PC87311AVF datasheet(34/78 Pages) NSC

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40 FDC Command Set Description (Continued)

Looking at the second command byte DC3 – DC0 corre-

spond to the four logical drives A 0 written to DCn sets

drive n to conventional mode and a 1 sets drive n to per-

pendicular mode Also the OW (Overwrite) bit offers addi-

tional control When OW e 1 changing the values of DC3 –

DC0 (drive configuration bits) is enabled When OW e 0

the internal values of DC3 – DC0 are unaffected regardless

of what is written to DC3 – DC0

The function of the DCn bits must also be qualified by set-

ting both WG and GAP to 0 If WG and GAP are used (ie

not set to 00) they will override whatever is programmed in

the DCn bits Table 4-4a below indicates the operation of

the FDC based on the values of GAP and WG Note that

when GAP and WG are both 0 the DCn bits are used to

configure each logical drive as conventional or perpendicu-

lar DC3 – DC0 are unaffected by a software reset but WG

and GAP are both cleared to 0 after a software reset A

hardware reset will reset all the bits to zero (conventional

mode for all drives) The Perpendicular Mode command bits

may be rewritten at any time

Note

When in the Perpendicular Mode for any drive at any data rate select-

ed by the DC0–3 bits write precompensation is set to zero

Perpendicular Recording type disk drives have a Pre-Erase

Head which leads the ReadWrite Head by 200 mm which

translates to 38 bytes at the 1 Mbs data transfer rate (19

bytes at 500 kbs) The increased spacing between the two

heads requires a larger GAP2 between the Address Field

and Data Field of a sector at 1 Mbs (See Perpendicular

Format in Table 4-1) This GAP2 length of 41 bytes (at

1 Mbs) will ensure that the Preamble in the Data Field is

completely ‘‘pre-erased’’ by the Pre-Erase Head Also dur-

ing Write Data operations to a perpendicular drive a portion

of GAP2 must be rewritten by the controller to guarantee

that the Data Field Preamble has been pre-erased (see

Table 4-4)

429 Read Data Command

The Read Data command reads logical sectors containing a

Normal Data AM from the selected drive and makes the

data available to the host mP After the last Command

Phase byte is written the controller will simulate the Motor

On time for the selected drive internally The user must turn

on the drive motor directly by enabling the appropriate drive

and motor select disk interface outputs with the Digital Out-

put Register (DOR)

If Implied Seeks are enabled the controller will perform a

Seek operation to the track number specified in the Com-

mand Phase The controller will also issue a Sense Interrupt

for the seek and wait the Head Settle time specified in the

Mode command

The correct ID information (track head sector bytes per

sector) for the desired sector must be specified in the com-

mand bytes See Table 4-5 Sector Size Selection for details

on the bytes per sector code In addition the End of Track

Sector Number (EOT) should be specified allowing the con-

troller to read multiple sectors The Data Length byte is a

don’t care and should be set to FF (hex)

TABLE 4-5 Sector Size Selection

Bytes per

Number of Bytes

Sector Code

in Data Field

128

256

512

1024

2048

4096

8192

16384

TABLE 4-4 Effect of Drive Mode and Data Rate on Format and Write Commands

Drive

GAP2 Length

Portion of GAP2

Data Rate

Mode

Written during

Re-Written by Write

Format

Data Command

250300500 kbs

Conventional

22 Bytes

0 Bytes

Perpendicular

22 Bytes

19 Bytes

1 Mbs

Conventional

22 Bytes

0 Bytes

Perpendicular

41 Bytes

38 Bytes

TABLE 4-4a Effect of GAP and WG on Format and Write Commands

Mode

GAP2 Length

Portion of GAP2

GAP

Description

Written during

Re-Written by Write

Format

Data Command

Conventional

22 Bytes

0 Bytes

Perpendicular

22 Bytes

19 Bytes

(s500 kbs)

Reserved

22 Bytes

0 Bytes

(Conventional)

Perpendicular

41 Bytes

38 Bytes

(1 Mbs)

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