Disk drive averaging phase-offset due to reader/writer gap in order to recover extended servo data

1. A disk drive comprising:

a disk comprising a plurality of servo tracks, wherein each servo track comprises a plurality of servo sectors;

a head actuated radially over the disk, wherein the head comprises a read element separated from a write element by a reader/writer gap; and

control circuitry operable to:

position the head over a first servo track;

synchronize a first clock to servo data in first and second servo sectors of the first servo track, wherein a length of the reader/writer gap comprises a non-integer number of cycles of the first clock at the first servo track;

measure a first phase-offset of the first clock for the first servo sector and measure a second phase-offset of the first clock for the second servo sector, wherein the first and second phase-offset correspond to an end of the reader/writer gap at the first servo track; and

average the first and second phase-offset to generate a first average phase-offset for the first servo track.

2. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to write extended servo data near a third servo sector of the first servo track in response to the first clock and the first average phase-offset.

3. The disk drive as recited in claim 2 , wherein the control circuitry is further operable to write extended servo data near a plurality of the servo sectors of the first servo track in response to the first clock and the first average phase-offset.

4. The disk drive as recited in claim 2 , wherein the control circuitry is further operable to:

generate a second clock phase offset from the first clock based on the first average phase-offset; and

write the extended servo data near the third servo sector in response to the second clock.

5. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to read extended servo data near a third servo sector of the first servo track in response to the first clock and the first average phase-offset.

6. The disk drive as recited in claim 5 , wherein the control circuitry is further operable to:

read the extended servo data to generate a read signal;

sample the read signal in response to the first clock to generate signal samples;

phase shift the signal samples in response to the first average phase-offset to generate phase-shifted signal samples; and

detect the extended servo data in response to the phase-shifted signal samples.

7. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to:

position the head over a second servo track;

synchronize the first clock to servo data in fourth and fifth servo sectors of the second servo track, wherein the length of the reader/writer gap comprises a non-integer number of cycles of the first clock at the second servo track;

measure a third phase-offset of the first clock for the fourth servo sector and measure a fourth phase-offset of the first clock for the fifth servo sector, wherein the third and fourth phase-offset correspond to an end of the reader/writer gap at the second servo track;

average the third and fourth phase-offset to generate a second average phase-offset for the second servo track;

write extended servo data near a sixth servo sector of the second servo track; and

perform one of:

write the extended servo data near the sixth servo sector in response to the first clock and the second average phase-offset; and

read the extended servo data near the sixth servo sector in response to the first clock and the second average phase-offset.

8. The disk drive as recited in claim 7 , wherein the control circuitry is further operable to:

position the head over a third servo track;

compute a third average phase-offset in response to the first and second average phase-offset;

write extended servo data near a seventh servo sector of the third servo track; and

perform one of:

write the extended servo data near the seventh servo sector in response to the first clock and the third average phase-offset; and

read the extended servo data near the seventh servo sector in response to the first clock and the third average phase-offset.

9. The disk drive as recited in claim 8 , wherein the control circuitry is further operable to write extended servo data near a plurality of the servo sectors of the third servo track in response to the first clock and the third average phase-offset.

10. A method of operating a disk drive, the disk drive comprising a disk comprising a plurality of servo tracks, wherein each servo track comprises a plurality of servo sectors, and a head actuated radially over the disk, wherein the head comprises a read element separated from a write element by a reader/writer gap, the method comprising:

positioning the head over a first servo track;

synchronizing a first clock to servo data in first and second servo sectors of the first servo track, wherein a length of the reader/writer gap comprises a non-integer number of cycles of the first clock at the first servo track;

measuring a first phase-offset of the first clock for the first servo sector and measure a second phase-offset of the first clock for the second servo sector, wherein the first and second phase-offset correspond to an end of the reader/writer gap at the first servo track; and

averaging the first and second phase-offset to generate a first average phase-offset for the first servo track.

11. The method as recited in claim 10 , further comprising writing extended servo data near a third servo sector of the first servo track in response to the first clock and the first average phase-offset.

12. The method as recited in claim 11 , further comprising writing extended servo data near a plurality of the servo sectors of the first servo track in response to the first clock and the first average phase-offset.

13. The method as recited in claim 11 , further comprising:

generating a second clock phase offset from the first clock based on the first average phase-offset; and

writing the extended servo data near the third servo sector in response to the second clock.

14. The method as recited in claim 10 , further comprising reading extended servo data near a third servo sector of the first servo track in response to the first clock and the first average phase-offset.

15. The method as recited in claim 14 , further comprising:

reading the extended servo data to generate a read signal;

sampling the read signal in response to the first clock to generate signal samples;

phase shifting the signal samples in response to the first average phase-offset to generate phase-shifted signal samples; and

detecting the extended servo data in response to the phase-shifted signal samples.

16. The method as recited in claim 10 , the method further comprising:

positioning the head over a second servo track;

synchronizing the first clock to servo data in fourth and fifth servo sectors of the second servo track, wherein the length of the reader/writer gap comprises a non-integer number of cycles of the first clock at the second servo track;

measuring a third phase-offset of the first clock for the fourth servo sector and measure a fourth phase-offset of the first clock for the fifth servo sector, wherein the third and fourth phase-offset correspond to an end of the reader/writer gap at the second servo track;

averaging the third and fourth phase-offset to generate a second average phase-offset for the second servo track;

writing extended servo data near a sixth servo sector of the second servo track; and

performing one of:

writing the extended servo data near the sixth servo sector in response to the first clock and the second average phase-offset; and

reading the extended servo data near the sixth servo sector in response to the first clock and the second average phase-offset.

17. The method as recited in claim 16 , further comprising:

positioning the head over a third servo track;

computing a third average phase-offset in response to the first and second average phase-offset;

writing extended servo data near a seventh servo sector of the third servo track; and

performing one of:

writing the extended servo data near the seventh servo sector in response to the first clock and the third average phase-offset; and

reading the extended servo data near the seventh servo sector in response to the first clock and the third average phase-offset.

18. The method as recited in claim 17 , further comprising writing extended servo data near a plurality of the servo sectors of the third servo track in response to the first clock and the third average phase-offset.