GM82C803CN Datasheet, PDF (56/84 Pages) List of Unclassifed Manufacturers – 2.88 MB FDC/ Dual UARTs with FIFO/PIO(EPP/ ECP)/ IDE Interface/ S-IR/ PnP

GM82C803CN
Relative Seek
The command is coded the same as for Seek except for the MSB of the first byte and the DIR bit.
DIR : Head Step Direction Control
0 = Step Head Out
1 = Step Head In
RTN : Relative Cylinder Number that determines how many tracks to step the head in or out from the current track
number.
This command differs from the Seek command in that it steps the head the absolute number of tracks specified in the c-
ommand 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
The controller will issue RTN number of step pulses and update the Present Cylinder Number register (PCN) for the sel-
ected drive. The one exception to this is if the Track 0 signal goes active, which indicated that the drive read/write head
is at the outermost track. In this case, the step pulses for the Relative Seek are terminated, and the PCN value is set acco-
rding to the actual number of step pulses issued. Bit 4 of Status Register 0 (EC) will be set if Relative Seek attempts to
step outward beyond Track 0.
After the step operation is complete, the controller will generate an interrupt. There is no result phase . No other comma-
nd except the Sense Interrupt command should be issued while a Relative Seek command is in progress. A Relative Seek
can be used instead of the normal Seek but the Host is required to calculate the difference between the current head loca-
tion 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.
Lock
This command allows the user full control of the FIFO parameters after a software reset. If the LOCK bit is set to 1, then
the FIFO, THRESH, and PRETRK bits in the Configure command are not affected by a software reset. If the LOCK is 0
(default after a hardware reset), then the above bits will be set to their default values retain the other FIFO parameter va-
lues (such as THRESH) after a software reset.
After the command byte is written, the result byte must be read before continuing to the next command. The extraction
of the Lock command is not performed until the result byte is read by the Host. If the part is reset after the command byte
is written but before the result byte is read, then the Lock command execution will not be performed. This is done to pre-
vent accidental execution of the Lock command.
Perpendicular Mode
This command is designed to support the unique Format and Write Data requirements of Perpendicular (Vertical) Recor-
ding disk drives(4Mbytes unformatted capacity). The Perpendicular Mode command will configure each of the four logi-
cal drives as a perpendicular of conventional disk drive. Configuration of the four logical disk drives is done via the D3-
0 bits or with the CAP and WG control bits This command should be issued during the initialization of the floppy contro-
ller.
Perpendicular Recording drives operate in Extra High Density mode at 1Mbps and are downward compatible with 1.44
Mbyte and 720 Kbyte drives at 500 Kbps(High Density) and 250 Kbps(Double Density) respectively. If perpendicular d-
rives are present in the system, this command should be issued during initialization of the FDC, which will configure eac-
h drive as perpendicular or conventional. Then when a drive is accessed for a Format or Write Data command, the FDC
will adjust the Format or Write Data parameters based on the data rate.