Crystals are typically used if the target IC has an integrated oscillator and on-chip phase-locked loops (PLLs) for internal timing. Crystals are cost-effective and are widely available. They can also be placed in close proximity to the IC, simplifying board layout. One of the drawbacks of crystals is that their frequency can vary significantly over temperature, exceeding the parts-per-million (ppm) stability requirements of some applications. In many stability-sensitive high-speed applications, crystal oscillators (XOs) are a better fit because they guarantee tighter temperature stability.