80
Perpendicular Mode
The Perpendicular Mode command should be issued prior to executing Read/Write/Format
commands that access a disk drive with perpendicular recording capability. With this command, the
length of the Gap2 field and VCO enable timing can be altered to accommodate the unique
requirements of these drives. Table 37 describes the effects of the WGATE and GAP bits for the
Perpendicular Mode command.
Upon a reset, the FDC will default to the conventional mode
(WGATE = 0, GAP = 0).
Selection of the 500 Kbps and 1 Mbps perpendicular modes is independent of the actual data rate
selected in the Data Rate Select Register. The user must ensure that these two data rates remain
consistent.
The Gap2 and VCO timing requirements for perpendicular recording type drives are dictated by the
design of the read/write head. In the design of this head, a pre-erase head precedes the normal
read/write head by a distance of 200 micrometers. This works out to about 38 bytes at a 1 Mbps
recording density. Whenever the write head is enabled by the Write Gate signal, the pre-erase head
is also activated at the same time. Thus, when the write head is initially turned on, flux transitions
recorded on the media for the first 38 bytes will not be preconditioned with the pre-erase head since it
has not yet been activated. To accommodate this head activation and deactivation time, the Gap2
field is expanded to a length of 41 bytes. The format field illustrates the change in the Gap2 field
size for the perpendicular format.
On the read back by the FDC, the controller must begin synchronization at the beginning of the sync
field. For the conventional mode, the internal PLL VCO is enabled (VCOEN) approximately 24 bytes
from the start of the Gap2 field. But, when the controller operates in the 1 Mbps perpendicular mode
(WGATE = 1, GAP = 1), VCOEN goes active after 43 bytes to accommodate the increased Gap2
field size. For both cases, and approximate two-byte cushion is maintained from the beginning of the
sync field for the purposes of avoiding write splices in the presence of motor speed variation.
For the Write Data case, the FDC activates Write Gate at the beginning of the sync field under the
conventional mode. The controller then writes a new sync field, data address mark, data field, and
CRC. With the pre-erase head of the perpendicular drive, the write head must be activated in the
Gap2 field to insure a proper write of the new sync field.
For the 1 Mbps perpendicular mode
(WGATE = 1, GAP = 1), 38 bytes will be written in the Gap2 space.
Since the bit density is
proportional to the data rate, 19 bytes will be written in the Gap2 field for the 500 Kbps perpendicular
mode (WGATE = 1, GAP =0).
It should be noted that none of the alterations in Gap2 size, VCO
timing, or Write Gate timing affect normal program flow. The information provided here is just for
background purposes and is not needed for normal operation.
Once the Perpendicular Mode
command is invoked, FDC software behavior from the user standpoint is unchanged.
The perpendicular mode command is enhanced to allow specific drives to be designated
Perpendicular recording drives. This enhancement allows data transfers between Conventional and