COM20020ILJP datasheet(36/72 Pages) MICROCHIP

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5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM

Datasheet

Revision 12-05-06

Page 36

SMSC COM20020I Rev D

DATASHEET

Table 6.10 - 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 COM20020ID. 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 COM20020ID 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 COM20020ID's

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

COM20020ID in a 'listen-only' mode, where the transmitter is

disabled and the COM20020ID 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

COM20020ID. 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 by the COM20020ID 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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