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DP83867E: How to Interpret the Amplitude Register from TDR Results

Matthew Morawiec
Prodigy 80 points
Part Number: DP83867E

Hello everyone,

I have just gotten TDR working on my PHY. I can run the test and, then read the location and amplitude registers. 

However, I am unsure how to scale/interpret these values, as I can find no information in the datasheet on how to do so.

For example, I have a 10 ft CAT6 cable with open termination connected to the PHY. On each of the 4 channels, I see one peak with location = 0b0001 0100 and amplitude = 0b111 1111.

In the data sheet, it says for the amplitude that "The value of these bits is translated into type of cable fault and-or interference." It says something similar for the location. 

How do I scale/interpret these values?

Thank you,

Matt

  • 0
    TI__Genius 9920 points

    Hi Matt,

    We have an app note on TDR for the DP83822 that you can use as reference. The register addresses may be slightly difference however the reference material should still be of use. Let me know if this clarifies things for you or if you still need further assistance.

    https://www.ti.com/lit/an/snla253/snla253.pdf

    Thanks,

    Cecilia

  • 0 in reply to Cecilia Reyes
    Prodigy 80 points

    Hello Cecilia,

    I have read SNLA253. However, there are a few problems...

    1) The location scaling does not work. I used 3m and 6m cables to test, and got decimal values of 20 and 24, respectively, in the PEAK_LOC registers. Using the scaling provided in SNLA253, this gives 9.85m and 13.25m respectively. This is pretty inaccurate, which leads me to believe it is not the correct way to scale.

    2) SNLA253 says to divide the decimal value of the amplitude by 256. The DP83822 allocates 8 bits to each amplitude, while the DP83867 only allocates 7 bits. Does this change the maximum value? 

    3) SNLA253 also calls out a register for the polarities of the amplitudes. This would be very useful for determining the termination condition of the cable. However, I could not find a register in the DP83867 datasheet that contained polarity information. Is this information not provided on the newer PHY?


    Hopefully you can understand why I am confused.

    Thanks

    Matt

  • 0 in reply to Matthew Morawiec
    TI__Genius 9920 points

    Hi Matt,

    My apologies this has caused you more confusion they may not be interchangeable docs.

    Let me sync with my team internally and give you my feedback in 1-2 days for more clarity.

    Thanks,

    Cecilia

  • 0 in reply to Cecilia Reyes
    TI__Genius 9920 points

    Hi Matt,

    I have provided you below with the interpretation of the location of the signal based on the location and its polarity. 

    TDR_PEAKS_SIGN_A_B

    Address offset

    0x0000 01A5

    Description

    Type

    RO

    Bits

    Field Name

    Description

    Type

    Reset

    15:10

    Reserved

    Ignored on read

    RO

    0x00

    9

    TDR_PEAKS_SIGN_B_4

    found peaks sign 4 in channel B

    RO

    0

    8

    TDR_PEAKS_SIGN_B_3

    found peaks sign 3 in channel B

    RO

    0

    7

    TDR_PEAKS_SIGN_B_2

    found peaks sign 2 in channel B

    RO

    0

    6

    TDR_PEAKS_SIGN_B_1

    found peaks sign 1 in channel B

    RO

    0

    5

    TDR_PEAKS_SIGN_B_0

    found peaks sign 0 in channel B

    RO

    0

    4

    TDR_PEAKS_SIGN_A_4

    found peaks sign 4 in channel A

    RO

    0

    3

    TDR_PEAKS_SIGN_A_3

    found peaks sign 3 in channel A

    RO

    0

    2

    TDR_PEAKS_SIGN_A_2

    found peaks sign 2 in channel A

    RO

    0

    1

    TDR_PEAKS_SIGN_A_1

    found peaks sign 1 in channel A

    RO

    0

    0

    TDR_PEAKS_SIGN_A_0

    found peaks sign 0 in channel A

    RO

    0

    TDR_PEAKS_SIGN_C_D

    Address offset

    0x0000 01A6

    Description

    Type

    RO

    Bits

    Field Name

    Description

    Type

    Reset

    15:10

    Reserved

    Ignored on read

    RO

    0x00

    9

    TDR_PEAKS_SIGN_D_4

    found peaks sign 4 in channel D

    RO

    0

    8

    TDR_PEAKS_SIGN_D_3

    found peaks sign 3 in channel D

    RO

    0

    7

    TDR_PEAKS_SIGN_D_2

    found peaks sign 2 in channel D

    RO

    0

    6

    TDR_PEAKS_SIGN_D_1

    found peaks sign 1 in channel D

    RO

    0

    5

    TDR_PEAKS_SIGN_D_0

    found peaks sign 0 in channel D

    RO

    0

    4

    TDR_PEAKS_SIGN_C_4

    found peaks sign 4 in channel C

    RO

    0

    3

    TDR_PEAKS_SIGN_C_3

    found peaks sign 3 in channel C

    RO

    0

    2

    TDR_PEAKS_SIGN_C_2

    found peaks sign 2 in channel C

    RO

    0

    1

    TDR_PEAKS_SIGN_C_1

    found peaks sign 1 in channel C

    RO

    0

    0

    TDR_PEAKS_SIGN_C_0

    found peaks sign 0 in channel C

    RO

    0
  • 0 in reply to Cecilia Reyes
    Prodigy 80 points

    Hi Cecilia,

    This is exactly what I needed for return pulse polarity! Thank you! Maybe this should be in the datasheet? :) 

    Do you have any information on the scaling formulas for the peak location and peak amplitude?

    Thanks!

    Matt

  • 0 in reply to Matthew Morawiec
    TI__Genius 9920 points

    Hi Matt yes! We are in the process of releasing this in our next datasheet refresh :)

    I believe this can be used for the peak location however for peak amplitude is there a specific reason or need for the peaks? Is that a design concern for you?

    Thanks,

    Cecilia

  • 0 in reply to Cecilia Reyes
    Prodigy 80 points

    Hi Cecilia,

    Knowing the scaling formula for the locations and the polarity is certainly more important than the amplitude, but if the information is there we may as well know how to scale that as well. If the amplitude scaling information is unavailable, that would not be the end of the world.

    Is there an update on the scaling formula for peak location?

    Thanks

    Matt

  • 0 in reply to Matthew Morawiec
    TI__Genius 9920 points

    Hi Matt,

    I don't believe I have the amplitude scaling to share. 

    According to past e2e posts it looks like the peak formula location from the app note was also recommended here: e2e.ti.com/.../825997

    However I know you mentioned that you were getting miscalculations so if you share the calculation I can also see if you are getting the right values as well.