Since the number of sectors is divisible by the motor pole number and there
must be integer number n sector of clock cycles for each servo sector, N must
be integer multiple of m sectors . The various numbers in the above equation
must be adjusted to determine an appropriate N.
Writing and Veri fi cation of the Clock Track
An INDEX mark, a special unique pattern, is written fi rst prior to writ-
ing the pre-programmed patterns for the clock track transitions. This unique
pattern is used by the circuit to recognize the beginning of the track. The
INDEX pattern, after it has been written, is sensed by the read head after
one revolution of the disk. As soon as the INDEX is detected, the write elec-
tronics is triggered to start writing the transition patterns for the clock track.
The writing of the transitions is done using a free running clock signal, and is
stopped as soon as the INDEX mark is detected again.
It should be noted that the number of cycles written on the clock track
(N) depends among many factors on the spindle speed and period of the free
running clock. Any variation in either the spindle speed or the frequency of the
clock causes the number of clock cycles written be different from pre-calculated
N. Such variation within the span of one revolution makes the transitions in
the clock track to be unequally spaced, which is not desired. It is also possible
that the gap between the clock transition writen last and the fi rst transition
of the INDEX does not satisfy the speci fi cations. These cases are known as
imperfect clock closure, and for both cases, the clock track must be re-written.
In case of re-writing the clock track, the speed of the spindle motor is dithered.
The process is repeated until a good clock track is written.
Once the clock track is written, a PLL is allowed to lock to the signal from
the clock track to produce a reference clock signal that is synchronized to the
tangential positions of the disk. The synchronized clock output of the PLL
divides the time of one revolution into N equal time slots. This clock signal
can also be used to de fi ne the boundaries of the sectors.
Since the clock tracks written offer a more accurate tangential position
signal than the spindle motor back EMF zero-crossing, the clock signal can be
used as speed feedback for accurate speed control during the process of servo
writing. For example, a few clock counts per revolution can be changed to
achieve the spindle speed dithering.
During the servo writing process, the clock signals are written by inserting
athin fi lm head into the HDD enclosure. This insertion is performed by a
mechanical unit of the clock head module of STW. After the process of servo
writing is over, the thin fi lm head is extracted. Thus, there is a need for an
opening in the drive enclosure to insert clock head. So the servo writing process
must be carried out in a clean room environment to minimize the harmful
effects caused by contaminations of the environment inside drive enclosure.
The opening is covered with a seal at the end of the servo writing, before the
drive is taken out of the clean room.