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
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 shown on page 61
illustrates the change in the Gap2 field size for the
perpendicular format.
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 as shown in Figure 4.
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 Perpendicular drives
without having to issue Perpendicular mode commands
between the accesses of the different drive types, nor
having to change write pre-compensation values.
When both GAP and WGATE bits of the
PERPENDICULAR MODE COMMAND are both
programmed to "0" (Conventional mode), then D0, D1,
D2, D3, and D4 can be programmed independently to "1"
for that drive to be set automatically to Perpendicular
mode. In this mode the following set of conditions also
apply:
1. The GAP2 written to a perpendicular drive during a
write operation will depend upon the programmed
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