COM20051 datasheet(36/82 Pages) SMSC

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Table 13 - Setup 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 ARCNET core. This bit, when reset, will set

the EXNACK bit after 128 NACKs to Free Buffer Enquiry. The

default is 128.

ET3

This bit, when set, scales down protocol timeout values of

Response Time and Idle Time but not Reconfiguration Time to

optimize network performance in short topologies. Provides a

scaling factor of ÷ 3. Defaults to a zero. Must be reset to be

ARCNET compliant.

Receive All

RCVALL

This bit, when set, allows the COM20051 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

COM20051's programmed node ID. This feature can be used

to put the COM20051 in a 'listen-only' mode, where the

transmitter is disabled and the COM20051 is not passing

tokens. Defaults low.

3,2,1

Clock

Prescaler Bits

3,2,1

CKP2,1,0

These bits are used to determine the data rate of the

COM20051. The following table is for a 40MHz crystal:

Response

CKP3 CKP2 CKP1

DIVISOR

CLOCK

2.5 Mbps

1.25 Mbps

625 Kbps

312.5 Kbps

128

156.25 Kbps

256

Reserved

NOTE: The lowest data rate achievable by the COM20051 is

156.25 Kbps. A divide by 256 is provided for those systems

that use faster clock speeds. Defaults to 000 or 2.5 Mbps.

Slow

Arbitration

Select

SLW-ARB

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

cycle times will increase when slow arbitration is selected.

NOTE: For 5 Mbps operation, SLOWARB must be set.

Defaults to low.

COM20051 Datasheet(PDF) 36 Page - SMSC Corporation

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