DP83867CR: TDR peaks location calculation

Hi,

Please use below two registers for Polarity. They got left out from DP83867 datasheet and will be added in next revision.

TDR_PEAKS_SIGN_A_B

TDR_PEAKS_SIGN_C_D

For Register mapping from DP83867 to DP83822, please refer to below e2e thread.

https://e2e.ti.com/support/interface/f/138/t/624167?DP83867IR-DP83867-TDR-enable

Regards,
Geet

Hi,

Thanks for the polarity registers !

About location registers, I found the thread you are pointing me to, it actually led me to snla253, but as I noted above, calculation doesnt give expected results.

Actual calculation for converting LOC value to meters must be slightly different for DP83867.

Thanks,

Hi,

Can you share your xxls you are using for the compuation ? I can ran it over to expert to confirm.

Regards,
Geet

DP83867_TDR.xlsx

Geet,

Here is a sample calculation for cables I had at hand.

Regards

Hi,

I am looking in this. I shall get back by friday.

Regards,

Geet

Hi Nicolas,

Thanks for your patience on this topic. We looked in further and found that the factor to substract for DP83867 is 17 instead of 8.

I am noting down the steps again and formula we used in lab for the quick reference. 

Regards,
Geet

How To use TDR on DP83867

TDR functionally of DP83867 can be used for detecting the location, amplitude, and nature of

the peak. Use the following script to execute TDR.

Script:

//start TDR

wd 1E 1

delay 2000

//TDR Done

//------------------

rd 1E

//TDR - Gen status

//------------------

rd 1A4

//TDR - read peak loc

//------------------

rd 190

rd 191

rd 192

rd 193

rd 194

rd 195

rd 196

rd 197

rd 198

rd 199

//TDR - read peak Amp

//------------------

rd 19A

rd 19B

rd 19C

rd 19D

rd 19E

rd 19F

rd 1A0

rd 1A1

rd 1A2

rd 1A3

Refer to the DP83867IR TDR registers.

Calculate distance to peak in meters:

• Run the script and check register 0x1E to verify that TDR is completed.

• Note the peak location values from register 0x190 to 0x199.

• Peak locations are stored in 8-bit Hex format in the above mentioned registers.

• DP83867 can store 5 peaks per channel.

• Convert the location from HEX to decimal.

• Use the following formula to get the location of the peak in meters.

Peak Location (in meters) = (Decimal value * 0.862)-17

For example,

Assume that after executing the TDR script, location for Peak 0 on Channel A (Register

0x190[7:0]) is recorded as 0x40. The decimal value is 64.

Peak location = (64 * 0.862) – 17

= 55.16 -17

= 38.16 meters

The location of the peak is approximately 38.16 meters from the PHY. This can be used to

detect break in cable, impedance discontinuity or cable length. For measuring cable length,

leave the other side of the cable unconnected and follow the procedure.

Geet,

Thanks for taking care of my issue.

If I apply the calculation you provide to the values I have in excel sheet I sent, LOC values of 21, 24 and 33 (decimal) gives lengths of 1.1 m, 3.7 m and 11.4 m, respectively. This puzzles me as this is quite far from actual cables lengths of 4,7 m, 7,4 m and 14.35m I measured.

If calculation is correct, what could cause such discrepancy ?

Regards,

Hi,

Can you please try out with larger cable lengths for me?

1. 30 meters

2. 50 meters

3. 70 meters.

Regards,

Geet

Hi,

I'm sorry, but I'll wont have access to these exact lengths in a close future. I would have to wait until next october or so.
Maybe you'll have better chances on your lab at Ti.

Regards,

Hi,

Got it. Unfortunately, we will not be able to perform the test internally right now. 

Regards,

Geet

Hi,

I am closing this thread. In case you need further assistance, please open new thread and provide reference to this thread.

Regards,

Geet