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DS90UB914A-Q1: 0x05 and 0x27 registers for impedance mismatch control

Yoshikazu Kawasaki
Expert 3510 points

Part Number: DS90UB914A-Q1

Hello,

My questions from my customer are related to the post shown below, but they're wondering about the 0x05 and 0x27 registers how they should use those.  Would you please give me your comments?

https://e2e.ti.com/support/interface/f/138/t/671002?tisearch=e2e-sitesearch&keymatch=DS90UB914A-Q1

1. Criteria for 0x05 register to use 0x00(default)

The datasheet says the followings, but would it be possible to provide criteria for using 0x00 by giving like insertion loss/return loss values?

0x00: Default
0xC0: Beneficial for shorter cable (< 6 meter)
applications that have system impedance
mismatch. Increases signal-to-noise ratio
(SNR) at low frequencies on forward channel to
alleviate impedance mismatch.

2. Changes in chip from 0x27=0x70

The datasheet says the followings, but what would change by 0x27=0x70 in chip?  0x05 register says SNR improvement, but would it also be SNR improvement?  They're worried if it might cause side effects by configuring 0x70, but wouldn't there be any side effects?

0x00: Default
0x70: Beneficial for longer cable (> 6 meter)
applications that have system impedance
mismatch on deserializer side.

Best Regards,

Yoshikazu Kawasaki

  • 0
    TI__Mastermind 30515 points
    Hi Yoshikazu,
    1. 0x05: for short cable, it would be better if you set 0x05 as 0xc0, but set 0x05 as 0x00 also can work.
    2. 0x27: if the cable is longer, 0x05 is set as 0x00, and 0x27 is set as 0x70 to better tolerate the link reflection due to impedance mismatch.

    regards,
    Steven
  • 0 in reply to Steven(Shenzhen) Shi
    TI__Expert 3510 points

    Hello Steven,

    Thank you very much for your comments.

    1. As you mentioned "set 0x05 as 0x00 also can work", 0x00 even for short cable of <6 meter should work, so what I'd like to know is the criteria for 0x00 or 0xC0 by insertion loss and/or return loss.  Is it possible to provide such values for <6 meter cable?

    2. As you mentioned "0x27 is set as 0x70 to better tolerate the link reflection due to impedance mismatch", but how will it improve the impedance mismatch?  0x05 register says SNR improvement, but what changes the internal of UB914A on 0x27 register by 0x70 configuration?  My customer is worried if it might cause side effects by configuring 0x70, but wouldn't there be any side effects?

    Best Regards,

    Yoshikazu Kawasaki

  • 0 in reply to Yoshikazu Kawasaki
    TI__Expert 3510 points

    Hello Steven,

    I'm sorry when you're busy, but I got reminder from my customer.  Would you please reply to me by tomorrow in your time?

    Best Regards,

    Yoshikazu Kawasaki

  • 0 in reply to Yoshikazu Kawasaki
    TI__Mastermind 30515 points
    Hi Yoshi,
    This setting has minor impact on the link quality based on our design and test, we provide one general guide for this parameter setting based on the real application.
    1. for shorter cable than 6m, it is recommended to set 0x05 to 0x0c.
    2. if the link is longer, generally the noise would become more due to reflection, loss, etc., and the SNR would be degraded. the reg. 0x27 is used here to better reduce this noise.

    best regards,
    Steven