FDC37C78-HT datasheet(54/82 Pages) MICROCHIP

54 / 82 page

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

time.

Relative

Seeks

may

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

(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). Any command issued will use the current

PCN value except for the Recalibrate command,

which

only

looks

for

the

TRACK0

signal.

Recalibrate will return an error if the head is farther

than 79 due to its limitation of issuing a maximum

of 80 step pulses. The user simply needs to issue

a second Recalibrate command.

The Seek

command and implied seeks will function correctly

within the 44 (D) track (299-255) area of the

"extended track area". It is the user's responsibility

not to issue a new track position that will exceed

the maximum track that is present in the extended

area. To return to the standard floppy range (0-

255) of tracks, a Relative Seek should be issued to

cross the track 255 boundary.

A Relative Seek can be used instead of the normal

Seek, but the host is required to calculate the

difference between the current head location 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.

Different FDC

commands will return different cylinder results

which may be difficult to keep track of with

software without the Read ID command.

Perpendicular Mode

The Perpendicular Mode command should be

issued

prior

executing

Read/Write/Format

commands

that

access

disk

drive

with

perpendicular recording capability.

With this

DIR

ACTION

Step Head Out

Step Head In

FDC37C78-HT Datasheet(PDF) 54 Page - Microchip Technology

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