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LMX2485E / Abnormal Ftest/LD output after short power off

S.Satoshi
Guru 21675 points
Other Parts Discussed in Thread: LMX2485E

Hi,

My customer found strange output from Ftest/LD after short power off. Please see below and give your comment.

[Behavior]

  • Ftest/LD output High before setting register so PLL looks locked when Vdd is turned off and on with short duration

[Procedure]

  • Power on and set register, then they can confirm desired operation.
  • Power off for 1sec and then power on again. at this time the Vdd goes down to 0V absolutely.
  • After power on, Ftest/LD pin output High before setting register.

Can you please give your comment why such behavior can be seen after short Vdd turned off?

Best Regards,

Sonoki / Japan Disty

  • 0
    TI__Mastermind 27581 points
    Sonoki-san,
    The LMX2485E does NOT have a power on reset. This could cause the Ftest/LD pin to go high and you may find a particular date code of device may behave a certain way, but this is not something I would assume all devices do.
    Regards,Dean
  • 0 in reply to Dean Banerjee
    Guru 21675 points

    Hi Dean,

    Usually the register in devices is set to default value by POR, but LMX2485E has no POR. Then my customer is asking how LMX2485E can set  to default value without POR. Can you please give your comment about this question?

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Mastermind 27581 points

    The LMX2485E does not have a POR.

    To answer your question, here are 2 considerations:

    1.  In many of our datasheets, we use the word "default" that we recommend the user to program, as opposed to a value that the device automatically sets.

    2.  On the LMX2485E, registers R0 - R3 are never set by the part and MUST be manually programmed by the user.   If we use the word "default" do describe anything about these registers, we really mean "recommended".    However, on registers R4 - R7, the ACCESS word in register R3 allows the user to force these registers to a value without programming.   The intention of the ACCESS word is to make it so the user only has to program 4 registers as opposed to 8, but has the option of programming all 8 for added functionality.   But in any case, the ACCESS word needs to be programmed itself to use this functionality. 

     

    Regards,

    Dean