COM20020ILJP datasheet(13/72 Pages) MICROCHIP

13 / 72 page

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

Datasheet

SMSC COM20020I Rev D

Page 13

Revision 12-05-06

DATASHEET

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.

4.3

Network Reconfiguration

A significant advantage of the COM20020ID 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

COM20020ID is turned on (creating a new active node on the network), or if the COM20020ID has not

received an INVITATION TO TRANSMIT for 420mS, or if a software reset occurs, the COM20020ID

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 interfere with the next

INVITATION TO

TRANSMIT, destroy the token and keep any other node from assuming control of the line.

When any COM20020ID senses an idle line for greater than 41

μS, which occurs only when the token Is

lost, each COM20020ID starts an internal timeout equal to 73

μs times the quantity 255 minus its own ID.

The COM20020ID 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

COM20020ID starts sending INVITATION TO TRANSMIT with the Destination ID (DID) equal to the

currently stored NID. Within a given network, only one COM20020ID will timeout (the one with the highest

ID number). After sending the INVITATION TO TRANSMIT, the COM20020ID waits for activity on the line.

If there is no activity for 37.4

μS, the COM20020ID increments the NID value and transmits another

INVITATION TO TRANSMIT using the NID equal to the DID. If activity appears before the 37.4

μS timeout

expires, the COM20020ID releases control of the line.

During NETWORK RECONFIGURATION,

INVITATIONS TO TRANSMIT are sent to all NIDs (1-255).

Each COM20020ID on the network will finally have saved a NID value equal to the ID of the COM20020ID

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 INVITATION TO TRANSMIT. Since this node does not respond, the previous node

times out and transmits another INVITATION TO TRANSMIT to an incremented ID and eventually a

response will be received.

The NETWORK RECONFIGURATION time depends on the number of nodes in the network, the

propagation delay between nodes, and the highest ID number on the network, but is typically within the

range of 12 to 30.5 mS.

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