RGMii TXC skew at 1000B, 125MHz IMX7 Registers

[davidsabalesky]

I am working on the RGMii interface between IMX7 and KSZ8795. I would like to know if there are clock registers to provide the necessary 1ns - 2.6ns delay on the TXC, transmit clock. The KSZ8795 is set to transmit with delay on RXC.

The RXC is handled and does not need to be modified.

I need to find either a register to set in the IMX7 for the delay or add a pcb trace of approximately 9in for the delay.

I prefer to set a register value instead of adding a lengthy trace on a highly constrained layout.

Please refer me to the documentation if available to skew the TXC to the PHY, RGMii mode.

I have the RGMii port working at 10/100base however I cannot get the port to operate at 1000base.

If anyone has information on the Linux driver for the KSZ8795 it is greatly appreciated.

Thank you,

David

[igorpadykov]

Hi David

unfortunately there is no way to program skew for TXC i.MX7 RGMII,

delay should be provided in hardware as described in Table 71. RGMII signal switching

specifications i.MX7D Datasheet, Footnote 3:

"PC board design will require clocks to be routed such that an additional
trace delay of greater than 1.5 ns and less than 2.0 ns will be added to the associated clock signal."

Best regards
igor
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[davidsabalesky]

Hi Igor,

Since my KSZ8795 has an egress delay of 1ns, I think it is prudent to add at minimum 500ps additional delay to meet the 1.5ns-2.0ns range in NOTE 3. Do you agree with this? A 750ps delay would put me in the middle of the range. This maybe a better selection for the delay.

Any suggestions is appreciated.

Thanks,

David

[davidsabalesky]

Thank you Igor.

As a pcb designer is is unfortunate that this feature is not part of the IMX7.

It would greatly help us reduce development time.

Have a wonderful day!