SCH3112I-NE SMSC [SMSC Corporation], SCH3112I-NE Datasheet - Page 90

• Manufacturer Part Number: SCH3112I-NE
• Description: LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
• Manufacturer: SMSC [SMSC Corporation]

LPC IO with 8042 KBC, Reset Generation, HWM and Multiple Serial Ports
Datasheet
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 usable 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 7.25 on page 75
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 Fields table illustrates the change in the Gap2
field size for the perpendicular format.
On the read back by the FDC, the controller must begin synchronization at the beginning of the sync
field. For the conventional mode, the internal PLL VCO is enabled (VCOEN) approximately 24 bytes
from the start of the Gap2 field. But, when the controller operates in the 1 Mbps perpendicular mode
(WGATE = 1, GAP = 1), VCOEN goes active after 43 bytes to accommodate the increased Gap2 field
size. For both cases, and approximate two-byte cushion is maintained from the beginning of the sync
field for the purposes of avoiding write splices in the presence of motor speed variation.
Rev 0.2 (09-28-04)
74
SMSC SCH311X
DATASHEET