Adaptive prewrite compensation apparatus and method

Abstract

A prewrite compensation circuit for use with a hard disk drive wherein surfaces of disks of the drive are formatted for writing data to the surfaces at different frequencies selected for different concentric zones on the surface. Data to be written is passed through a shift register to a series of delay elements and a multiplexer that passes the output of a delay element selected in relation to a data word received by the multiplexer to a write circuit that writes the data to the disc. The delay words, or elements of delay words, are stored in latches that are selectively connected to the multiplexer, or a delay word accumulator connected to the multiplexer, in relation to the pattern of bits surrounding a bit to be written. A delay calibration circuit includes an inverter and a timing circuit that connects the inverter across the delay elements, or a series of delay elements having the same electrical characteristics, for a series of time intervals prior to writing to a new zone to form a ring oscillator whose oscillations clock a counter during each time interval. Following each time interval, the counter contents are entered into a selected latch.