In terms of the process for a crystal oscillator to start, the immediate frequency generated is not stable or constant. In turn, the clock frequency of the crystal oscillator is not stable during this initial period. As a result, some microcontrollers rely on an oscillator start-up timer (OST) so that the electronic device remains in its reset mode until the crystal oscillator is warmed up and operating at its desired frequency. The time that it takes the crystal oscillator to achieve this stable frequency state is the oscillator’s start-up time. Depending on the particular oscillator that is operating within a device, the start-up time will vary and is specified on the datasheet for the individual product.
