[nakamoto_damacy]

no, we use atomic clocks now... j/k

piezoelectric refers to generation of electricity from pressure applied to the material... the inverse of that effect is what generates oscillation.. quartz has a natural resonant frequency determined by its shape, size, and the way it’s cut, and when you apply AC it oscillates at a specific frequency.. the applied electricity causes is the material to deform.. that is the basic physical effect used in oscillators

MEMS oscillators are increasingly replacing quartz in compact, rugged, or integrated designs.

PLL-based frequency synthesis is used to scale a low-frequency reference (e.g., 25 MHz crystal) up to CPU/GPU GHz speeds.

[willis936]

MEMS are made on a different process than other silicon devices, which slightly increases their cost. They also need to have hermetically sealed packaging, same as quartz. Together there is little fundamental savings to be had with MEMS, but they do offer a higher ceiling on performance. I don't see crystals going away anytime soon.

Also, if you get a MEMS in a small epoxy / CSP package be weary of gases that permeate the packaging material, such as helium.

https://hackaday.com/2018/10/31/helium-can-stop-your-iphone-...