LPC47B34X datasheet(72/250 Pages) SMSC

72 / 250 page

As an example, assume that a floppy drive has 300 useable tracks. The host needs to read track 300

and the head is on any track (0-255). If a Seek command is issued, the head will stop at track 255. If

a Relative Seek command is issued, the FDC will move the head the specified number of tracks,

regardless of the internal cylinder position register (but will increment the register). If the head was on

track 40 (d), the maximum track that the FDC could position the head on using Relative Seek will be

295 (D), the initial track + 255 (D). The maximum count that the head can be moved with a single

Relative Seek command is 255 (D).

The internal register, PCN, will overflow as the cylinder number crosses track 255 and will contain 39

(D). The resulting PCN value is thus (RCN + PCN) mod 256. Functionally, the FDC starts counting

from 0 again as the track number goes above 255 (D). It is the user's responsibility to compensate

FDC functions (precompensation track number) when accessing tracks greater than 255. The FDC

does not keep track that it is working in an "extended track area" (greater than 255). Any command

issued will use the current PCN value except for the Recalibrate command, which only looks for the

TRACK0 signal. Recalibrate will return an error if the head is farther than 79 due to its limitation of

issuing a maximum of 80 step pulses. The user simply needs to issue a second Recalibrate

command. The Seek command and implied seeks will function correctly within the 44 (D) track (299-

255) area of the "extended track area". It is the user's responsibility not to issue a new track position

that will exceed the maximum track that is present in the extended area.

To return to the standard floppy range (0-255) of tracks, a Relative Seek should be issued to cross the

track 255 boundary.

A Relative Seek can be used instead of the normal Seek, but the host is required to calculate the

difference between the current head location and the new (target) head location. This may require the

host to issue a Read ID command to ensure that the head is physically on the track that software

assumes it to be. Different FDC commands will return different cylinder results which may be difficult

to keep track of with software without the Read ID command.

Perpendicular Mode

The Perpendicular Mode command should be issued prior to executing Read/Write/Format commands

that access a disk drive with perpendicular recording capability. With this command, the length of the

Gap2 field and VCO enable timing can be altered to accommodate the unique requirements of these

drives.

Table 27 describes the effects of the WGATE and GAP bits for the Perpendicular Mode

command. Upon a reset, the FDC will default to the conventional mode (WGATE = 0, GAP = 0).

Selection of the 500 Kbps and 1 Mbps perpendicular modes is independent of the actual data rate

selected in the Data Rate Select Register. The user must ensure that these two data rates remain

consistent.

The Gap2 and VCO timing requirements for perpendicular recording type drives are dictated by the

design of the read/write head.

In the design of this head, a pre-erase head precedes the normal

read/write head by a distance of 200 micrometers. This works out to about 38 bytes at a 1 Mbps

recording density. Whenever the write head is enabled by the Write Gate signal, the pre-erase head is

also activated at the same time. Thus, when the write head is initially turned on, flux transitions

recorded on the media for the first 38 bytes will not be preconditioned with the pre-erase head since it

has not yet been activated. To accommodate this head activation and deactivation time, the Gap2

field is expanded to a length of 41 bytes. The format field shown on Page 58 illustrates the change in

the Gap2 field size for the perpendicular format.

LPC47B34X Datasheet(PDF) 72 Page - SMSC Corporation

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