COM20020D datasheet(34/72 Pages) SMSC

34 / 72 page

Table 11 - Setup 1 Register

BIT

BIT NAME

SYMBOL

DESCRIPTION

Pulse1 Mode

P1MODE

This bit determines the type of PULSE1 output driver used

in Backplane Mode. When high, a push/pull output is used.

When low, an open drain output is used. The default is

open drain.

Four NACKS

FOUR

NACKS

This bit, when set, will cause the EXNACK bit in the

Diagnostic Status Register to set after four NACKs to Free

Buffer Enquiry are detected by the COM20020D. This bit,

when reset, will set the EXNACK bit after 128 NACKs to

Free Buffer Enquiry. The default is 128.

Reserved

Do not set.

Receive All

RCVALL

This bit, when set, allows the COM20020D to receive all

valid data packets on the network, regardless of their

destination ID.

This mode can be used to implement a

network monitor with the transmitter on- or off-line. Note

that ACKs are only sent for packets received with a

destination ID equal to the COM20020D's programmed

node ID. This feature can be used to put the COM20020D

in a 'listen-only' mode, where the transmitter is disabled and

the COM20020D is not passing tokens. Defaults low.

3,2,1

Clock Prescaler Bits

3,2,1

CKP3,2,1

These bits are used to determine the data rate of the

COM20020D. The following table is for a 20 MHz crystal:

(Clock Multiplier is bypassed)

CKP3

CKP2

CKP1

DIVISOR

128

SPEED

2.5Mbs

1.25Mbs

625Kbs

312.5Kbs

156.25Kbs

NOTE:

The

lowest

data

rate

achievable

the

COM20020D is 156.25Kbs. Defaults to 000 or 2.5Mbs. For

Clock Multiplier output clock speed greater than 20 MHz,

CKP3, CKP2 and CKP1 must all be zero.

Slow Arbitration

Select

SLOWARB

This bit, when set, will divide the arbitration clock by 2.

Memory cycle times will increase when slow arbitration is

selected.

NOTE:

For clock multiplier output clock speeds greater

than 40 MHz, SLOWARB must be set. Defaults to low.

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