PC87303 datasheet(62/114 Pages) NSC

62 / 114 page

50 FDC Functional Description (Continued)

250 kbps

TLC12074 – 21

300 kbps

TLC12074 – 22

500 kbps

TLC12074 – 23

1 Mbps

TLC12074 – 24

FIGURE 5-2 PC87303 Dynamic Window Margin Performance

(Typical performance at VDD e 50V 25 C)

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)

PC87303 Datasheet(PDF) 62 Page - National Semiconductor (TI)