PC87332VLJ-5 NSC [National Semiconductor], PC87332VLJ-5 Datasheet - Page 62

• Manufacturer Part Number: PC87332VLJ-5
• Description: PC87332VLJ (3.3V/5V) and PC87332VLJ-5 (5V) (SuperI/OTM III Premium Green) Floppy Disk Controller, Dual UARTs, IEEE1284 Parallel Port, and IDE Interfac
• Manufacturer: NSC [National Semiconductor]

5 0 FDC Functional Description
5 5 CRYSTAL OSCILLATOR
The FDC is clocked by a single 24 MHz signal An on-chip
oscillator is provided to enable the attachment of a crystal
or a clock signal
A parallel resonant crystal is preferred if at all possible In
some cases a series resonant crystal can be used but care
must be taken to ensure that the crystal does not oscillate
at a sub-harmonic frequency The oscillator is able to work
with high profile low profile and surface mount type crystal
enclosures External bypass capacitors (5 pF to 10 pF)
should be connected from XTAL1 and XTAL2 to GND If an
external oscillator circuit is used it must have a duty cycle of
at least 40%–60% and minimum input levels of 2 4V and
0 4V The controller should be configured so that the exter-
nal oscillator clock is input into the X1 OSC pin and XTAL2
is left unconnected
5 6 PERPENDICULAR RECORDING MODE
The FDC is fully compatible with perpendicular recording
mode disk drives at all data rates These perpendicular
mode drives are also called 4 Mbyte (unformatted) or
2 88 Mbyte (formatted) drives which refers to their maxi-
mum storage capacity Perpendicular recording will orient
the magnetic flux changes (which represent bits) vertically
on the disk surface allowing for a higher recording density
than the conventional longitudinal recording methods With
this increase in recording density comes an increase in the
data rate of up to 1 Mbps thus doubling the storage capaci-
ty In addition the perpendicular 2 88M drive is read write
compatible with 1 44M and 720k diskettes (500 kbps and
250 kbps respectively)
The 2 88M drive has unique format and write data timing
requirements due to its read write head and pre-erase head
design (see Figure 5-3 ) Unlike conventional disk drives
which have only a read write head the 2 88M drive has
both a pre-erase head and read write head With conven-
FIGURE 5-2 Read Data Algorithm State Diagram
(Continued)
62
tional disk drives the read write head by itself is able to
rewrite the disk without problems For 2 88M drives a pre-
erase head is needed to erase the magnetic flux on the disk
surface before the read write can write to the disk surface
The pre-erase head is activated during disk write operations
only i e Format and Write Data commands
In 2 88M drives the pre-erase head leads the read write
head by 200
(19 bytes at 500 kbps) For both conventional and perpen-
dicular drives WGATE is asserted with respect to the posi-
tion of the read write head With conventional drives this
means that WGATE is asserted when the read write head is
located at the beginning of the Data Field preamble With
the 2 88M drives since the preamble must be pre-erased
before it is rewritten WGATE should be asserted when the
pre-erase head is located at the beginning of the Data Field
preamble This means that WGATE should be asserted
when the read write head is at least 38 bytes (at 1 Mbps)
before the preamble See Table 4-6 for a description of the
WGATE timing for perpendicular drives at the various data
rates
Because of the 38 byte spacing between the read write
head and the pre-erase head at 1 Mbps the GAP2 length of
22 bytes used in the standard IBM disk format is not long
enough There is a new format standard for 2 88M drives at
1 Mbps called the Perpendicular Format which increases
the GAP2 length to 41 bytes (see Figure 4-7 ) The Perpen-
dicular Mode command will put the floppy controller into
perpendicular recording mode which allows it to read and
write perpendicular media Once this command is invoked
the read write and format commands can be executed in
the normal manner The perpendicular mode of the floppy
controller will work at all data rates adjusting the format and
write data parameters accordingly See Section 4 1 8 for
more details
m which translates to 38 bytes at 1 Mbps
TL C 11930 – 9