 The normal system hedges are to have an auxiliary oscillator using some cheap, robust scheme such as a ring of inverters or a resistor-capacitor one-shot timer.After a reset (perhaps caused by a watchdog timer), the system may default to these, only switching in the sensitive crystal oscillator once timing measurements have proven it to be stable.It is also common in high-reliability systems to measure the clock frequency by comparing it to an external standard, usually a communications clock, the power line, or a resistor-capacitor timer.