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DP83TC811S-Q1: Coefficients of TDR Result

Hajime Kubo
Prodigy 10 points
Part Number: DP83TC811S-Q1
Other Parts Discussed in Thread: DP83TD510E

Hi,

We are testing the TDR function of  DP83TC811S-Q1(100BASE-T1 PHY) and DP83TD510E(10BASE-T1L PHY).

The DP83TC811S-Q1 datasheet says the fault location value is obtained as raw value * 1.5[m]

TDRR Register 0x016B
 bit[7:0] Fault Location
 Fault Location:
 To calculate fault location in meters, convert binary field to
 decimal form and multiply by 1.5.

The DP83TD510E datasheet says fault location value in meters can be obtained from register 0x30C.
Also, we found it seems to be 12.5[m] times raw values stored in resisters 0x305 to 0x307.

 TDR_Fault_Status 0x30C Bits [9:0] store fault location in meters

However, I think that the TDR location results depend on the propagation delay of the cable being tested.
How are these coefficients (1.5m and 12.5m) derived ?
Do you assume specific nominal propagation delay value? (e.g. 5.0ns/m)

Regards,

  • 0
    TI__Expert 4720 points

    Hello Hajime, 

    I understand there is a difference in coefficients however these involve internal mechanisms within the PHY which I can't disclose publicly.

    Besides the coefficient being different do you see an issue when doing TDR testing?

    Regards,
    Avtar 

  • 0 in reply to Avtar Dhaliwal
    Prodigy 10 points

    Hello Avtar,

    Thanks for your reply.
    We don't see any other issues besides that.

    However, if you don't provide any information about the propagation delay values,
    we always have large uncertainty in TDR location results.

    DP83TD510E datasheet also states (in p.31):
    > For all TDR measurements, the transformation between time of arrival and physical distance is done by the
    > external host using minor computations (such as multiplication, addition and lookup tables). The host must know
    > the expected propagation delay of the cable, which depends, among other things, on the cable category (for
    > example, CAT5, CAT5e, or CAT6).

    We don't request that all internal calculations to be exposed.
    But we just need minimum information to execute this computation.

    Application note snla334 (for 1000BASE-T PHYs) provides an equation to get fault location in meters
    and it includes correction factor on the propagation delay value.
    > Fault_location = |(peak_indx(i)-offset) * 8 / (2*prop_dly)|

    An equation like this is enough for us.
    Please consider it.

    Best regards,
    Hajime

  • +1 in reply to Hajime Kubo
    TI__Expert 4720 points

    Hello Hajime,

    The propagation delay values will be different between DP83867 and DP83TC811S and we do not have the values for DP83TC811S.

    If you have trouble testing TDR please let us know.

    Regards,

    Avtar 

  • 0 in reply to Avtar Dhaliwal
    Prodigy 10 points

    Hello Avtar,

    O.K.
    For these PHYs, a convenient way to get precise results is not offered like snla334 for DP83867 does.
    Therefore, if we need precise results, we have to perform a calibration for each kind of cable on our side.
    Am I correct my understanding?


    Thanks,
    Hajime

  • 0 in reply to Hajime Kubo
    TI__Expert 4720 points

    Hello Hajime,

    Yes your understanding is correct.

    Regards,

    Avtar