Patent ID: 6954324

Claim:
A disk drive comprising: (a) a disk comprising a plurality of tracks, wherein each track comprises a plurality of data sectors and a plurality of servo wedges; (b) a head actuated over the disk; (c) a spindle motor for rotating the disk at an operating speed in response to a spindle control current, the spindle motor comprising a plurality of windings which generate a back electromotive force (BEMF) voltage; (d) a BEMF detector for generating a BEMF signal by comparing the BEMF voltage to a threshold; (e) a current modulator for generating a PWM signal representing the spindle control current; (f) BEMF detection window circuitry for periodically disabling the PWM signal for a predetermined interval to attenuate noise in the BEMF voltage while the BEMF detector compares the BEMF voltage to the threshold; and (g) a disk controller for: measuring a BEMF speed error responsive to the BEMF signal during a BEMF spindle speed control mode; updating the spindle control current in response to the BEMF speed error to drive the disk at the operating speed; disabling the BEMF detection window circuitry to reduce acoustic noise and switching to a wedge spindle speed control mode; measuring a wedge speed error in response to the servo wedges; maintaining the disk at the operating speed by updating the spindle control current in response to the wedge speed error; if an error condition is detected, enabling the BEMF detection window circuitry and maintaining the disk at the operating speed by updating the spindle control current in response to the BEMF speed error generated from the BEMF signal; and when the error condition subsides, disabling the BEMF detection window circuitry and maintaining the disk at the operating speed by updating the spindle control current in response to the wedge speed error generated from the servo wedges.