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DP83TC812S-Q1: Initiate Link in Loopback - Link Status bit

Jonathan Jank
Intellectual 955 points
Part Number: DP83TC812S-Q1

HI,

I have a question about loopback modes for the DP83TC812S PHY. There are a few different loopback modes (MII, PCS, Digital, Analog, etc…). When I put the unit into loopback mode I also lose sync (BMSR register, bit 2). I’m wondering if there’s a loopback mode that does not interfere with Link Status. If no, I’m wondering if there’s a means to initiate the link process.

Thanks,

  • 0
    TI__Mastermind 27915 points

    Hi,

    For MAC facing (Data coming from and redirecting back to the MAC) loopback modes PCS, Digital, and Analog, the PHY will show link high. However, this comes with a few notes. The link that the PHY is saying is connected is with itself. It will not be linked up with the link partner (if applicable) on the other side of the cable. The link status bit is also a latch low bit. This means that for accurate depiction of link, this register will need to be read twice. This is due to PHY initially breaking link while setting up loopback, then once loopback is successfully implemented, link will be established. 

    Sincerely,

    Gerome

  • 0
    Prodigy 60 points

    and others. Is there a way to initiate the link process with the external partner. I am putting the PHY in loopback mode and performing a simple loopback test. Transmitting a few bytes. After I confirm the reception of the bytes I put the PHY back into Normal Mode. After that I'd like to read the link status bit in the BMSR register. Unfortunately the link status bit seems to take a while to change to 1. Is there a way to initiate the link process after putting the PHY back into Normal Mode. It seems to take a while and I'm not finding a means to control or initiate it. 

  • 0 in reply to Tim New
    TI__Mastermind 27915 points

    Hi Tim,

    For clarity, which type of lookback are you implementing on the PHY (Reverse, Analog, Digital, XGMII, etc)?

    Sincerely,

    Gerome

  • 0 in reply to Gerome Cacho
    Prodigy 60 points

    I am running just the XMII loopback (write BMCR reg to 0x6100), but I am open to using an alternate loopback mode. 

  • 0 in reply to Tim New
    Prodigy 60 points

    in short what I initially wanted to do is put the PHY into loopback (any loopback mode... I'm not picky) and also get valid LINK Status data. But I found that all the loopback modes interfere with link status temporarily. 

  • 0 in reply to Tim New
    TI__Mastermind 27915 points

    Hi Tim,

    Can you try reverse loopback? This will take data coming from the MDI and bring it through some of the PHY's blocks and reroute back out through the MDI. More information is found in section 8.3.1.5.6 of the datasheet.

    Sincerely,

    Gerome