Disk drive accounting for fractional clock cycle when measuring reader/writer gap

1. A disk drive comprising:

a disk;

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:

synchronize a disk-locked clock to a rotation of the disk, wherein:

the disk-locked clock comprises a plurality of clock cycles; and

the reader/writer gap spans a first number of the clock cycles comprising a first integer of the clock cycles plus a fraction of one of the clock cycles;

determine a reference clock cycle of the disk-locked clock;

delay a write operation relative to the reference clock cycle by a write delay comprising a second number of clock cycles comprising a second integer of the clock cycles plus the fraction of one of the clock cycles;

write a sync mark on the disk after the write delay; and

read the sync mark to estimate the first number of clock cycles.

2. The disk drive as recited in claim 1 , wherein the second integer of clock cycles is greater than the first integer of clock cycles.

3. The disk drive as recited in claim 1 , wherein the write delay causes the sync mark to substantially align with an edge of one of the clock cycles.

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

measure a read delay based on when the read element reaches the reference clock cycle and when the read element reads the sync mark; and

estimate the first number of clock cycles based on the write delay, the read delay, and the fraction of one of the clock cycles.

5. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to estimate the first number of clock cycles according to:

D_w−D_r+D_fraction

where:

D_w represents the second integer of clock cycles;

D_r represents the read delay; and

D_fraction represents the fraction of one of the clock cycles.

6. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to measure the fraction of one of the clock cycles.

7. A method of operating a disk drive comprising a head actuated radially over a disk, wherein the head comprises a read element separated from a write element by a reader/writer gap, the method comprising:

synchronizing a disk-locked clock to a rotation of the disk, wherein:

the disk-locked clock comprises a plurality of clock cycles; and

the reader/writer gap spans a first number of the clock cycles comprising a first integer of the clock cycles plus a fraction of one of the clock cycles;

determining a reference clock cycle of the disk-locked clock;

delaying a write operation relative to the reference clock cycle by a write delay comprising a second number of clock cycles comprising a second integer of the clock cycles plus the fraction of one of the clock cycles;

writing a sync mark on the disk after the write delay; and

reading the sync mark to estimate the first number of clock cycles.

8. The method as recited in claim 7 , wherein the second integer of clock cycles is greater than the first integer of clock cycles.

9. The method as recited in claim 7 , wherein the write delay causes the sync mark to substantially align with an edge of one of the clock cycles.

10. The method as recited in claim 7 , further comprising:

measuring a read delay based on when the read element reaches the reference clock cycle and when the read element reads the sync mark; and

estimating the first number of clock cycles based on the write delay, the read delay, and the fraction of one of the clock cycles.

11. The method as recited in claim 10 , further comprising estimating the first number of clock cycles according to:

D_w−D_r+D_fraction

where:

D_w represents the second integer of clock cycles;

D_r represents the read delay; and

D_fraction represents the fraction of one of the clock cycles.

12. The method as recited in claim 7 , further comprising measuring the fraction of one of the clock cycles.