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DS90UB953-Q1: How to strap into Coax mode on power-on?

Christopher Buck24
Intellectual 455 points
Part Number: DS90UB953-Q1
Other Parts Discussed in Thread: DS90UB954-Q1, ,

Hello,

I have a design which implements the DS90UB953-Q1 however I am unable to establish a link over coax with the deserializer (DS90UB954-Q1) upon power-on due to the DS90UB953 powering up into "TP" mode instead of Coax mode.  I am having trouble locating any information in the datasheet which describes how to strap coax mode.  Curiously, this TP/coax mode configuration is not an issue when I use the DS90UB953-Q1EVM, as it appears to strap into coax mode.  Upon further inspection it appears the EVM is not using suggested strap resistors for neither the MODE nor IDX divider network.  Is this the reason the evaluation module is able to strap into coax mode?  Can TI please provide documentation for how to replicate this functionality?

Thank you,
Christopher

  • 0
    TI__Mastermind 30515 points

    Hi Chris,

    regarding the coax or STP mode, for 953 and 954 link, the register setting doesn't impact their performance. that is to day, with same setting, 953/954 can work at either coax or STP mode. You can refer to TI's new 954 d/s in www.ti.com,, the mode pin strap setting had removed the coax/stp setting. in reg. 0x6D, the coax_mode can be set either 0 or 1 in your STP mode.

    best regards,

    Steven

  • 0 in reply to Steven(Shenzhen) Shi
    Intellectual 455 points

    Hi Steven,

    Thank you for the response.  I went and tested my setup again, and found that despite what you're saying, my connection is dependent on the 0x03 register setting of the 953.  On power up, there is no link lock.  On the 953, I set register 0x03 bit 7, then bit 6, and only then do the 954 and 953 achieve link lock.  As soon as I change 953 register 0x03 bit 7 back to zero, the link goes down and loses lock.  Therefore, I am fairly convinced the 953 0x03 register does impact performance.

    I also tested the register you appear to be referring to, the 954 0x6D register.  Specifically bit 2.  This register does not appear to make any difference--the same behavior exists where the link cannot be established unless 953 0x03 bit 6&7 are set regardless of the value of 954 0x6D bit 2.

    Can TI please advise how to strap DS90UB953 into coax mode?

    Thank you,
    Christopher

  • 0 in reply to Christopher Buck24
    TI__Mastermind 30515 points

    Hi Christopher,

    thanks for update. your test result surprised me :(. In our WW customer, most customers select coax cable as 953's output phy medium, and don't see any issue here.

    For your issue, may I know more details on your application conditions? 1. what is the clocking tree in your case? 2. what is the cable type and length, what is the S11/S21?

    in our 953 d/s, you can see 0x03[7:6] are reserved, which is not needed be set here. If we recall our initial design, you can check below strapped items from IDX pin, the origin has coax option but this setting is removed in our finally released part. If you still need set 953 in your "coax mode", you can try set 0x03[7:6] = 2b'11, these two bits are described as below register note.

    Table1. original IDX's strap mode with coax but it is removed in final version

    #

    Ratio
    VR2/VDD18

    Target VR2 (V)

    Suggested Resistor Pull-Up R1 (1% tol)

    Suggested Resistor Pull-Down R2 (1% tol)

    I2C Address

    Cable

    I2C Supply

    1

    0

    0

    OPEN

    40.2

    0x30

    COAX

    1.8V

    2

    0.208

    0.374

    118

    30.9

    0x32

    COAX

    1.8V

    3

    0.323

    0.582

    107

    51.1

    0x30

    STP

    1.8V

    4

    0.44

    0.792

    113

    88.7

    0x32

    STP

    1.8V

    5

    0.553

    0.995

    82.5

    102

    0x30

    COAX

    3.3 V

    6

    0.668

    1.202

    68.1

    137

    0x32

    COAX

    3.3 V

    7

    0.789

    1.42

    56.2

    210

    0x30

    STP

    3.3 V

    8

    1

    1.8

    13.3

    OPEN

    0x32

    STP

    3.3 V

    Table2 0x03[7:6] description

    0x03 MODE_SEL          
        7 FPD COAX MODE_OV RW 1'b0 0: Cable Mode setting from the strapped IDX pin
     1: Register Cable mode overrides strapped value
        6 FPD COAX MODE RW 1'b0 Cable Mode configuration
     This field always indicates the current value of cable mode. When bit 7 of this register is 0, this field is read_only and shows the strapped cable mode. When bit 7 of this register is 1, this field is read/write and can use to set the cable mode configuraiton
    0 : STP mode
    1 : Coax mode

     

    regards,

    Steven

  • 0 in reply to Steven(Shenzhen) Shi
    Intellectual 455 points

    Hi Steven,

    Thank you for the detailed response and the original IDX strap table.  This was very helpful.

    With some further testing based on the information you provided, I have determined another component on the IDX/CLKOUT line is leaking more current than expected, causing the divider voltage to decrease, which is placing the 953 into a range reserved for an STP mode.  Depopulating this offending component appears to allow the 953 to boot into coax mode, which avoids needing the aforementioned workaround in register 0x03.

    Thank you for your assistance in troubleshooting this issue!

    Sincerely,
    Christopher