without extra reads or writes. 

    So shorting D+ and D- before opening the diode, then reconnecting the diode and lastly releasing the short from D+ to D- will work.  

    There is no spec available on how long the D+ can be open before TMP468 will report an open fault. 

    Thanks,