PC87332VLJ datasheet(62/98 Pages) NSC

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50 FDC Functional Description (Continued)

TLC11930 – 9

FIGURE 5-2 Read Data AlgorithmState Diagram

55 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 24V and

04V The controller should be configured so that the exter-

nal oscillator clock is input into the X1OSC pin and XTAL2

is left unconnected

56 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

288 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 288M drive is readwrite

compatible with 144M and 720k diskettes (500 kbps and

250 kbps respectively)

The 288M drive has unique format and write data timing

requirements due to its readwrite head and pre-erase head

design (see

Figure 5-3 ) Unlike conventional disk drives

which have only a readwrite head the 288M drive has

both a pre-erase head and readwrite head With conven-

tional disk drives the readwrite head by itself is able to

rewrite the disk without problems For 288M drives a pre-

erase head is needed to erase the magnetic flux on the disk

surface before the readwrite can write to the disk surface

The pre-erase head is activated during disk write operations

only ie Format and Write Data commands

In 288M drives the pre-erase head leads the readwrite

head by 200 mm which translates to 38 bytes at 1 Mbps

(19 bytes at 500 kbps) For both conventional and perpen-

dicular drives WGATE is asserted with respect to the posi-

tion of the readwrite head With conventional drives this

means that WGATE is asserted when the readwrite head is

located at the beginning of the Data Field preamble With

the 288M 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 readwrite 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 readwrite

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 288M 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 418 for

more details

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