Back to back Gigabit Ethernet Phys???

John Turner

Other Parts Discussed in Thread: DP83865

I found an old thread detailing how I can use the RMII interface to wire up to PHYs back to back to make a repeater.

My question is, are there any gigabit PHYs that I can use to do the same thing? Any reference designs showing how to wire them up? Here is the link to the thread I found for reference:

over 6 years ago

0 Rob Rodrigues over 6 years ago

TI__Mastermind 19780 points

Hi John,

The RGMII interface is capable of a back-to-back connection without any sort of modification. SGMII is also capable of this if you disable the auto-negotiation feature.

I am attaching a schematic with a reference back-to-back connection using RGMII and the DP83867. The thing to keep in mind with RGMII back-to-back is properly setting the required clk to data delay. The clock should be nominally 2ns delay from the data. The DP83867 does this by default, but in back-to-back, the delay inserted by both PHYs cancels each other out.

In the reference schematic, I have used the strap option to disable the TX delay insertion on each PHY. *NOTE* The strap option also disables the DP83867 from advertising 10BASE-Te mode. If you still want 10BASE-Te supported, then LED_1 should be placed in mode 2 with a resistor divider.

DP83867_RGMII_Back-to-Back.pdf

Best Regards,

0 John Turner over 6 years ago in reply to Rob Rodrigues

Prodigy 240 points

Thanks! This is probably a dumb question, but I assume I still need to provide the 25MHz clock to both PHYs, correct?

0 Rob Rodrigues over 6 years ago in reply to John Turner

TI__Mastermind 19780 points

Hi John,

Yes you do. My schematic should not be considered complete. It only shows the RGMII connection. You still need to include all the supporting passives, reference clock, and supplies as shown in the EVM.

www.ti.com/.../dp83867ergz-r-evm

Best Regards,

0 John Turner over 6 years ago in reply to Rob Rodrigues

Prodigy 240 points

Any reason I couldn't do this with the DP83865? I only ask because I have that part already available in an Altium library.

0 Rob Rodrigues over 6 years ago in reply to John Turner

TI__Mastermind 19780 points

Hi John,

I strongly recommend you use the DP83867. The DP83865 is a significantly older, larger, and more power hungry design. It also is much harder to implement in a system.

Best Regards,

Because of the holidays, TI E2E™ design support forum responses will be delayed from Dec. 25 through Jan. 2. Thank you for your patience.

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Back to back Gigabit Ethernet Phys???