A timing circuit especially designed for use in implantable medical devices provides both a low frequency clock and a high frequency clock. The low frequency clock and high frequency clock are compared each time the oscillator producing the high frequency clock is enabled and the result of the comparison is used to retrim the high frequency oscillator to maintain a stable output frequency, even when subjected to drift. Additional circuitry is provided for signaling an oscillator fault in the event that an error signal resulting from the comparison of the low frequency clock with the high frequency clock exceeds a predetermined limit value. Digital trim values are stored for fast and controlled oscillator start-up.