How to route Ethernet PCB ? [closed]

Question

I am new to PCB routing. In one of my projects, I need to be able to exchange data between nano-computers (https://www.variscite.com/wp-content/uploads/2018/01/DART-6UL_DART-6UL-5G_Datasheet.pdf) using an Ethernet connection. I will use a simple RJ45 connector soldered on my card (https://docs-emea.rs-online.com/webdocs/0fae/0900766b80fae0ad.pdf).

My question is now : how am I suppose to route the 8 Ethernet pins to my nano-computer ?

I do not want to route differential pair all the way to the nano-computer. I have searched and read about magnetics, transformers, rmii, PHY, MAC... But it is all confused in my head and I don't know which one I should use.

Is there a classic way to achieve this PCB routing ? (at least schematic routing first)

Thank you all

Answer 1

I think the routing is at the moment the least of your concerns. Usually, providers of SOMs also provide schematics for a base board. The least painful way to get a working board is to copy those schematics (at least the parts you require). I would guess this information is hidden behind the customer registration item on their webpage.

In any case, the module already contains everything including MAC + PHY (KSZ...) for one ethernet port according to their description, so you probably won't need those. I would guess (according to the pin description) that you would then route the pins to a transformer (1:1 winding ratio) and then to the actual connector. If you want to save on parts, you can get a connector with integrated magnetics (typically called magjack). Again, refer to the module's vendor on how to connect them correctly. The PDF is a little bit on the low information side.

If the track between RJ45 connector and module connector is very short, you can probably (!) get away with simply routing the tracks as you like. For everything longer, you will need to match the impedance closely to 100R Differential or 50R Single-Ended. Make sure that the trace lengths match at all costs.

Answer 2

You can use the onboard phy, but you would also need to supply external magnetics. You can use an external phy (like if you needed to use fiber or a different ethernet interface) on the either of the RMII interfaces supplied on the SOM.

It would probably be wise to use the onboard phy:

The DART-6UL/DART-6UL-5G features Micrel™ KSZ8081RNL Ethernet PHY. KSZ8081 is a single supply 10Base-T/100BaseTX Ethernet physical-layer transceiver for transmission and reception of data over standard CAT-5 unshielded twisted pair (UTP) cable.

Here is the datasheet for the KSZ8081RNL

Typically you place magnetics between the phy and the outside world (as per the spec), if the phy's are on the same board, you may not want to use magnetics. Read the datasheet of the KSZ8081 and follow the guidelines. I'd estimate someone who has not implemented an ethernet interface would need about 30 to 60 hours to read and understand all of the material required to correctly implement the interface. Lucky for you the phy is already on the board so using that should save you some time. You need to make sure the routing is done using impedance controlled (usually 100Ω check the datasheet) traces.

Answer 3

Ethernet PHY (physical layer) on your board is implemented with Microchip's KSZ8081 controller. Check Fig. 7-16 at page 58 of the datasheet: Pins TXP, TXM, RXP and RXM corresponds to pins ETH_RXDP, ETH_RXDN, ETH_TXDP and ETH_TXDN at J1 connector of your board (page 19 of the datasheet you provided). I suggest you to use RJ45 connector with integrated transformer and common mode choke, as this one.

Besides RJ45 connector you need only two additional capacitors, so routing is fairly simple. Keep tracks as short as possible and the same length.