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COM20022I

10Mbps ARCNET controller with 2Kx8 on-board RAM

SMSC -> Microchip 

Selecting Clock Frequencies Above 2.5 Mbps

To realize a 10 Mbps network, an external 80

MHz clock must be input. However, since 80

MHz is 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 in the Setup2 register,

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

is supplied to the ARCNET controller.

CKUP1

0

0

1

1

CKUP0

0

1

0

1

CLOCK FREQUENCY (DATA RATE)

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

40 MHz (Up to 5Mbps)

Reserved

80 MHz (Only 10Mbps)

This clock multiplier is powered-down

(bypassed) on default. After changing the

CKUP1 and CKUP0 bits, the ARCNET core

interfere

with the next INVITATION TO

TRANSMIT, destroy the token and keep any other

node from assuming control of the line.

operation is stopped and the internal PLL in the

clock generator is awakened and it starts to

generate the 40 MHz or 80 MHz. The lock out

time of the internal PLL is 8uSec typically. After

more than 8 #sec (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 a hardware

reset.

When any COM20022 senses an idle line for

greater than 20.5 S, which occurs only when the

token Is lost, each COM20022 starts an internal

timeout equal to 36.5 s times the quantity 255

minus its own ID. The COM20022 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 no line activity, the

COM20022 starts sending INVITATION TO

TRANSMIT with the Destination ID (DID) equal to

the currently stored NID. Within a given network,

only one COM20022 will timeout (the one with the

The EF bit in the SETUP2 register must be set

when the data rate is over 5 Mbps.

highest ID number). After sending the

INVITATION TO TRANSMIT, the COM20022

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

NETWORK RECONFIGURATION

A significant advantage of the COM20022 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 COM20022 is turned on (creating

a new active node on the network), or if the

for 18.7 S, the COM20022 increments the NID

value and transmits another INVITATION TO

TRANSMIT using the NID equal to the DID. If

activity appears before the 18.7 S timeout

expires, the COM20022 releases control of the

line. During NETWORK RECONFIGURATION,

INVITATIONS TO TRANSMIT are sent to all NIDs

(1-255).

COM20022 has not received an INVITATION TO

TRANSMIT for 210mS, or if a software reset

occurs, the COM20022 causes a NETWORK

RECONFIGURATION by sending a

RECONFIGURE BURST consisting of 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

Each COM20022 on the network will finally have

saved a NID value equal to the ID of the

COM20022 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

10

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