Fault tolerant sync mark detection while servo writing a disk drive from spiral tracks

1. A method of writing product servo sectors to a disk of a disk drive, the disk drive comprising control circuitry and a head disk assembly (HDA) comprising the disk, an actuator arm, a head connected to a distal end of the actuator arm, and a voice coil motor for rotating the actuator arm about a pivot to position the head radially over the disk, the disk comprising a plurality of spiral tracks, wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark, the method comprising:

using the head internal to the disk drive to read the spiral tracks to generate a read signal;

processing the read signal to detect a plurality of the sync marks in a spiral track crossing;

selecting one out of the plurality of detected sync marks based on a reliability of the detected sync marks;

generating a timing recovery measurement in response to the selected sync mark;

synchronizing a servo write clock in response to the timing recovery measurement;

processing the read signal representing the high frequency signal in the spiral track crossing to generate a position error signal (PES) used to maintain the head along a substantially circular target path; and

using the servo write clock and the head internal to the disk drive to write the product servo sectors along the circular target path.

2. The method as recited in claim 1 , wherein:

two sync marks are detected in the spiral track crossing; and

the sync mark that generates the highest amplitude read signal is selected as having the high reliability.

3. The method as recited in claim 1 , wherein:

three sync marks are detected in the spiral track crossing; and

the second sync mark is selected as having the high reliability.

4. The method as recited in claim 1 , wherein:

four sync marks are detected in the spiral track crossing;

a timing recovery measurement is generated for at least the second and third sync marks; and

one of the second and third sync marks is selected as having the high reliability based on the timing recovery measurements.

5. The method as recited in claim 4 , wherein the sync mark that generates the minimum timing recovery measurement is selected as having the high reliability.

6. A disk drive comprising:

(a) a disk comprising a plurality of spiral tracks, wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark;

(b) an actuator arm;

(c) a head connected to a distal end of the actuator arm;

(d) a voice coil motor for rotating the actuator arm about a pivot to position the head radially over the disk; and

(e) control circuitry for writing a plurality of product servo sectors to the disk to define a plurality of radially spaced, concentric data tracks by:

using the head internal to the disk drive to read the spiral tracks to generate a read signal;

processing the read signal to detect a plurality of the sync marks in a spiral track crossing;

selecting one out of the plurality of detected sync marks based on a reliability of the detected sync marks;

generating a timing recovery measurement in response to the selected sync mark;

synchronizing a servo write clock in response to the timing recovery measurement;

processing the read signal representing the high frequency signal in the spiral track crossing to generate a position error signal (PES) used to maintain the head along a substantially circular target path; and

using the servo write clock and the head internal to the disk drive to write the product servo sectors along the circular target path.

7. The disk drive as recited in claim 6 , wherein:

two sync marks are detected in the spiral track crossing; and

the sync mark that generates the highest amplitude read signal is selected as having the high reliability.

8. The disk drive as recited in claim 6 , wherein:

three sync marks are detected in the spiral track crossing; and

the second sync mark is selected as having the high reliability.

9. The disk drive as recited in claim 6 , wherein:

four sync marks are detected in the spiral track crossing;

a timing recovery measurement is generated for at least the second and third sync marks; and

one of the second and third sync marks is selected as having the high reliability based on the timing recovery measurements.

10. The disk drive as recited in claim 9 , wherein the sync mark that generates the minimum timing recovery measurement is selected as having the high reliability.

11. A method of writing product servo sectors to a disk of a disk drive, the disk drive comprising control circuitry and a head disk assembly (HDA) comprising the disk, an actuator arm, a head connected to a distal end of the actuator arm, and a voice coil motor for rotating the actuator arm about a pivot to position the head radially over the disk, the disk comprising a plurality of spiral tracks, wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark, the method comprising:

using the head internal to the disk drive to read the spiral tracks to generate a read signal;

processing the read signal to detect a plurality of the sync marks in a spiral track crossing;

generating a plurality of timing recovery measurements corresponding to the plurality of detected sync marks;

filtering the plurality of timing recovery measurements to generate a filtered timing recovery measurement;

processing the high frequency signal in the spiral track crossing to generate a fine timing recovery measurement;

combining the fine timing recovery measurement with the filtered timing recovery measurement to generate a combined timing recovery measurement;

synchronizing a servo write clock in response to the combined timing recovery measurement;

processing the read signal representing the high frequency signal in the spiral track crossing to generate a position error signal (PES) used to maintain the head along a substantially circular target path; and

using the servo write clock and the head internal to the disk drive to write the product servo sectors along the circular target path.

12. The method as recited in claim 11 , wherein filtering the plurality of timing recovery measurements comprises computing an average of the timing recovery measurements for the spiral track crossing.

13. The method as recited in claim 11 , wherein filtering the plurality of timing recovery measurements comprises computing a weighted average of the timing recovery measurements for the spiral track crossing.

14. A disk drive comprising:

(a) a disk comprising a plurality of spiral tracks, wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark;

(b) an actuator arm;

(c) a head connected to a distal end of the actuator arm;

(d) a voice coil motor for rotating the actuator arm about a pivot to position the head radially over the disk; and

(e) control circuitry for writing a plurality of product servo sectors to the disk to define a plurality of radially spaced, concentric data tracks by:

using the head internal to the disk drive to read the spiral tracks to generate a read signal;

processing the read signal to detect a plurality of the sync marks in a spiral track crossing;

generating a plurality of timing recovery measurements corresponding to the plurality of detected sync marks;

filtering the plurality of timing recovery measurements to generate a filtered timing recovery measurement;

processing the high frequency signal in the spiral track crossing to generate a fine timing recovery measurement;

combining the fine timing recovery measurement with the filtered timing recovery measurement to generate a combined timing recovery measurement;

synchronizing a servo write clock in response to the combined timing recovery measurement;

processing the read signal representing the high frequency signal in the spiral track crossing to generate a position error signal (PES) used to maintain the head along a substantially circular target path; and

using the servo write clock and the head internal to the disk drive to write the product servo sectors along the circular target path.

15. The disk drive as recited in claim 14 , wherein the control circuitry filters the timing recovery measurements by computing an average of the timing recovery measurements for the spiral track crossing.

16. The disk drive as recited in claim 14 , wherein the control circuitry filters the timing recovery measurements by computing a weighted average of the timing recovery measurements for the spiral track crossing.