Many microprocessors include real time clocks as an integrated function but external RTCs are still widely used since power and accuracy often are effected when the internal RTC is used. Often power is lower with an external Real Time Clock because the RTC function requires time to be continuously tracked and the microprocessor cannot be put to sleep if it is performing this function with a calculator. In addition, the absolute accuracy of the timing crystal is not a requirement for microprocessor operation but for a real time clock it is critical to use accurate crystals to ensure real time accuracy.