MII inpedance matching for LAN9313i

Question

I am designing an ethernet switch using the LAN9313i and the ADIN1100. For my understanding MII traces should be routed to match 50 Ω.

However, Microchip recommends 68 Ω in their routing checklist and their reference schematic. I can't find any matching resistors. So my understanding is that the chip has internal matching resistors. Analog Devices, on the other hand, has matching resistors in their reference schematic. So thinking that these are the matching resistors to match 50 Ω.

But if I use 68 Ω traces they are getting very thick and the distance is only 2 cm. I wonder if all that is necessary for such a small distance, and if I really need matching resistors.

https://ww1.microchip.com/downloads/en/DeviceDoc/LAN9313I_XVTQFP_Rev_B_Routing_Checklist.pdf

https://ww1.microchip.com/downloads/en/DeviceDoc/9313sch.pdf

https://www.analog.com/media/en/technical-documentation/data-sheets/adin1100.pdf

https://www.analog.com/en/products/adin1100.html#product-overview

Answer 1

But if I use 68 Ω traces they are getting very thick and the distance is only 2 cm. I wonder if all that is necessary for such a small distance, and if I really need matching resistors.

Looks like you are using 10-BaseT (10MHz) at 10MHz PCB effects wouldn't be critical as a wavelength would be 100's of cm (through air). If you are using 10-baseT, it wouldn't even be worth it to do length matching.

Even at 100MHz a wavelength would be 166cm on fr4. So I wouldn't worry about it.

I would probably route the signals as best you can. Shouldn't need any matching resistors (the last design I did with a 100BaseT switch did not need them).