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I've saw so many reference designs use just a crystal to generate the clock for ethernet phys, though they can be generated with a crystal oscillator too. Such as KSZ9021's 'XI' and 'XO' pin.

Are there some special reason that a crystal is better than a oscillator here? Or just because crystal is cheaper than a oscillator?

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If the PHY has a crystal driver built in, then using a separate oscillator just for the PHY is rather silly.

A crystal with caps is usually cheaper, smaller, and takes less power than a whole separate crystal oscillator module.

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  • \$\begingroup\$ Depending on the developer experience, it might be easier to use a whole oscillator to reach required clock stability. \$\endgroup\$ – AndrejaKo Jul 11 '18 at 12:14
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    \$\begingroup\$ If you pick a standard footprint, the oscillators are usually pretty much interchangeable with little thought, crystals (especially SMT ones) can be a bit more of an annoyance when (not if) your chosen part goes end of life. This for me is a good enough reason to place a 7*5mm 4 pin oscillator, they are second sourced by LOTS of vendors. \$\endgroup\$ – Dan Mills Jul 11 '18 at 12:28
  • \$\begingroup\$ I fully agree with both my pre-commenters, but do think Olin is right to give the pro-arguments for crystals. Olin, have my upvote! \$\endgroup\$ – Marcus Müller Jul 11 '18 at 13:11
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One needs to take into account that Ethernet PHY requires a fairy accurate frequency, typically less than +-50ppm.

Using a crystal with PHY-embedded driver usually require more expensive crystals, and crystal loading/tracing/gain needs more careful engineering/tuning than an ordinary MCU would require.

The stand-alone crystal oscillator chips, on the other hand, while usually more expensive, usually guarantee the accuracy without any need for external components, and therefore are easy to deal with. But they might need another GPIO from a system to shut it down in low-power applications.

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