PC87322VF Datasheet, PDF (48/84 Pages) National Semiconductor (TI) – Floppy Disk Controller with Dual UARTs, Enhanced Parallel Port, and IDE Interface

5 0 FDC Functional Description (Continued)
0 4V The controller should be configured so that the exter-
nal oscillator clock is input into the X1 OSC pin and XTAL2
is left unconnected
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 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 5-4 ) 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
In 2 88M drives the pre-erase head leads the read write
head by 200 mm which translates to 38 bytes at 1 Mb s (19
bytes at 500 kb s) For both conventional and perpendicular
drives WGATE is asserted with respect to the position of
the read write head With conventional drives this means
that WGATE is asserted when the read write head is locat-
ed at the beginning of the Data Field preamble With the
FIGURE 5-3 Read Data Algorithm State Diagram
TL C 11870 – 16
FIGURE 5-4 Perpendicular Recording Drive R W Head and Pre-Erase Head
TL C 11870 – 17
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