FDC37C78-HT View Datasheet(PDF) - Microchip Technology

Part Name

Description

Manufacturer

FDC37C78-HT

Floppy Disk Controller

Microchip Technology 

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.

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).

Relative Seek

The internal register, PCN, will overflow as the

cylinder number crosses track 255 and will contain

The command is coded the same as for Seek, 39 (D). The resulting PCN value is thus (RCN +

except for the MSB of the first byte and the DIR PCN) mod 256. Functionally, the FDC starts

bit.

counting from 0 again as the track number goes

above 255 (D). It is the user's responsibility to

DIR Head Step Direction Control

compensate FDC functions (precompensation

track number) when accessing tracks greater than

DIR

ACTION

255. The FDC does not keep track that it is

0 Step Head Out

1 Step Head In

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.

RCN Relative Cylinder Number that determines Recalibrate will return an error if the head is farther

how many tracks to step the head in or out than 79 due to its limitation of issuing a maximum

from the current track number.

of 80 step pulses. The user simply needs to issue

a second Recalibrate command. The Seek

The Relative Seek command differs from the Seek command and implied seeks will function correctly

command in that it steps the head the absolute within the 44 (D) track (299-255) area of the

number of tracks specified in the command "extended track area". It is the user's responsibility

instead of making a comparison against an not to issue a new track position that will exceed

internal register. The Seek command is good for the maximum track that is present in the extended

drives that support a maximum of 256 tracks. area. To return to the standard floppy range (0-

Relative Seeks cannot be overlapped with other 255) of tracks, a Relative Seek should be issued to

Relative Seeks. Only one Relative Seek can be cross the track 255 boundary.

active at a time. Relative Seeks may be

overlapped with Seeks and Recalibrates. Bit 4 of A Relative Seek can be used instead of the normal

Status Register 0 (EC) will be set if Relative Seek Seek, but the host is required to calculate the

attempts to step outward beyond Track 0.

difference between the current head location and

the new (target) head location. This may require

As an example, assume that a floppy drive has the host to issue a Read ID command to ensure

300 useable tracks. The host needs to read track that the head is physically on the track that

300 and the head is on any track (0-255). If a software assumes it to be. Different FDC

Seek command is issued, the head will stop at commands will return different cylinder results

track 255. If a Relative Seek command is issued, which may be difficult to keep track of with

the FDC will move the head the specified number software without the Read ID command.

of tracks, regardless of the internal cylinder Perpendicular Mode

position

register

(but

will

The Perpendicular Mode command should be

issued prior to executing Read/Write/Format

commands that access a disk drive with

perpendicular recording capability. With this

54

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