pc8477b National Semiconductor Corporation, pc8477b Datasheet - Page 35

• Manufacturer Part Number: pc8477b
• Description: Advanced Floppy Disk Controller
• Manufacturer: National Semiconductor Corporation

5 0 Functional Description
5 5 CRYSTAL OSCILLATOR
The PC8477B is clocked by a single 24 MHz signal for the
250 kb s 300 kb s 500 kb s and 1 Mb s 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
Mode
Effective Series
Shunt Capacitance
Recommended Crystals
NEL Frequency Controls
SaRonix
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 2 0V and
0 8V The controller should be configured so that the exter-
nal oscillator clock is input into the XTAL1 CLK pin and
XTAL2 is left unconnected
5 6 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
Resistance (ESR)
FIGURE 5-4 PC8477B Dynamic Window Margin Performance with
500 kb s
24 MHz
Parallel Resonant (preferred)
Fundamental Mode
Less than 50X
Less than 7 pF
NEL-C5480N
NEL-C2800N
NMP240
(Continued)
(Typical performance at V
TL F 11332 – 11
24 MHz
24 MHz
24 MHz
35
CC
(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
curves and measured at V
data separator should be able to tolerate at least
MSV and 60% window margin
5 7 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
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 Mb s thus doubling the storage capaci-
ty In addition the perpendicular 2 88M drive is read write
compatible with 1 44M and 720k diskettes (500 kb s and
250 kb s respectively)
The 2 88M drive has unique format and write data timing
requirements due to its read write head and pre-erase head
design (see Figure 5-5 ) Unlike conventional disk drives
which have only a read write head the 2 88M drive has
both a pre-erase head and read write head With conven-
tional disk drives the read write head by itself is able to
rewrite the disk without problems For 2 88M drives a pre-
erase head is needed to erase the magnetic flux on the disk
surface before the read write can write to the disk surface
The pre-erase head is activated during disk write operations
only i e Format and Write Data commands
g
e
3% at frequency of 1 kHz These are typical performance
5 0V 25 C)
g
3% ISV at 1 kHz
CC
1 Mb s
e
5 0V and 25 C A good
TL F 11332 – 12
g
6%