LPC47N227TQFP Standard Microsystems (SMSC), LPC47N227TQFP Datasheet - Page 65

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LPC47N227TQFP

Manufacturer Part Number
LPC47N227TQFP
Description
Manufacturer
Standard Microsystems (SMSC)
Datasheet

Specifications of LPC47N227TQFP

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The Relative Seek command differs from the Seek command in that it steps the head the absolute
number of tracks specified in the command instead of making a comparison against an internal register.
The Seek command is good for drives that support a maximum of 256 tracks. Relative Seeks cannot be
overlapped with other Relative Seeks. Only one Relative Seek can be active at a time. Relative Seeks
may be overlapped with Seeks and Recalibrates. Bit 4 of Status Register 0 (EC) will be set if Relative
Seek attempts to step outward beyond Track 0.
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.
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.
Table 27 describes the effects of the WGATE and GAP bits for the Perpendicular Mode
DIR
0
1
Step Head Out
Step Head In
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