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help! Ethernet PHY Loopback problem

Discussion in 'Electronic Design' started by [email protected], Jun 14, 2007.

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  1. Guest

    Hi guys,
    A problem concering broadcom PHY 5461 loopback
    mode(internal loopback).from mac ----phy----mac
    the phy works well on 100M loopback, but cannot works in 1000M
    mode,i am sure regiser setting are write correctly.

    A strange problem is :
    when i ping from another swith(not in loopback mode), 5461phy
    can received data correctly , and mac connected with 5461 return arp
    requset to phy, but, 5461 cannot transmitted the data,using
    oscilliscope, i can see the data on 5461 TX pins, it means mac
    succefully transmit data to 5461,

    now we consider loopback again,since we can receive data, and mac
    can transmit data, why the phy cannot loop the data back to MAC

    any advice from you would be great help!
  2. Jason,

    I've never worked with the 5461 but most PHYs have different register
    write sequences for different loopback modes. Draw a picture of what
    you want to do and check your register write sequence again. The fact
    that 100BASE-TX works but 1000BASE-T does not seems to indicate that
    1000BASE-T is not auto-negotiating properly (1000BASE-T is master/
    slave timing system.) You cannot force 1000BASE-T to link, you must

    Kadir "Solid Gold" Suleyman
  3. Ben Jackson

    Ben Jackson Guest

    The clocking schemes for 10/100 (MII) are different from 1000 (GMII).
    The clocks also go in different directions. At 10/100, the MAC is
    driven by the PHY's TXCLK. At 1000, the PHY is driven by the MAC's

    One simple test would be that if you can achieve a gigabit link and
    *receive* packets but not *send* them.
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