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DP83869HM: DP83869 in gigabit FMC mode need to be able to run copper Ethernet at either gigabot or 100Mbit speed

Part Number: DP83869HM
Other Parts Discussed in Thread: DP83869,

I have a board with a DP83869 setup in gigabit FMC mode, it works great when the device connected to the copper Ethernet supports gigbit, but it does not work when device does not support gigabit.

I have LED0 set low, LED1 and LED2 set low. According to the datasheet this should allow autonegotiation on the copper Ethernet of both 100M and gigabit. This does in fact happen, both speed devices bring up a propper link, BUT the optical connection to the other box does not work when a 100Mb device is connected. Looking at the signal going to the SFP cage the signals look identical in both cases but SOMETHING is different because the other box does not connect when 100Mb device is plugged int o the FMC box.

I tried setting the fiber force mode but that didn't make any difference. The datasheet says that when LED0 is 0 fiber autonegotiation is enabled and 1 is fiber force mode. But elsewhere it says the fiber speed is not dependent on autonegotiation. This is a bit confusing.

SO is there anyway to make it work, that when a 100Mbit device is plugged into the copper Ethernet jack that the optical will properly connect to the other box that is just gigabit optical?

John S.

  • Hi John,

    As defined by IEEE, Fiber medium can not be downgraded from 1G to 100M using auto-negotiation. It has to be be known upfront on speed of 1G or 100M.

    Hence on DP83869HM, we are offering two seperate modes:

    1. 1G Media Convertor

    2. 100M Media Convertor

    For 100M, you have to configure DP83869HM in 100M MC mode. Let me know if you need further assitance.

    Regards,
    Geet

  • Hi Geet,

    I want the fiber connection to stay at gigabit, it is the copper Ethernet connection I want to run at 100M. I changed the straps to advertise 100/1000 on the copper Ethernet, and that seems to work. When I 100M device into the RJ45 it does seem to connect at 100M. Looking at the LVDS signal going into the SFP module the data rate doesn't seem to have changed, BUT the 100M device does not connect to the network (ie DHCP does not deliver an IP address). With the straps the same, when I plug a gigbit device into the RJ45 it connects at gigbit speed and it DOES get an IP address. The LEDs from the DP83869, the 100M device and the FMC on the other side of the optical cable all look like everything is fine.

    The DP83869 has obviously connected at 100M on the copper Ethernet port, the fiber looks like it has stayed at the same frequency, but SOMETHING has changed on the fiber side since the 100Mbit device cannot get an IP address.

    In the datasheet in section 9.5.1.8 for the 1000M Media converter if LED1 and LED2 are strapped low this is supposed to advertise 1000/100 to the link partner on the copper Ethernet port. I may be reading this wrong but this seems to indicate to me that it should work with fiber at 1000 and copper Ethernet at 100. This is what I'm trying to do but it does not seem to work.

    Table 18 shows in the top two lines "Fiber Auto Negotiation" and "Fiber Force Mode", It certainly looks like this is different than the Copper Ethernet autonegotiation, any hint what these two things actually mean?

    Thanks,

    John S.

  • Hi John,

    At system level, keeping Fiber at 1Gigabit while copper on 100M will create data rate inbalance and system may loose packets. I don't it's a good strategy to with. My recommendation would be to choose 100M Media Convertor mode, incase your Cu will be at 100M mode.

    Incase you are not sure on Cu link and then add a external micrcontroller which can look at Link Status of Copper Speed and adapt the Fiber speed according to Cu.

    On Fiber AutoNeg vs Force Mode, IEEE supports link establishement of 1G Fiber can be done using Auto-Neg or Force Mode. DP83869 can be configured to choose any.

    Regards,

    Geet

  • Hi,

    I hope you get it working now. I am closing this thread. Incase you need further assistance, kindly open new thread and provide references to this thread.

    Regards,
    Geet