ethernet PHY thoughts (compatibilty, drivers etc.)

[rjmaris]

Thoughts pop up after having created an STM324F-based board with a LAN8720A PHY. I have selected this PHY because its compatible part 8742A is being used in a NUCLEO-F429ZI board. As such I wouldn't risk that ethernet won't work.

In general, I would prefer to feel free to make a choice out of different PHYs based on the knowledge that different PHY's share standardized registers and a bunch of vendor specific registers. Without a PHY driver, it makes extra sense to carefully strap PHY pins to my needs to avoid the need to initialize (vendor specific) registers.
If anyone could confirm that I do not need to hesitate about using PHYs that are untested in boards supported by Zephyr (when I practice careful strapping rules), then I'll consider other compact RMII-only-devices like KSZ8081 and DP83825i.

This topic is also related to the fact that drivers (which should open the door to using extra features) are mostly in modules/hal and hence with availability dependent on used SoC vendor.

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The reason for my thoughts: 1) answers maybe helpful for other designers as well, and 2) I just encounter two nullseries boards with PHY problems: blinking LED where no link is active. When I measure an eye-diagram, it looks not so nice like sine wave as compared to another board equipped with KSZ8051 - even on TX pins. I fear that this is related to the 8720/42 requirement of secondary transformer taps connected via resistors to 3.3V instead of floating while DC-level is controlled by the PHY itself. I'd consider this scheme also more robust (while also saving four termination resistors).