Hello, I was looking into a solution for a automotive PHY: TJA1101 and found this thread.
I have not used TJA1101, but I did get the datasheet from
NXP's web page. So please keep in mind these answers are based on a quick look at the datasheet (no actual experience with TJA1101) and familiarity with Teensy's ethernet peripheral (based on experience) and ethernet PHY connections in general.
Q: are all lanes relevant to have the required speed?
100 Mbit ethernet PHY chips use 2 different connections, call MII and RMII. They are functionally the same.
MII clocks at 25 MHz and uses 4 data signals in each direction. RMII clocks at 50 MHz and uses 2 data signals in each direction. RMII also uses fewer extra signals.
RMII uses 7 signals and 1 pin for 50 MHz clock. Most PHY chips can either create the 50 MHz clock (usually uses a 25 MHz crystal) or configure their clock pin as an input. On MicroMod and Teensy 4.1, the main processor creates the 50 MHz, so you need to configure the PHY chip to receive the clock. Usually this is done by connecting resistors to certain pins, which the PHY chip uses at startup to configure itself.
PHY chips also have 2 pins (MDIO, MDC) for configuration and status, like checking whether the link is up. Usually use of the MDIO config is optional. Almost all PHY chips alllow configuring all the important features using resistors. As you read the datasheet, you'll find both ways documented, and usually the MIDO config offers a lot of extra non-essential stuff.
When you first get started with a new PHY chip, best to simply get it working first with resistor-only config. I believe the info you need is on page 23. You would want to configure for "RMII mode (50 MHz input on REF_CLK)"