Is it useful to connect crystal metal outside layer to GND to increase its noise filter ability and make the crystal more stable?
Generally speaking, to connect the crystal shell to ground is only for the purpose
of structure stability: Anti-shaking! Circuit characteristics can fully suppress EMI
interference. Only high frequency crystal, EMI only need to focus on the CPU timing
problems areas caused by the Sasser-shaped spikes on the overshoot! In less demanding
place, actually, too much consideration is unnecessary, as long as the basic circuit
margin to ensure reasonable mix component parameters, layout good is enough!