Abstract: An externally tunable clock circuit (10) and a method for adjusting the calibration of a clock circuit with an externally defined time period. A voltage controlled oscillator (VCO) (40) provides clock signals of variable frequency, and a counter (42) receives timing and control signals to provide a digital count indicative of clock signals during a predetermined period. The input voltage to the VCO (40) is adjusted based on differences between the oscillator clock count and a predetermined value. Calibration of the clock circuit (10) is done by counting clock signals from the VCO (40) during an externally defined time period, then comparing the clock signal count with a predetermined value that represents a desired frequency. The VCO frequency is then adjusted according to the difference between the clock signal count and the predetermined value.