At Command Register Descriptions - Samsung SP0401N Product Manual

DISK DRIVE OPERATION
6.4

AT Command Register Descriptions

Commands are issued to the drive by loading the pertinent registers in the command block with the needed
parameters, and then writing the command code to the Command register. The manner in which a command
is accepted varies. There are three classes of command acceptance (see Table 6-4), all based on the fact that
to receive a command, BSY=0:
•
Upon receipt of a Class 1 command, the drive sets BSY within 400 nsec.
•
Upon receipt of a Class 2 command, the drive sets BSY within 400 nsec, sets up the sector buffer for a
write operation, sets DRQ within 700 µsec, and clears BSY within 400 nsec of setting DRQ.
•
Upon receipt of a Class 3 command, the drive sets BSY within 400 nsec, sets up the sector buffer for a
write operation, sets DRQ within 20 msec, and clears BSY within 400 nsec of setting DRQ.
NOTE: DRQ may be set so quickly on Class 2 and Class 3 that the BSY transition is too short for BSY=1 to
be recognized.
If a new command is issued to a drive which has an uncompleted command (subsequently referred to as
Old_Command) in progress, the drive immediately responds to the new command (subsequently referred to
as New_Command), even if execution of the Old_Command could have been completed.
Extended commands are unique to the 48-bit Address feature set.
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