PC87311AVF datasheet(46/78 Pages) NSC

46 / 78 page

50 FDC Functional Description (Continued)

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 Mbs thus doubling the storage capaci-

ty In addition the perpendicular 288M drive is readwrite

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

250 kbs respectively)

The 288M drive has unique format and write data timing

requirements due to its readwrite head and pre-erase head

design (see

Figure 5-4 ) 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 Mbs (19

bytes at 500 kbs) For both conventional and perpendicular

drives WGATE is asserted with respect to the position of

the readwrite head With conventional drives this means

that WGATE is asserted when the readwrite head is locat-

ed at the beginning of the Data Field preamble With the

TLF11362 – 11

FIGURE 5-3 Read Data AlgorithmState Diagram

TLF11362 – 12

FIGURE 5-4 Perpendicular Recording Drive RW Head and Pre-Erase Head

PC87311AVF Datasheet(PDF) 46 Page - National Semiconductor (TI)