Calculate skew timing with Cyclone V and ksz9031

[qwitza]

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

[Nurina]

Hello,

Thank you for using Intel Communities.

Please allow some time while I investigate your case.

Thanks,

Nurina

[Nurina]

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]

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]

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

[qwitza]

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]

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