Disk drive writing a sync mark seam in a bootstrap spiral track

1. A disk drive comprising:

a disk;

a head actuated over the disk;

a spindle motor operable to rotate the disk; and

control circuitry operable to:

measure a spindle back electromotive force (BEMF) voltage generated by the spindle motor;

write a bootstrap spiral track to the disk, wherein the bootstrap spiral track comprises a high frequency signal interrupted by sync marks;

write a first sync mark while writing a first part of the bootstrap spiral track;

write a second sync mark while writing a second part of the bootstrap spiral track; and

switch between writing the first sync mark and the second sync mark in response to the spindle BEMF voltage, thereby generating a sync mark seam in the bootstrap spiral track.

2. The disk drive as recited in claim 1 , wherein the control circuitry is operable to switch between writing the first sync mark and the second sync mark in response to zero crossings in the spindle BEMF voltage.

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

position the head at one of an outer diameter of the disk and an inner diameter of the disk; and

launch the head toward a middle diameter of the disk while writing the first part of the bootstrap spiral track.

4. The disk drive as recited in claim 3 , wherein the control circuitry is further operable to:

first synchronize a disk locked clock to zero crossings in the spindle BEMF voltage; and

launch the head toward a middle diameter of the disk when the disk locked lock reaches a first value.

5. The disk drive as recited in claim 4 , wherein the control circuitry is operable to switch between writing the first sync mark and the second sync mark when the disk locked clock reaches a second value.

6. The disk drive as recited in claim 5 , wherein the second value corresponds to a target interval between a first and second zero crossings in the spindle BEMF voltage.

7. The disk drive as recited in claim 6 , wherein the target interval is halfway between the first and second zero crossings in the spindle BEMF voltage.

8. The disk drive as recited in claim 5 , wherein after writing the bootstrap spiral to the disk, the control circuitry is further operable to:

second synchronize the disk locked clock to zero crossings in the spindle BEMF voltage;

clock a counter in response to the disk locked clock;

detect the sync mark seam in the bootstrap spiral track; and

initialize the counter in response to detecting the sync mark seam.

9. The disk drive as recited in claim 8 , wherein the control circuitry is further operable to initialize the counter in response to a zero crossing in the spindle BEMF voltage after detecting the sync mark seam.

10. The disk drive as recited in claim 3 , wherein the control circuitry is further operable to:

accelerate the head toward the middle diameter of the disk while writing the first part of the bootstrap spiral track; and

proximate to switching between writing the first sync mark and the second sync mark, move the head toward the middle diameter of the disk at a substantially constant velocity.

11. A disk drive comprising:

a disk comprising a spiral track, wherein the spiral track comprises a high frequency signal interrupted by sync marks including a first sync mark over a first part of the spiral track and a second sync mark over a second part of the spiral track thereby forming a sync mark seam in the spiral track;

a head actuated over the disk; and

control circuitry operable to:

move the head radially over the disk;

read the spiral track to generate a read signal;

integrate the read signal representing the high frequency signal;

when the first sync mark is detected, accumulate the integrated read signal into a first weighted measure;

when the second sync mark is detected, accumulate the integrated read signal into a second weighted measure; and

detect the sync mark seam in response to the first and second weighted measures.

12. The disk drive as recited in claim 11 , wherein the control circuitry is further operable to detect the sync mark seam proximate to when an amplitude of the second weighted measure crosses an amplitude of the first weighted measure.

13. A method of operating a disk drive comprising, the disk drive comprising a disk, a head actuated over the disk, and a spindle motor operable to rotate the disk, the method comprising:

measuring a spindle back electromotive force (BEMF) voltage generated by the spindle motor;

writing a bootstrap spiral track to the disk, wherein the bootstrap spiral track comprises a high frequency signal interrupted by sync marks;

writing a first sync mark while writing a first part of the bootstrap spiral track;

writing a second sync mark while writing a second part of the bootstrap spiral track; and

switching between writing the first sync mark and the second sync mark in response to the spindle BEMF voltage, thereby generating a sync mark seam in the bootstrap spiral track.

14. The method as recited in claim 13 , further comprising switching between writing the first sync mark and the second sync mark in response to zero crossings in the spindle BEMF voltage.

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

positioning the head at one of an outer diameter of the disk and an inner diameter of the disk; and

launching the head toward a middle diameter of the disk while writing the first part of the bootstrap spiral track.

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

first synchronizing a disk locked clock to zero crossings in the spindle BEMF voltage; and

launching the head toward a middle diameter of the disk when the disk locked lock reaches a first value.

17. The method as recited in claim 16 , further comprising switching between writing the first sync mark and the second sync mark when the disk locked clock reaches a second value.

18. The method as recited in claim 17 , wherein the second value corresponds to a target interval between a first and second zero crossings in the spindle BEMF voltage.

19. The method as recited in claim 18 , wherein the target interval is halfway between the first and second zero crossings in the spindle BEMF voltage.

20. The method as recited in claim 17 , wherein after writing the bootstrap spiral to the disk, further comprising:

second synchronizing the disk locked clock to zero crossings in the spindle BEMF voltage;

clocking a counter in response to the disk locked clock;

detecting the sync mark seam in the bootstrap spiral track; and

initializing the counter in response to detecting the sync mark seam.

21. The method as recited in claim 20 , further comprising initializing the counter in response to a zero crossing in the spindle BEMF voltage after detecting the sync mark seam.

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

accelerating the head toward the middle diameter of the disk while writing the first part of the bootstrap spiral track; and

proximate to switching between writing the first sync mark and the second sync mark, moving the head toward the middle diameter of the disk at a substantially constant velocity.

23. A method of operating a disk drive, the disk drive comprising:

a disk comprising a spiral track, wherein the spiral track comprises a high frequency signal interrupted by sync marks including a first sync mark over a first part of the spiral track and a second sync mark over a second part of the spiral track thereby forming a sync mark seam in the spiral track; and

a head actuated over the disk,

the method comprising:

moving the head radially over the disk;

reading the spiral track to generate a read signal;

integrating the read signal representing the high frequency signal;

when the first sync mark is detected, accumulating the integrated read signal into a first weighted measure;

when the second sync mark is detected, accumulating the integrated read signal into a second weighted measure; and

detecting the sync mark seam in response to the first and second weighted measures.

24. The method as recited in claim 23 , further comprising detecting the sync mark seam proximate to when an amplitude of the second weighted measure crosses an amplitude of the first weighted measure.