PC87306 Datasheet, PDF (60/110 Pages) National Semiconductor (TI) – PC87306 SuperI/OTM Enhanced Sidewinder Lite Floppy Disk Controller, Keyboard Controller, Real-Time Clock, Dual UARTs, Infrared Interface, IEEE 1284 Pa

5 0 FDC Functional Description (Continued)
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1 Mbps
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FIGURE 5-2 PC87306 Dynamic Window Margin Performance
(Typical performance at V_DD e 5 0V 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
5 5 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 2 4V and
0 4V The controller should be configured so that the exter-
nal oscillator clock is input into the X1 OSC pin
5 6 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
2 88 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 2 88M drive is read write
compatible with 1 44M and 720k diskettes (500 kbps and
250 kbps respectively)
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