In an integrated circuit, a clock monitor circuit detects when an analog clock signal output by an on-chip crystal oscillator has stabilized. The clock monitor circuit uses an envelope follower circuit to monitor the envelope of the analog clock signal and compare the amplitude of the envelope with a predetermined amplitude value. When the predetermined value is reached and the envelope has remained steady for a predetermined time, an oscillator okay signal is generated. If an oscillator okay signal is not detected within another predetermined time, then an oscillator failure signal may be generated.