COM20020D datasheet(9/72 Pages) SMSC

9 / 72 page

Selecting Clock Frequencies Above 2.5 Mbps

To realize a 5 Mbps network, an external 40 MHz

clock must be input. However, since 40 MHz is

near the frequency of FM radio band, it is not

practical for use for noise emission reasons.

Therefore,

higher

frequency

clocks

are

generated from the 20 MHz crystal as selected

through

two

bits

the

Setup2

CKUP[1,0] as shown below. The selected clock

is supplied to the ARCNET controller.

CKUP1

CKUP0

CLOCK FREQUENCY (DATA RATE)

20 MHz (Up to 2.5Mbps) Default (Bypass)

40 MHz (Up to 5Mbps)

Reserved

This clock multiplier is powered-down (bypassed)

on default. After changing the CKUP1 and

CKUP0 bits, the ARCNET core operation is

stopped and the internal PLL in the clock

generator is awakened and it starts to generate

the 40 MHz. The lock out time of the internal PLL

is 8uSec typically. After more than 8

µsec (this

wait time is defined as 1 msec in this data sheet),

it is necessary to write command data '18H' to

the command register to re-start the ARCNET

core operation.

This clock generator is called

“clock multiplier”.

Changing the CKUP1 and CKUP0 bits must be

one time or less after releasing hardware reset.

The EF bit in the SETUP2 register must be set

when the data rate is over 5 Mbps.

NETWORK RECONFIGURATION

A significant advantage of the COM20020D is its

ability to adapt to changes on the network.

Whenever a new node is activated or deactivated,

a NETWORK RECONFIGURATION is performed.

When a new COM20020D is turned on (creating

new

active node on the network), or if the

COM20020D has not received an INVITATION

TO TRANSMIT for 420mS, or if a software reset

occurs, the COM20020D causes a NETWORK

RECONFIGURATION

sending

RECONFIGURE

BURST

consisting

eight

marks and one space repeated 765 times. The

purpose of this burst is to terminate all activity on

the network. Since this burst is longer than any

other type of transmission, the burst will interfere

with

the

INVITATION TO TRANSMIT,

destroy the token and keep any other node from

assuming control of the line.

When any COM20020D senses an idle line for

greater than 41

µS, which occurs only when the

token Is lost, each COM20020D starts an internal

timeout equal to 73

µs times the quantity 255

minus its own ID. The COM20020D starts network

reconfiguration by sending an invitation to transmit

first to itself and then to all other nodes by

decrementing the destination Node ID.

If the

timeout

expires

with

line

activity,

the

COM20020D starts sending INVITATION TO

TRANSMIT with the Destination ID (DID) equal to

the currently stored NID. Within a given network,

only one COM20020D will timeout (the one with

the highest ID number). After sending the

INVITATION TO TRANSMIT, the COM20020D

waits for activity on the line. If there is no activity

for 37.4

µS, the COM20020D increments the NID

value and transmits another INVITATION TO

TRANSMIT using the NID equal to the DID.

activity

appears

before

the

37.4

µS timeout

expires, the COM20020D releases control of the

line.

During NETWORK RECONFIGURATION,

INVITATIONS TO TRANSMIT are sent to all NIDs

(1-255).

Each COM20020D on the network will finally have

saved a NID value equal to the ID of the

COM20020D that it released control to. At this

point, control is passed directly from one node to

the next with no wasted INVITATIONS TO

TRANSMIT being sent to ID's not on the network,

until the next NETWORK RECONFIGURATION

occurs. When a node is powered off, the previous

node attempts to pass the token to it by issuing an

COM20020D Scheda tecnica(PDF) 9 Page - SMSC Corporation

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