37C67X Datasheet, PDF (60/194 Pages) SMSC Corporation – ENHANCED SUPER I/O CONTROLLER WITH FAST IR

CONTROL COMMANDS
Control commands differ from the other
commands in that no data transfer takes place.
Three commands generate an interrupt when
complete: Read ID, Recalibrate, and Seek. The
other control commands do not generate an
interrupt.
Read ID
The Read ID command is used to find the
present position of the recording heads. The
FDC stores the values from the first ID field it is
able to read into its registers. If the FDC does
not find an ID address mark on the diskette after
the second occurrence of a pulse on the
nINDEX pin, it then sets the IC code in Status
Register 0 to "01" (abnormal termination), sets
the MA bit in Status Register 1 to "1", and
terminates the command.
The following commands will generate an
interrupt upon completion. They do not return
any result bytes. It is highly recommended that
control commands be followed by the Sense
Interrupt Status command. Otherwise, valuable
interrupt status information will be lost.
Recalibrate
This command causes the read/write head
within the FDC to retract to the track 0 position.
The FDC clears the contents of the PCN
counter and checks the status of the nTR0 pin
from the FDD. As long as the nTR0 pin is low,
the DIR pin remains 0 and step pulses are
issued. When the nTR0 pin goes high, the SE
bit in Status Register 0 is set to "1" and the
command is terminated. If the nTR0 pin is still
low after 79 step pulses have been issued, the
FDC sets the SE and the EC bits of Status
Register 0 to "1" and terminates the command.
Disks capable of handling more than 80 tracks
per side may require more than one Recalibrate
command to return the head back to physical
Track 0.
The Recalibrate command does not have a
result phase. The Sense Interrupt Status
command must be issued after the Recalibrate
command to effectively terminate it and to
provide verification of the head position (PCN).
During the command phase of the recalibrate
operation, the FDC is in the BUSY state, but
during the execution phase it is in a NON-BUSY
state. At this time, another Recalibrate
command may be issued, and in this manner
parallel Recalibrate operations may be done on
up to four drives at once.
Upon power up, the software must issue a
Recalibrate command to properly initialize all
drives and the controller.
Seek
The read/write head within the drive is moved
from track to track under the control of the Seek
command. The FDC compares the PCN, which
is the current head position, with the NCN and
performs the following operation if there is a
difference:
PCN < NCN: Direction signal to drive set to
"1" (step in) and issues step pulses.
PCN > NCN: Direction signal to drive set to
"0" (step out) and issues step pulses.
The rate at which step pulses are issued is
controlled by SRT (Stepping Rate Time) in the
Specify command. After each step pulse is
issued, NCN is compared against PCN, and
when NCN = PCN the SE bit in Status Register
0 is set to "1" and the command is terminated.
During the command phase of the seek or
recalibrate operation, the FDC is in the BUSY
state, but during the execution phase it is in the
NON-BUSY state. At this time, another Seek or
Recalibrate command may be issued, and in
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