Jazz answered your question perfectly. But I'll answer with another question. How are you going to interface the UC300eth with the servo? The thought process behind that question is, if you're targeting 400kHz signalling then you can face a couple of challenges for correct operation. Look at the respective devices - and cables - in the signal path between servo and the UC300eth - make sure you're not setting yourself up for a fall. If you believe you need 400kHz (and there's some advantage, just make sure you balance that against the disadvantages). Look at establishing a differential drive to the servo and either avoid, or be very aware of any component in that signal chain.