- What it is
A self‑contained 3‑ or 4‑pin package that integrates a quartz crystal and the oscillator circuitry (inverter, buffers, level‑shift). - Function
Simply apply power (VCC & GND) and you get a buffered square‑wave (TTL/CMOS/CMOS‑LV) out. - Key specs
- Frequency tolerance: often ±50–100 ppm
- Stability: a little worse than a bare crystal (e.g. ±100 ppm over temperature)
- Startup time: specified (e.g. < 1 ms to valid clock)
from STM32 Crystal Guideline
it can reduce drive strength of crystal. If you overdrive crystal, you will get harmonic and it can influence timing accuracy of the device.