FDC37C669-MT datasheet(67/162 Pages) MICROCHIP

67 / 162 page

Selection of the 500 Kbps and 1 Mbps perpendicular

modes is independent of the actual data rate selected in

the Data Rate Select Register. The user must ensure

that these two data rates remain consistent.

The

Gap2

and

VCO

timing

requirements

for

perpendicular recording type drives are dictated by the

design of the read/write head. In the design of this head,

a pre-erase head precedes the normal read/write head by

a distance of 200 micrometers. This works out to about

38 bytes at a 1 Mbps recording density. Whenever the

write head is enabled by the Write Gate signal, the pre-

erase head is also activated at the same time. Thus,

when the write head is initially turned on, flux transitions

recorded on the media for the first 38 bytes will not be

preconditioned with the pre-erase head since it has not

yet been activated. To accommodate this head activation

and deactivation time, the Gap2 field is expanded to a

length of 41 bytes. The format field shown on page 61

illustrates the change in the Gap2 field size for the

perpendicular format.

On the read back by the FDC, the controller must begin

synchronization at the beginning of the sync field. For the

conventional mode, the internal PLL VCO is enabled

(VCOEN) approximately 24 bytes from the start of the

Gap2 field. But, when the controller operates in the 1

Mbps perpendicular mode (WGATE = 1, GAP = 1),

VCOEN goes active after 43 bytes to accommodate the

increased Gap2 field size.

For both cases, and

approximate two-byte cushion is maintained from the

beginning of the sync field for the purposes of avoiding

write splices in the presence of motor speed variation.

For the Write Data case, the FDC activates Write Gate at

the beginning of the sync field under the conventional

mode. The controller then writes a new sync field, data

address mark, data field, and CRC as shown in Figure 4.

With the pre-erase head of the perpendicular drive, the

write head must be activated in the Gap2 field to insure a

proper write of the new sync field.

For the 1 Mbps

perpendicular mode (WGATE = 1, GAP = 1), 38 bytes will

be written in the Gap2 space. Since the bit density is

proportional to the data rate, 19 bytes will be written in the

Gap2 field for the 500 Kbps perpendicular mode

(WGATE = 1, GAP =0).

It should be noted that none of the alterations in Gap2

size, VCO timing, or Write Gate timing affect normal

program flow. The information provided here is just for

background purposes and is not needed for normal

operation. Once the Perpendicular Mode command is

invoked, FDC software behavior from the user standpoint

is unchanged.

The perpendicular mode command is enhanced to allow

specific

drives

designated

Perpendicular

recording

drives.

This enhancement allows data

transfers between Conventional and Perpendicular drives

without having to issue Perpendicular mode commands

between the accesses of the different drive types, nor

having to change write pre-compensation values.

When

both

GAP

and

WGATE

bits

the

PERPENDICULAR

MODE

COMMAND

are

both

programmed to "0" (Conventional mode), then D0, D1,

D2, D3, and D4 can be programmed independently to "1"

for that drive to be set automatically to Perpendicular

mode. In this mode the following set of conditions also

apply:

1. The GAP2 written to a perpendicular drive during a

write operation will depend upon the programmed

FDC37C669-MT Datasheet(PDF) 67 Page - Microchip Technology

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