5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Datasheet
CKUP1
CKUP0
CLOCK FREQUENCY (DATA RATE)
20 MHz (Up to 2.5Mbps) Default (Bypass)
40 MHz (Up to 5Mbps)
0
0
1
1
0
1
0
1
Reserved
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 41μS, which occurs only when the token Is
lost, each COM20020ID starts an internal timeout equal to 73μs 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 37.4μS, the COM20020ID increments the NID value and transmits another
INVITATION TO TRANSMIT using the NID equal to the DID. If activity appears before the 37.4μS 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.
SMSC COM20020I Rev D
Page 13
Revision 12-05-06
DATASHEET
SMSC [ SMSC CORPORATION ]
