Circuit arrangement for the determination of the constant speed rotation

Abstract

A circuit arrangement for a flexible magnetic disk drive having an index sensor for optically sensing an index hole defined eccentrically in the magnetic disk, with the index sensor putting out an electric index pulse of less than a predetermined duration each time it detects the index hole during the rotation of the magnetic disk. The index sensor is prone to put out invalid pulses of not less than the predetermined duration as a result of the displacement of the magnetic disk relative to the index sensor. For detecting the constant speed rotation of the magnetic disk on the basis of the valid index pulses only, the circuit arrangement comprises an index pulse spacing detector circuit for producing an index pulse spacing signal indicative of whether the spacings between the successive index sensor output pulses are less than a predetermined limit or not, a latching circuit for putting out a constant speed rotation signal in response to the index pulse spacing signal when the spacing between any two consecutive index sensor output pulses is less than the predetermined limit, and an index pulse duration detector circuit for inhibiting the production of the constant speed rotation signal by the latching circuit upon detection of an invalid pulse even if the spacing between this invalid pulse and the next index sensor output pulse is less than the predetermined limit.