PC87307VUL National Semiconductor, PC87307VUL Datasheet - Page 68

• Manufacturer Part Number: PC87307VUL
• Description: PC87307/PC97307 Plug and Play Compatible and PC97 Compliant SuperI/O
• Manufacturer: National Semiconductor

www.national.com
The controller maximizes the internal digital data separator
by implementing a read algorithm that enhances the lock
characteristics of the fully digital Phase-Locked Loop (PLL).
The algorithm minimizes the effect of bad data on the syn-
chronization between the PLL and the data.
It does this by forcing the fully digital PLL to re-lock to the
clock reference frequency any time the data separator at-
tempts to lock to a non-preamble pattern. See the state di-
agram of this read algorithm in Figure 5-3.
5.2.3
The FDC is fully compatible with perpendicular recording
mode disk drives at all data transfer rates. These perpendic-
ular drives are also called 4 Mbyte (unformatted) or 2.88
Mbyte (formatted) drives. This refers to their maximum stor-
age capacity.
Perpendicular recording orients the magnetic flux changes
(which represent bits) vertically on the disk surface, allow-
ing for a higher recording density than conventional longitu-
dinal recording methods. This increased recording density
increases data rate by up to 1 Mbps, thereby doubling the
storage capacity. In addition, the perpendicular 2.88 MB
drive is read/write compatible with 1.44 MB and 720 KB dis-
kettes (500 Kbps and 250 Kbps respectively).
The 2.88 MB drive has unique format and write data timing
requirements due to its read/write head and pre-erase head
design. This is illustrated in Figure 5-4.
Unlike conventional disk drives which have only a
read/write head, the 2.88 MB drive has both a pre-erase
head and read/write head. With conventional disk drives,
the read/write head, itself, can rewrite the disk without prob-
lems. 2.88 MB drives need a pre-erase head to erase the
magnetic flux on the disk surface before the read/write head
can write to the disk surface. The pre-erase head is activat-
ed during disk write operations only, i.e. FORMAT and
WRITE DATA commands.
Not
sixth bit.
Bit is
preamble.
Read Gate = 1
FIGURE 5-3. Read Algorithm State Diagram
Perpendicular Recording Mode Support
Wait six bits.
first bit that
preamble
to data.
Wait for
is not a
locking
Read Gate = 0
PLL
bit.
marks not found.
Three address
preamble.
to clock.
PLL idle
Bit is not
locked
The Digital Floppy Disk Controller (FDC) (Logical Device 3)
three address
mark bytes.
completed.
Operation
Check for
data field.
address mark.
Read ID
field or
Three address
marks found.
Not third
68
In 2.88 MB drives, the pre-erase head leads the read/write
head by 200 m, which translates to 38 bytes at 1 Mbps (19
bytes at 500 Kbps).
For both conventional and perpendicular drives, WGATE is
asserted with respect to the position of the read/write head.
With conventional drives, this means that WGATE is assert-
ed when the read/write head is located at the beginning of
the preamble to the data field.
With 2.88 MB drives, since the preamble must be erased
before it is rewritten, WGATE should be asserted when the
pre-erase head is located at the beginning of the preamble
to the data field. This means that WGATE should be assert-
ed when the read/write head is at least 38 bytes (at 1 Mbps)
before the preamble. Tables 5-15 and 5-16 on page 95
show how the perpendicular format affects gap 2 and, con-
sequently, WGATE timing, for different data rates.
Because of the 38-byte spacing between the read/write
head and the pre-erase head at 1 Mbps, the gap 2 length of
22 bytes used in the standard IBM disk format is not long
enough. The format standard for 2.88 MB drives at 1 Mbps
called the Perpendicular Format, increases the length of
gap 2 to 41 bytes. See Figure 5-20 on page 91.
The PERPENDICULAR MODE command puts the Floppy
Disk Controller (FDC) into perpendicular recording mode,
which allows it to read and write perpendicular media. Once
this command is invoked, the read, write and format com-
mands can be executed in the normal manner. The perpen-
dicular mode of the FDC functions at all data rates,
adjusting format and write data parameters accordingly.
See “The PERPENDICULAR MODE Command” on page
94 for more details.
5.2.4
The FDC sets the data rate in two ways. For PC compatible
software, the Configuration Control Register (CCR) at offset
07h programs the data rate for the FDC. The lower bits D1
and D0 in the CCR set the data rate. The other bits should
be set to zero. Table 5-6 on page 75 shows how to encode
the desired data rate.
The lower two bits of the Data rate Select Register (DSR) at
offset 04h can also set the data rate. These bits are encod-
ed like the corresponding bits in the CCR. The remainder of
the bits in the DSR have other functions. See the descrip-
tion of the DSR in Section 5.3.6 on page 75 for more details.
The data rate is determined by the last value written to ei-
ther the CCR or the DSR. Either the CCR or the DSR can
override the data rate selection of the other register. When
the data rate is selected, the micro-engine and data sepa-
rator clocks are scaled appropriately.
ID Field
End of
Read/
Write
Head
FIGURE 5-4. Perpendicular Recording Drive
Data Rate Selection
Read/Write Head and Pre-Erase Head
Intersector
Gap 2
(38 bytes @ 1 Mbps)
200 m
= 41 x 4Eh
Data Field
Preamble
Pre-
Erase
Head