PC87306 datasheet(60/110 Pages) NSC

60 / 110 page

50 FDC Functional Description (Continued)

250 kbps

TLC12379 – 31

300 kbps

TLC12379 – 32

500 kbps

TLC12379 – 33

1 Mbps

TLC12379 – 34

FIGURE 5-2 PC87306 Dynamic Window Margin Performance

(Typical performance at VDD e 50V 25 C)

window margin (or bit jitter) that the data separator can han-

dle with no read errors Thus the area under the dynamic

window margin curves in

Figure 5-2 is the range of MSV and

bit jitter that the FDC can handle with no read errors The

FDC internal analog data separator has a much better per-

formance than comparable digital data separator designs

and does not require any external components

Note

The dynamic window margin curves were generated using a FlexStar

FS-540 Floppy Disk Simulator and a proprietary dynamic window

margin test program written by National Semiconductor

The controller takes best advantage of the internal analog

data separator by implementing a sophisticated read algo-

rithm This ID search algorithm shown in

Figure 5-3 en-

hances the PLL’s lock characteristics by forcing the PLL to

relock to the crystal reference frequency any time the data

separator attempts to lock to a non-preamble pattern This

algorithm ensures that the PLL is not thrown way out of lock

by write splices or bad data fields

55 CRYSTAL OSCILLATOR

The FDC is clocked by a single 24 MHz signal

An external oscillator circuit 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

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)

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