Spinpoint M8 Product Manual REV 2.7
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5.3 Servo System
The Servo System controls the position of the read/write heads and holds them on track during read/write
operations. The Servo System also compensates for MR write/read offsets and thermal offsets between heads on
different surfaces and for vibration and shock applied to the drive.
The Spinpoint M8 hard disk drive is an Embedded Sector Servo System. Positioning information is radially located
in evenly spaced servo sectors on each track.
Radial position information can be provided from these sectors for each data head. Because the drive incorporates
multiple data zones and each zone has a different bit density, split data fields are necessary for optimal use of the
non-servo area of the disk. The servo area remains phase-coherent across the surface of the disk, even though the
disk has various data zones. The main advantage of the Embedded Sector Servo System is that it eliminates the
problems of static and dynamic offsets between heads on different surfaces. The Spinpoint M8 hard disk drive
Servo System is classified as a digital servo system because track-following and seek control, bias cancellation, and
other typical tasks are done in a Digital Signal Processor (DSP).
The Servo system has three modes of operation: track-following mode, settle mode, and velocity control mode.
1. Track-following mode is used when heads are “on-track.” This is a position loop with an
integrator in the compensation.
2. Settle mode is used for all accesses; head switches, short-track seeks and long-track seeks.
Settle mode is a position loop with velocity damping. Settle mode does not use feed forward.
3. Velocity control mode is used for acceleration and deceleration of the actuator for seeking of
two or more tracks. A seek operation of this length is accomplished with a velocity control
loop. The drive’s ROM stores the velocity profile in a look-up table.
5.4 Read and Write Operations
The following two sections describe the read and write channels.
5.4.1 The Read Channel
The drive has one read/write head for each of the data surfaces. The signal path for the Read Channel starts at the
read/write heads. When the magnetic flux transitions recorded on a disk pass under the head, they generate low-
amplitude, differential output voltages. The read/write head transfers these signals to the flexible circuit’s amplifier,
which amplifies the signal.
The flexible circuit transmits the pre-amplified signal from the HDA to the PCBA. The EPRML channel on the
PCBA shapes, filters, detects, synchronizes, and decodes the data from the disk. The Read/Write IC then sends the
resynchronized data output to the 88i9422 (Rev3.1) DSP & Interface/Disk Controller.
The 88i9422 Disk Controller manages the flow of data between the Data Synchronizer on the Read/Write IC and
its AT Interface Controller. It also controls data access for the external RAM buffer. The ENDEC of 88C9410
decodes the LDPC with post-processor format to produce a serial bit stream. This NRZ (Non Return to Zero) serial
data is converted to 12-bits.
The Sequencer module identifies the data as belonging to the target sector. After a full sector is read, the
88i9422 checks to see if the firmware needs to apply an ECC algorithm to the data. The Buffer Control section of
the 88i9422 stores the data in the cache and transmits the data to the AT bus.
