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LP877451Q1EVM: LP877451 WD can't be disabled

Ruijie Zhu
Prodigy 40 points
Part Number: LP877451Q1EVM
Other Parts Discussed in Thread: AWR2944

Dear Experts,

We are using LP877451-Q1 to power AWR2944 and set a pull-down resistor R14 on the  CS_SPI/WD_DIS of 877451.

The test results of our two batches of PCBA produced are different, but the designs and components are the same.

The waveforms of CS_SPI/WD_DIS are different, and abnormal PCBA's NRSTOUT will continue to output reset.

     

We believe that the reason for this issue is that WD has not been disabled on abnormal PCBA.

I have changed R14 to 2.2K/1k/200R/100R/0R, while the WD can't be disabled and the NRSTOUT always  reset, the waveform of CS_SPI/WD_DIS has not be changed.

   

Please help to see what is causing this problem, and how can we set the value of R14.

Thanks.

  • 0
    TI__Expert 8990 points

    Hello Zhu,

    Thanks for reaching out. 

    The 2.2K pull down resistor should work. Typical recommendation has been 4.5 K or less so, I think this is okay. One thing you must check on the abnormal board is layout. Is the R14 routed firmly to GND and to the CS_SPI/WD_DIS pin?

    And another thing is are you using the same devices on both PCBs? Please let me know the part numbers.

    Also if you could provide me following information related to PMICs on both PCBs that would be helpful whether are you using different NVM versions. 

    1. TI_NVM_ID 

    2. TI_NVM_REV 

    Best regards,

    Ishtiaque Panhwar

  • 0 in reply to Ishtiaque Panhwar
    Prodigy 40 points

    Hi 

    This is our PCB and we have a via to GND.

    The following figure shows the screen printing of PMIC on two batches of PCBA.

       

    Thanks.

  • 0 in reply to Ruijie Zhu
    TI__Expert 8990 points

    Hi Zhu,

    Thanks for sharing the information. 

    It seems that in layout R14 is grounded. Then only thing needs to be checked that doesn't that pulled up before going to sensing mode during start up. I see in the schematic PU is not populated but may be need to check as well. 

    Also from the top marking It's bit difficult to recognize the previous version of OTP so, if possible can you read the following resistors information.  

    1. TI_NVM_ID 

    2. TI_NVM_REV 

    Best regards,

    Ishtiaque Panhwar