PC8477B datasheet(35/54 Pages) NSC

35 / 54 page

50 Functional Description (Continued)

500 kbs

TLF11332 – 11

1 Mbs

TLF11332 – 12

FIGURE 5-4 PC8477B Dynamic Window Margin Performance with g3% ISV at 1 kHz

(Typical performance at VCC e 50V 25 C)

55 CRYSTAL OSCILLATOR

The PC8477B is clocked by a single 24 MHz signal for the

250 kbs 300 kbs 500 kbs and 1 Mbs data rates An

on-chip oscillator is provided to enable the attachment of a

crystal or a clock signal If a crystal is used the following

parameters are required

Crystal Specifications

Frequency

24 MHz

Mode

Parallel Resonant (preferred)

Fundamental Mode

Effective Series

Resistance (ESR)

Less than 50X

Shunt Capacitance

Less than 7 pF

Recommended Crystals

NEL Frequency Controls

NEL-C5480N

24 MHz

NEL-C2800N

24 MHz

SaRonix

NMP240

24 MHz

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 circuit is able to

utilize high profile low profile and surface mount type crys-

tal enclosures External bypass capacitors (5 pF to 15 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 20V and

08V The controller should be configured so that the exter-

nal oscillator clock is input into the XTAL1CLK pin and

XTAL2 is left unconnected

56 DYNAMIC WINDOW MARGIN PERFORMANCE

The performance of the data separator is measured by its

ability to read and decode incoming pulses shifted away

from the nominal position The percentage window margin

indicates how much bit shift the data separator will tolerate

and still be able to read correctly For a Dynamic Window

Margin test all the bits in the data pattern are subject to jitter

(as they would be in a real floppy drive) and the frequency

of the data stream is subject to changes arising from motor

speed variations

Typical dynamic margin performance

curves for the PC8477B are listed in

Figure 5-3 Thse mea-

surements are taken using a FlexStar FS-540 Disk Simula-

tor with a repetitive ‘‘DB6’’ data pattern The graphs indicate

motor speed variation (MSV) vs bit jitter tolerance for the

floppy controller For reliable performance with tape drives

the data separator needs to be able to track to instanta-

neous changes as well

Figure 5-4 shows jitter tolerance vs

MSV with an added instantaneous speed variation (ISV) of

3% at frequency of 1 kHz These are typical performance

curves and measured at VCC e 50V and 25 C A good

data separator should be able to tolerate at least g6%

MSV and 60% window margin

57 PERPENDICULAR RECORDING MODE

The PC8477B is fully compatible with perpendicular record-

ing 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 5-5 ) 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

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