PC8477B datasheet(24/54 Pages) NSC

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40 Command Set Description (Continued)

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)

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 Ta-

ble 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

The controller then starts the Data Separator and waits for

the Data Separator to find the next sector Address Field

The controller compares the Address Field ID information

(track head sector bytes per sector) with the desired ID

specified in the Command Phase If the sector ID bytes do

not match then the controller waits for the Data Separator

to find the next sector Address Field The ID comparison

process repeats until the Data Separator finds a sector Ad-

dress Field ID that matches that in the command bytes or

until an error occurs Possible errors are

1 The mP aborted the command by writing to the FIFO If

there is no disk in the drive the controller will hang up

The mP must then take the controller out of this hung

state by writing a byte to the FIFO This will put the con-

troller into the Result Phase

2 Two index pulses were detected since the search began

and no valid ID has been found If the track address ID

differs the WT bit or BT bit (if the track address is FF hex)

will be set in ST2 If the head sector or bytes per sector

code did not match the ND bit is set in ST1 If the Ad-

dress Field AM was never found the MA bit is set in ST1

3 The Address Field was found with a CRC error The CE

bit is set in ST1

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