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Ethernet PHY DP83640 speed led ON although no cable connected ?

jonds
Prodigy 110 points
Other Parts Discussed in Thread: DP83640, DP83620

On a new PCB we are using the TI DP83640 Eth PHY. On this PHY we are using the LED SPEED output (connected to the yellow led in the RJ45 connector). According to the datasheet, the output indicates that the link is operating in 100 Mbit/s mode (yellow led on) or in 10Mbit/s mode (led off). This works fine. (this also means that the strapping options, led polarity, etc are handled properly).

However we also observe the following unexpected behavior: when the RJ45 cable is NOT connected, the yellow led is still ON.

Although this is not really a violation of what is in the datasheet (There is no description in the datasheet of what happens when the link is down), it is a bit unlogical and it is different from what most other PHYs are doing (speed led off when cable removed).

Could someone confirm this behavior?

  • 0
    TI__Mastermind 31600 points

    I have moved your post to the Ethernet forum so it will get the necessary visibility.

    I believe I saw a similar post recently.  I will try to find that and post the link.

    Patrick

  • 0 in reply to Patrick OFarrell
    TI__Mastermind 31600 points

    Please see the corresponding post at:

    <Link removed.  Information from the link copied to subsequent post below.>

    The post is related to the DP83620, but the functionality for the DP83640 is the same.  Similarly, the key question to confirm is the same.

    Based on the default configuration, Auto-Negotiation will be enabled.  Can you confirm that the device is configured for Auto-Negotiation?

    Patrick

  • 0 in reply to Patrick OFarrell
    Prodigy 110 points

    the link you suggest doesn't seem to work? (I get an error 'Group not found')

    I can confirm that we are using the DP83640 in Auto-Negotiation mode (pin 28 open, relying on the internal PU).

    Jon

     

     

     

  • 0 in reply to jonds
    TI__Mastermind 31600 points

    My apologies.  I edited the previous post to remove the bad link and I have pasted the information below:

    I can provide some quick feedback on the functionality of the speed LED.  The functionality you are describing is most likely the correct, expected functionality. 

    The speed LED indicates that the device is operating in 100M mode or is capable of operating in 100M mode.  When the device is configured for Auto-Negotiation, it must be able to correctly receive 100M scrambled idles.  Therefore, the 100M receiver is enabled and the speed LED is lit. 

    Based on the default configuration, Auto-Negotiation will be enabled.  Can you confirm that the device is configured for Auto-Negotiation?  In other words, can you confirm that the customer has not changed the default configuration by means of register writes?

    Please let me know if you have any questions.

    Patrick

  • 0 in reply to Patrick OFarrell
    Prodigy 110 points

    OK, so this seems to confirm that the unexpected behavior is normal.

    I think it would be useful to add it to the datasheet. Apparently I'm not the first one dealing with this issue...

    Thanks,

    Jon

     

  • 0 in reply to jonds
    TI__Mastermind 31600 points

    Jon,

    You are absolutely right.  Given that this question has been raised previously, it makes sense to clarify the functionality. 

    Below is the current content of the datasheet:

    My first thought would be to make the change in this pin description.  Perhaps adding the following:

    The speed LED indicates that the device is operating in 100Mb/s mode or is capable of operating in 100Mb/s mode.  When the device is configured for Auto-Negotiation, prior to establishment of link, the speed LED is lit.  After link establishment or when the speed is forced, the speed LED is lit when the device is in 100Mb/s and is extinguished when the device is in 10Mb/s.

    What are your thoughts?  Is this the best place for this information?  Is the wording sufficient and clear?

    Patrick

  • 0 in reply to Patrick OFarrell
    Prodigy 110 points

    Hi Patrick,

    I guess it should be added into that table in paragraph 3.6, and in paragraph 5.5 where the led functionality is elaborated in detail (except for that speed led ;-)

    I would not focus too much on 'auto-negotiation mode', because I guess the behavior will be the same (led ON) when the mode is forced to 100 Mbit/s and no link is connected. I didn't verify this though...

    I would simply put it as follows (in that table):

    "SPEED LED: The LED is ON when device is in 100 Mb/s (even when no link is present!) and OFF when in 10 Mb/s. Functionality of this LED is independent of mode selected."

    In paragraph 5.5 a more detailed description (like yours) could be added.

    Jon

     

  • 0 in reply to jonds
    Prodigy 70 points

    We are facing the same issue with DP83620SQ PHY. As a workaround, trying to manually configure the Speed LED behaviour using PHY registers seems to work. However, at power ON the LED still gets ON for around 4 seconds and then goes OFF as the register is configured in the software.