PRETRK - Pre-Compensation Start Track Number. Programmable from track 0 to 255. Defaults to track 0. A “00”
selects track 0; “FF” selects track 255.
VERSION
The Version command checks to see if the controller is an enhanced type or the older type (765A). A value of 90 H is
returned as the result byte.
RELATIVE SEEK
The command is coded the same as for Seek, except for the MSB of the first byte and the DIR bit.
Table 44 - Head Step Direction Control
DIR
0
1
ACTION
Step Head Out
Step Head In
RCN - Relative Cylinder Number that determines how many tracks to step the head in or out from the current track
number.
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.
Table 45 describes the affects 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-
SMSC DS – FDC37N3869
Page 52
Rev. 10/25/2000
SMSC [ SMSC CORPORATION ]
