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goes high, the SE bit in Status Register 0 is set to "1" and the command is terminated. If the nTRK0
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
this manner, parallel seek operations may be done on up to four drives at once.
Note that if implied seek is not enabled, the read and write commands should be preceded by:
1) Seek command - Step to the proper track
2) Sense Interrupt Status command - Terminate the Seek command
3) Read ID - Verify head is on proper track
4) Issue Read/Write command.
The Seek command does not have a result phase. Therefore, it is highly recommended that the
Sense Interrupt Status command is issued after the Seek command to terminate it and to provide
verification of the head position (PCN). The H bit (Head Address) in ST0 will always return to a "0".
When exiting POWERDOWN mode, the FDC clears the PCN value and the status information to zero.
Prior to issuing the POWERDOWN command, it is highly recommended that the user service all
pending interrupts through the Sense Interrupt Status command.