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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

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

    Hello,


    We do not receive any response from you on the previous question/reply/answer provided. 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


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

    Hello,

    thanks a lot for the great answer and all the work!

    I ve test a lot of different skew parameter. I started with the calculated skew parameter.

    procedure:

    1. send an file pc => cyclone 5 processor
    2. count the ack errors with wireshark
    3. receive an file pc <= cyclone 5 processor
    4. count the ack errors with wireshark
    5. if number of tries < 10 goto 1
    6. else change the skew parameter goto 1)

    the file is 1gb big, and i ve get the best skew values with:

    rxdX 420
    txdX 360
    rxc 1680
    txc

    1740

    i think i can only verify the parameter with an oscilloscope, but at the moment i have not the necessary equipment for that.

    thanks again for the support and the endurance.

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

    Hello,

    i have additional questions.

    we tested now more than 10 boards with our new device tree, and have the following result.

    File embedded system => PC:

    if we copy an file from our embedded system to the pc, we have no or less retransmission, < 5 retransmission per copy.

    all 10 boards have the same result

    File PC => embedded system:

    8 boards:

    if we copy an file from our embedded system to the pc, we have no or less retransmission, < 55 retransmission per copy.

    2 boards:

    have really a lot of > 400 ack retransmissions, and ack fast retransmissions.

    Result:

    the ack retransmission error is always from pc to our embedded system, which connection is bad.

    Where can we start troubleshooting?

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

      Hi,

      This thread has already transitioned to community support.

      Please post a new thread for Intel support.

      Thanks,

      Regards,

      Nurina

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

    Hello,

    the retransmission error results from an to low voltage supply for the ksz9031.

    DVDDL was 0,1mV to low.

    thanks for your support.

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