10 Mbps ARCNET (ANSI 878.1) Controller with 2Kx8 On-Board RAM
Datasheet
CKUP1
CKUP0
CLOCK FREQUENCY (DATA RATE)
0
0
1
1
0
1
0
1
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 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 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 a 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 COM20022I 3V 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
COM20022I 3V is turned on (creating a new active node on the network), or if the COM20022I 3V has not
received an INVITATION TO TRANSMIT for 210mS, or if a software reset occurs, the COM20022I 3V
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 COM20022I 3V senses an idle line for greater than 20.5μS, which occurs only when the token
Is lost, each COM20022I 3V starts an internal timeout equal to 36.5μs times the quantity 255 minus its
own ID. The COM20022I 3V 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 COM20022I 3V starts sending INVITATION TO TRANSMIT with the Destination ID (DID)
equal to the currently stored NID. Within a given network, only one COM20022I 3V will timeout (the one
with the highest ID number). After sending the INVITATION TO TRANSMIT, the COM20022I 3V waits for
activity on the line. If there is no activity for 18.7μS, the COM20022I 3V increments the NID value and
transmits another INVITATION TO TRANSMIT using the NID equal to the DID. If activity appears before
the 18.7μS timeout expires, the COM20022I 3V releases control of the line. During NETWORK
RECONFIGURATION, INVITATIONS TO TRANSMIT are sent to all NIDs (1-255).
Each COM20022I 3V on the network will finally have saved a NID value equal to the ID of the COM20022I
3V 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 6 to 15.3 mS.
Revision 02-27-06
Page 12
SMSC COM20022I 3V
DATASHEET
SMSC [ SMSC CORPORATION ]
