PC87303 datasheet(39/114 Pages) NSC

39 / 114 page

40 FDC Command Set Description (Continued)

FRD

FIFO Read Disable for mP read transfers from con-

troller Default after a software reset if LOCK is 0 If

LOCK is 1 FRD retains its value after a software re-

set

Note

This bit is only valid if the FIFO is enabled in the Configure

command If the FIFO is not enabled in the Configure com-

mand then this bit is a don’t care

0 e Enable FIFO mP read transfers during the Exe-

cution Phase use the internal FIFO

1 e

Disable FIFO All read data transfers take place

without the FIFO

BST

Burst Mode Disable Default after a software reset if

LOCK is 0 If LOCK is 1 BST retains its value after a

software reset

Note

This bit is only valid if the FIFO is enabled in the Configure

command If the FIFO is not enabled in the Configure com-

mand then this bit is a don’t care

0 e Burst mode enabled for FIFO Execution Phase

data transfers

1 e

Non-Burst mode enabled The DRQ or IRQ6 pin

is strobed once for each byte to be transferred

while the FIFO is enabled

R255

Recalibrate Step Pulses The bit determines the

maximum number of recalibrate step pulses the con-

troller issues before terminating with an error De-

fault after a software reset

0 e Maximum of 85 recalibrate step pulses If ETR

1 controller issues 3925 recalibrate step

pulses maximum

1 e

Maximum of 255 recalibrate step pulses If ETR

1 controller issues 4095 maximum recali-

brate step pulses

DENSEL

Density Select Pin Configuration This 2-bit value

configures the Density Select output to one of

three possible modes The default mode config-

ures the DENSEL pin according to the state of

the IDENT input pin after a data rate has been

selected That is if IDENT is high the DENSEL

pin is active high for the 500 kbps1 Mbps data

rates If IDENT is low the DENSEL pin is active

low for the 500 kbps1 Mbps data rates See Ta-

ble 4-3 In addition to these modes the DENSEL

output can be set to always low or always high as

shown in Table 4-4 This allows the user more

flexibility with new drive types

TABLE 4-3 DENSEL Default Encoding

Data Rate

DENSEL Pin Definition

IDENT e 1

IDENT e 0

250 kbps

Low

High

300 kbps

Low

High

500 kbps

High

Low

1 Mbps

High

Low

When TUP bit 1 e 0 a Data Rate of 1 Mbps is selected

TABLE 4-4 DENSEL Encoding

Bit 1

Bit 0

DENSEL

Pin Definition

Pin Low

Pin High

Undefined

DEFAULT

BFR

CMOS Disk Interface Buffer Enable

0 e Drive output signals configured as standard 4 mA

push-pull outputs (actually 40 mA sink 4 mA

source)

1 e

Drive output signals configured as 40 mA open-

drain outputs

WLD

Scan Wild Card

0 e An FFh from either the mP or the disk during a

Scan command is interpreted as a wildcard

character that always matches true

1 e

The Scan commands do not recognize FFh as a

wildcard character

Head

Settle

Time allowed for readwrite head to settle after a

seek during an Implied Seek operation This is con-

trolled as shown in Table 4-5 by loading a 4-bit value

for N (The default value for N is 8)

TABLE 4-5 Head Settle Time Calculation

Data Rate

Multiplier

Head Settle

(kbitssec)

(4 Bit Value)

Time (ms)

250

N c 8

0 – 120

300

N c 6666

0 – 100

500

N c 4

0–60

1000

N c 2

0–30

Read Gate Diagnostic

0 e Enable DSKCHG disk interface input for normal

operation

1 e

Enable DSKCHG to act as an external Read Gate

input signal to the Data Separator This is intend-

ed as a test mode to aid in evaluation of the Data

Separator

PUMP Pulse Output Diagnostic

0 e Enable MFM output pin for normal operation

1 e

Enable the MFM output to act as an internal serial

data in signal

417 NSC Command

The NSC command can be used to distinguish between the

FDC versions and the 82077 The Result Phase byte

uniquely identifies the floppy controller as a PC87334 which

returns a value of 73h The 82077 and DP8473 return a

value of 80h signifying an invalid command The lower four

bits of this result byte are subject to change by National

and reflects the particular version of the floppy disk control-

ler part

Command Phase

00011000

Execution Phase None

Result Phase

01110011

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