What does a crystal do in an electronic circuit?

Answer 1

A quartz crystal oscillator (often shortened to "crystal" or even "xtal") is an important part of many electronic circuits.
Inside its metal can, it mechanically resonates at a very precise frequency.
(The manufacturer writes the frequency of the crystal on the outside of the can).

The 3 most common applications for that frequency are: pacing (synchronization), timekeeping, and radio communication frequency matching.

An "oscillator circuit" uses that crystal and a few other components to generate a metronome-like electrical signal called the "clock" (often shortened to CLK).

That clock signal is typically sent to practically every digital chip in the box.
Those chips use that clock for synchronization.
The clock pulse tells each chip that all the outputs from the other chips have settled, and its inputs are now valid.
The system designer specifically chooses a crystal that will give a pace slow enough that even the slowest chip has settled at the end of each step.

Each chip takes a certain amount of time from when it gets a new input to the time its outputs stabilize with its new output.

timekeeping ...

radio communication ...


rare exceptions:

A few digital systems don't use a global clock signal, such as asynchronous computers.

A few electronic circuits use piezoelectric crystals (silicon or PZT crystals) as buzzers and ultrasonic transducers.

A few electronic circuits use other kinds of crystals -- photonic crystals, ferroelectric crystals, etc.

Each integrated circuit is made from a slice of crystal, but we call them "chips" or "ICs".