FDC37C78-HT Standard Microsystems (SMSC), FDC37C78-HT Datasheet - Page 55

• Manufacturer Part Number: FDC37C78-HT
• Manufacturer: Standard Microsystems (SMSC)

Specifications of FDC37C78-HT

• Package Type: TQFP
• Lead Free Status / RoHS Status: Compliant

command, the length of the Gap2 field and VCO
enable timing can be altered to accommodate the
unique requirements of these drives. Table 31
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.
recording density.
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.
accommodate
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.
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
This works out to about 38 bytes at a 1 Mbps
is
this
Whenever the write head is
enabled
head
activation
(VCOEN)
and
To
55
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 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
Perpendicular Mode command is invoked, FDC
software behavior from the user standpoint is
unchanged.
The perpendicular mode command is enhanced to
allow
Perpendicular
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.
specific
for
normal
drives
recording
operation.
to
be
drives.
Once
designated
This
the