COM20019IP datasheet(35/81 Pages) SMSC

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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 COM20019. 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 COM20019 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

COM20019's programmed node ID. This feature can be

used to put the COM20019 in a 'listen-only' mode, where

the transmitter is disabled and the COM20019 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

COM20019. The following table is for a 20 MHz crystal:

CKP3

CKP2

CKP1

DIVISOR

128

SPEED

312.5 Kbs

156.25 Kbs

NOTE: The lowest data rate achievable by the COM20019

is 156.25 Kbs. Defaults to 011 or 312.5 Kbps.

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.

Defaults to low.

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