FDC37M81 SMSC [SMSC Corporation], FDC37M81 Datasheet - Page 65

• Manufacturer Part Number: FDC37M81
• Description: PC98/99 Compliant Enhanced Super I/O Controller with Keyboard/Mouse Wake-Up
• Manufacturer: SMSC [SMSC Corporation]

The maximum count that the head can be
moved with a single Relative Seek command is
255 (D).
The internal register, PCN, will overflow as the
cylinder number crosses track 255 and will
contain 39 (D). The resulting PCN value is thus
(RCN + PCN) mod 256. Functionally, the FDC
starts counting from 0 again as the track
number goes above 255 (D). It is the user's
responsibility to compensate FDC functions
(precompensation
accessing tracks greater than 255. The FDC
does not keep track that it is working in an
"extended track area" (greater than 255). Any
command issued will use the current PCN value
except for the Recalibrate command, which only
looks for the TRACK0 signal. Recalibrate will
return an error if the head is farther than 79 due
to its limitation of issuing a maximum of 80 step
pulses. The user simply needs to issue a second
Recalibrate command. The Seek command and
implied seeks will function correctly within the
44 (D) track (299-255) area of the "extended
track area". It is the user's responsibility not to
issue a new track position that will exceed the
maximum track that is present in the extended
area.
To return to the standard floppy range (0-255) of
tracks, a Relative Seek should be issued to
cross the track 255 boundary.
A Relative Seek can be used instead of the
normal Seek, but the host is required to
calculate the difference between the current
head location and the new (target) head
location. This may require the host to issue a
Read ID command to ensure that the head is
physically on the track that software assumes it
to be.
different cylinder results which may be difficult
to keep track of with software without the Read
ID command.
Different FDC commands will return
track
number)
when
65
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.
command, the length of the Gap2 field and VCO
enable timing can be altered to accommodate
the unique requirements of these drives. Table
28 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.
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
Fields table 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
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.
PLL
VCO
is
enabled
With this
(VCOEN)
In the