PC8477B datasheet(23/54 Pages) NSC

23 / 54 page

40 Command Set Description (Continued)

If IDENT is low the DENSEL pin is active low for the

500 kbs or 1 Mbs data rates In addition to these modes

the DENSEL output can be set to always low or always high

as shown in Table 4-2 This will allow the user more flexibili-

ty with new drive types

Note

The DENSEL output values shown below are with the INVERT pin

tied low If the INVERT pin is tied high the outputs shown below have

the opposite polarity

TABLE 4-2 DENSEL Decoding

Bit 1

Bit 0

DENSEL Pin Definition

low

high

undefined

DEFAULT

TABLE 4-3 DENSEL Default Encoding

Data Rate

DENSEL (default)

IDENT e 1

IDENT e 0

250 kbs

low

high

300 kbs

low

high

500 kbs

high

low

1 Mbs

high

low

BFR

CMOS Disk Interface Buffer Enable

0 e Drive output signals configured as standard 4 mA

push-pull outputs (actually 48 mA sink

4 mA

source) (default)

1 e

Drive output signals configured as 48 mA open-

drain outputs

WLD

Scan Wild Card

0 e An FF (hex) from either the mP or the disk during a

Scan command is interpreted as a wildcard charac-

ter that will always match true (default)

1 e

The Scan commands do not recognize FF (hex) as

a wildcard character

Head Settle

Time allowed for readwrite head to settle af-

ter a seek during an Implied Seek operation

Data Rate

HST

Range

250 kbs

N x 8

0 – 120 ms

300 kbs

N x 667

0 – 100 ms

500 kbs

N x 4

0 – 60 ms

1 Mbs

N x 2

0 – 30 ms

Note

N e 8 (default)

HST e Head Settle Time

Read Gate Diagnostic

0 e Enable DSKCHG disk interface input for normal op-

eration (default)

1 e

Enable DSKCHG to act as an external Read Gate

input signal to the Data Separator This is intended

as a test mode to aid in evaluation of the Data Sep-

arator

PUMP Pulse Output Diagnostic

0 e Enable MFM output pin for normal operation (de-

fault)

1 e

Enable the MFM output to act as the active low out-

put of the Data Separator charge pump This signal

consists of a series of pulses indicating when the

phase comparator is making a phase correction

This Pump output will be active low for a pump up or

pump down signal from the phase comparator and

is intended as a test mode to aid in the evaluation of

the Data Separator

427 NSC Command

The NSC command can be used to distinguish between the

PC8477B version and the Intel 82077AA The result Phase

byte uniquely identifies the floppy controller as a PC8477B

which returns a value of 73 hex The 82077AA and DP8473

return a value 80h signifying an invalid command The lower

four bits of this result byte are subject to change by NSC

and will reflect the particular version of the PC8477B part

Note

The PC8477A will return a value of 72h in the result phase of the

NSC command

428 Perpendicular Mode Command

The Perpendicular Mode command is designed to support

the unique Format and Write Data requirements of Perpen-

dicular (Vertical) Recording disk drives (4 Mbytes unformat-

ted capacity) The Perpendicular Mode command will con-

figure each of the four logical drives as a perpendicular or

conventional disk drive Configuration of the four logical disk

drives is done via the D3 – D0 bits or with the GAP and WG

control bits This command should be issued during the ini-

tialization of the floppy controller

Perpendicular Recording drives operate in ‘‘Extra High Den-

sity’’ mode at 1 Mbs and are downward compatible with

144 Mbyte and 720 kbyte drives at 500 kbs (High Density)

and 250 kbs (Double Density) respectively If perpendicular

drives are present in the system this command should be

issued during initialization of the floppy controller which will

configure each drive as perpendicular or conventional

Then when a drive is accessed for a Format or Write Data

command the floppy controller will adjust the Format or

Write Data parameters based on the data rate (see Table

4-4)

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

perpendicular mode Also the OW (Overwrite) bit offers ad-

ditional 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 indicates the operation of the

PC8477B 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 via the

DC3–DC0 bits write precompensation is set to zero

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