I've designed a circuit board with a Microchip switch (KSZ9897) going to a Ethernet-to-VDSL module (LX200V20).
The PHY ports between the two devices are connected using a transformerless connection as shown on Figure 4 in this application note (http://ww1.microchip.com/downloads/en/AppNotes/ANLAN120-UNG.pdf).
By default with auto-negotiation & auto MDI-X enabled, the switch establishes a 10 Mbps, full-duplex link. I am expecting a 100 Mbps link since both devices are 100-BaseTX compatible and I've confirmed that by decoding the LCW (link-code-word) from their fast link pulses.
I have also tried a forced speed connection (disabling auto-negotiation). For 100 Mbps & Full-duplex, I can see some idle traffic on both RX and TX pairs, but the link does not come up. For 10 Mbps & Full-duplex, the link does come up and work. I have also tried disabling auto MDI-X, but that does not seem to change much either.
My question is what could be causing the link to only establish at 10 Mbps? I have attached some scope pics to help isolate the problem. The scopes are all from one of the RX/TX pairs between the switch and the AC coupling caps.
- Scope 1) Forced 100 Mbps idle traffic
- Scope 2) LCW from VDSL module (left burst), LCW from switch (right burst)
- Scope 3) NLP (single burst of LCW) from VDSL module