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DS80PCI800 Redriver settings

Alex Kolesnyk
Prodigy 150 points
Other Parts Discussed in Thread: DS80PCI800

Hello,

can you recommend the redriver settings: de-emphasis, equalizer, signal detect threshold level for the system which consist of the PC riser card with DS80PCI800 chips, external PCIe cable 1m length, DUT card with also DS80PCI800 chips. Gen.3 link, width is x8. What additional information need to know that to set proper mode of the de-emphasis ,equalizer etc.
In datasheet, for example, for de-emphasis settings there are information for trace 20inch max. What should be settings when the cable 1m is present?

Thanks.

Alex

  • 0
    TI__Mastermind 29655 points

    Hi Alex,

    Could you provide a block diagram of your arrangement from TX to RX so that we can see where the DS80PCI800 is being placed in the signal path? From your description, it looks like you are driving 1m of cable and not receiving a signal after 1m of cable. Is this correct? Is there any additional trace length between the signal source and the redriver?

    The datasheet EQ and De-emphasis settings on p. 13-14 are suggestions based on prior data that suggest minimized jitter. It is expected that from there, the user should tweak the EQ gain or de-emphasis attenuation in order to achieve the best performance for his/her system. This is because there are various other factors particular to the user's system that can introduce additional jitter.

    Note that EQ settings are typically advised for trace/cable length prior to the redriver, whereas de-emphasis settings are typically advised for trace/cable length after the redriver.

    If you are driving a 1m shielded coax cable of that length, 0dB or -1.5dB de-emphasis should suffice. SD_TH levels can remain the same as the default. EQ settings should be made in accordance to the amount of trace/cable length you have before the redriver.

    Thanks,

    Michael 

  • 0 in reply to Michael Lu (Santa Clara)
    Prodigy 150 points

    Hi Michael,

    thanks for reply. The block diagram is in attachment. I have the next signal path length:

    - on side A the path length from edge connector to redrivers is 2.8'', from redrivers to iPASS connector is 1.8''

    - on side B the path length from edge connector to redrivers is 4.2'', from redrivers to iPASS connector is 2.2''

    Now let's consider that the path length on the motherboard and on the backplane are zero.

    So, for example, for A.TX the length prior redriver is known (2.8'') and what is the length after redriver? 1.8'' + 1m? or should use only length on FR4? or de-emphasis should be chosen by experimental way?

    The same question for A.RX. The length before redriver is 1m+1.8'', but in table on page 13 there are no such length. What level should use in this case?

    Thanks,

    Alex

    3833.doc1.pdf

  • 0 in reply to Alex Kolesnyk
    TI__Mastermind 29655 points

    Hi Alex,

    Thank you for the block diagram!

    The trace lengths prior to the repeater on both the A.TX and B.TX are minimal (<5 in. each), so you should not need to increase the EQ beyond 0x00 (note that there is still approx. 4.9dB gain @4GHz at the lowest EQ setting). Similarly, for the trace lengths after the repeater on both A.RX and B.RX, de-emphasis should be kept at 0dB.

    You are right that there is no specific EQ/DEM setting defined for the redriver to drive a 1m cable + 1.8" FR4 trace (or 1m cable + 2.2" FR4 trace). However, these short distances imply that the lowest settings for EQ (B.RX and A.RX) and De-emphasis (A.TX and B.TX) should work well. Finding the optimal settings will require further experimentation with the EQ and DEM, but beginning with the minimum should be a good place to start.

    Ultimately, the best way to determine the settings most appropriate for this case is to examine the insertion loss characteristic of the 1m cable + trace with a network analyzer. Then, select a boost or de-emphasis that best counters this loss.

    Thanks,

    Michael

  • 0 in reply to Michael Lu (Santa Clara)
    Prodigy 150 points

    Hi Michael,

    I will try to check with your suggestions.

    Thank you for your help.

    Regards,

    Alex