FDC37C78-HT Standard Microsystems (SMSC), FDC37C78-HT Datasheet - Page 54

• Manufacturer Part Number: FDC37C78-HT
• Manufacturer: Standard Microsystems (SMSC)

Specifications of FDC37C78-HT

• Package Type: TQFP
• Lead Free Status / RoHS Status: Compliant

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.
DIR
RCN
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
Relative Cylinder Number that determines
how many tracks to step the head in or out
from the current track number.
Head Step Direction Control
DIR
0
1
register
Step Head Out
Step Head In
ACTION
(but
will
54
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.
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.
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.
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.
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.
The FDC does not keep track that it is
Functionally, the FDC starts
Different FDC
The Seek
With this