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PC8477BV-1 Datasheet(PDF) 35 Page - National Semiconductor (TI) |
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PC8477BV-1 Datasheet(HTML) 35 Page - National Semiconductor (TI) |
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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 g 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 35 |
Similar Part No. - PC8477BV-1 |
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Similar Description - PC8477BV-1 |
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