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qwitza's avatar
qwitza
Icon for Occasional Contributor rankOccasional Contributor
3 years ago

Calculate skew timing with Cyclone V and ksz9031

hello,

i have troubles with the ethernet bandwith and retransmissions (ack). after some diagnostic i think it is an timing issue.

the wire length from the fpga -> knz9031 is:

mil mm m delay delay
RX_DV 1692,83 42,997882 4,30E-02 227,5E-12 228ps
RX_CLK 1649,35 41,89349 4,19E-02 221,7E-12 222ps
RXD0 1684,72 42,791888 4,28E-02 226,4E-12 226ps
RXD1 1648,24 41,865296 4,19E-02 221,5E-12 222ps
RXD2 1681 42,6974 4,27E-02 225,9E-12 226ps
RXD3 1677,12 42,598848 4,26E-02 225,4E-12 225ps
TX_EN 1625,87 41,297098 4,13E-02 218,5E-12 219ps
GTX_CLK 1629,75 41,39565 4,14E-02 219,0E-12 219ps
TXD0 1633,94 41,502076 4,15E-02 219,6E-12 220ps
TXD1 1657,65 42,10431 4,21E-02 222,8E-12 223ps
TXD2 1657,33 42,096182 4,21E-02 222,7E-12 223ps
TXD3 1655,12 42,040048 4,20E-02 222,4E-12 222ps

all wires are the same length, in every wire is an 22r resistor. the design is bought and not designed by us.

i found the following knowledge base entry:

https://www.intel.com/content/www/us/en/support/programmable/articles/000079123.html

=> TX_CLK to TXD/TX_CTL output data delay is specified as -0.85 to 0.15 ns

Datasheet 9031:
For the receive path (KSZ9031RNX to MAC), the KSZ9031RNX adds 1.2 ns typical delay to the RX_CLK output pin, with respect to RX_DV and RXD[3:0] output pins.

We use kernel v4 with u-boot and devicetree, after a lot of measuring and test,

i got the best result with this skew timing:

&gmac1 {
	status = "okay";
	phy-mode = "rgmii";
  /* 0 bis 900*/
	rxd0-skew-ps = <420>;
	rxd1-skew-ps = <420>;
	rxd2-skew-ps = <420>;
	rxd3-skew-ps = <420>;
	rxdv-skew-ps = <420>;

	txd0-skew-ps = <360>;
	txd1-skew-ps = <360>;
	txd2-skew-ps = <360>;
	txd3-skew-ps = <360>;
	txen-skew-ps = <360>;
/* 0 bis 1860*/
	rxc-skew-ps = <1680>;
	txc-skew-ps = <1740>;

	max-frame-size = <3800>;
};

But how could i calculate the skew timing?

Which timing should be the best, with mathematics

Thanks

35 Replies

  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    2 years ago

    Hello,


    I’m glad that your question has been addressed, I now transition this thread to community support. If you have a new question, please login to https://supporttickets.intel.com , view details of the desire request, and post a feed/response within the next 15 days to allow me to continue to support you. After 15 days, this thread will be transitioned to community support. The community users will be able to help you on your follow-up questions.


    p/s: If any answer from community or Intel support are helpful, please feel free to mark as solution, give Kudos and rate 4/5 survey


    Regards,

    Nurina


  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    3 years ago

    Hello,


    Thank you for using Intel Communities.

    Please allow some time while I investigate your case.


    Thanks,

    Nurina


  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    3 years ago

    Hi,


    As far as I know, connecting it to an oscilloscope would be easiest. The calculation could be quite difficult.

    Let me get back to you on the calculation method.


    Regards,

    Nurina



    • qwitza's avatar
      qwitza
      Icon for Occasional Contributor rankOccasional Contributor
      2 years ago

      hi,

      but it have to be possible to calculate the best timing,

      or have i use really an oscilloscope for the timing?

      our oscilloscope is too slow, for that speed

  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    3 years ago

    Also, may I know where are you adding these skew timing code? Is this directly applied to the KSZ9031 to adjust the skew?


    • qwitza's avatar
      qwitza
      Icon for Occasional Contributor rankOccasional Contributor
      2 years ago

      Hello,

      we change the skew time in the device tree.

      i think that it is applied, because the transfer speed and the retransmission error change with.

      thanks a lot

  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    2 years ago

    Hi,


    I'm still waiting for a response from our internal team regarding the calculation. I'll let you know once I receive updates.


    Regards,

    Nurina


  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    2 years ago

    Hello,


    I'm still waiting for internal team's response. I'll let you know of any updates.


    Regards,

    Nurina


  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    2 years ago

    Hello,


    I'm still waiting for internal team's response. I'll let you know of any updates.


    Regards,

    Nurina


  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    2 years ago

    Hello,


    Are you using KSZ9031MNX or KSZ9031RNX?


    • qwitza's avatar
      qwitza
      Icon for Occasional Contributor rankOccasional Contributor
      2 years ago

      Hello,

      we are using the KSZ9031RNX.

      thanks

  • Nurina's avatar
    Nurina
    Icon for Regular Contributor rankRegular Contributor
    2 years ago

    Also, just to confirm- you have already adjusted the skew setting on the PHY and you have found the best result from there?

    So now you want a method to calculate the best skew timing for confirmation? How did you know the adjustment gave the best result in the first place?


    • qwitza's avatar
      qwitza
      Icon for Occasional Contributor rankOccasional Contributor
      2 years ago

      Hello,

      in the first run, we just try different values and measured the data rate.

      afte we ve more concerned with the topic, we found the information about calculating the delay.

      and now we are not sure how we get the values.

      i think with all the information from the first post, it should be possible to get the best delay settings.

      but the fpga need some special settings

      https://www.intel.com/content/www/us/en/support/programmable/articles/000079123.html

      thanks