US Patent 7315489 Method and apparatus for time measurement

A method is provided for accurate time measurement. Time is first measured with a first oscillator. At designated intervals, a second oscillator is activated for a period of time based on the first oscillator. The second oscillator is more accurate than the first oscillator. Pulses are then counted from the second oscillator during the period of time. The second oscillator is then turned off after the period of time. The count from the second oscillator is used as a new measurement of the period of time of the first oscillator.