LPC47M10X datasheet(52/188 Pages) SMSC

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Page 52

Configure Default Values:

EIS - No Implied Seeks

EFIFO - FIFO Disabled

POLL - Polling Enabled

FIFOTHR - FIFO Threshold Set to 1 Byte

PRETRK - Pre-Compensation Set to Track 0

EIS - Enable Implied Seek. When set to "1", the FDC will perform a Seek operation before executing a read or write

command. Defaults to no implied seek.

EFIFO - A "1" disables the FIFO (default). This means data transfers are asked for on a byte-by-byte basis. Defaults to

"1", FIFO disabled. The threshold defaults to "1".

POLL - Disable polling of the drives. Defaults to "0", polling enabled. When enabled, a single interrupt is generated

after a reset. No polling is performed while the drive head is loaded and the head unload delay has not expired.

FIFOTHR - The FIFO threshold in the execution phase of read or write commands. This is programmable from 1 to 16

bytes. Defaults to one byte. A "00" selects one byte; "0F" selects 16 bytes.

PRETRK - Pre-Compensation Start Track Number. Programmable from track 0 to 255. Defaults to track 0. A "00"

selects track 0; "FF" selects track 255.

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

Head Step Direction Control

RCN

Relative Cylinder Number that determines how many tracks to step the head in or out from the current track

number.

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 register

(but will 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. Functionally, the FDC starts 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. The FDC does not keep track that it is working in an "extended track area" (greater

than 255).

DIR

ACTION

Step Head Out

Step Head In

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