COM20020I-HT SMSC, COM20020I-HT Datasheet - Page 12

• Manufacturer Part Number: COM20020I-HT
• Description: Network Controller & Processor ICs ARCNET Contrllr
• Manufacturer: SMSC

Specifications of COM20020I-HT

• Product: Controller Area Network (CAN)
• Number Of Transceivers: 1
• Data Rate: 5 Mbps
• Supply Voltage (max): 5.5 V
• Supply Voltage (min): 4.5 V
• Supply Current (max): 40 mA
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Mounting Style: SMD/SMT
• Package / Case: TQFP-48
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

The EF bit in the SETUP2 register must be set when the data rate is over 5 Mbps.
4.3
A significant advantage of the COM20020I 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 COM20020I 3V is turned
on (creating a new active node on the network), or if the COM20020I 3V has not received an INVITATION TO
TRANSMIT for 420mS, or if a software reset occurs, the COM20020I 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 COM20020I 3V senses an idle line for greater than 41μS, which occurs only when the token Is lost, each
COM20020I 3V starts an internal timeout equal to 73μs times the quantity 255 minus its own ID. The COM20020I 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 COM20020I 3V starts sending
INVITATION TO TRANSMIT with the Destination ID (DID) equal to the currently stored NID. Within a given network,
only one COM20020I 3V will timeout (the one with the highest ID number). After sending the INVITATION TO
TRANSMIT, the COM20020I 3V waits for activity on the line. If there is no activity for 37.4μS, the COM20020I 3V
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 COM20020I 3V releases control of the line.
NETWORK RECONFIGURATION, INVITATIONS TO TRANSMIT are sent to all NIDs (1-255).
Each COM20020I 3V on the network will finally have saved a NID value equal to the ID of the COM20020I 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 12 to 30.5 mS.
4.4
Broadcasting gives a particular node the ability to transmit a data packet to all nodes on the network simultaneously.
ID zero is reserved for this feature and no node on the network can be assigned ID zero. To broadcast a message,
the transmitting node's processor simply loads the RAM buffer with the data packet and sets the DID equal to zero.
Figure 4 illustrates the position of each byte in the packet with the DID residing at address 0X01 or
Hex of the current page selected in the "Enable Transmit from Page fnn" command. Each individual node has the
ability to ignore broadcast messages by setting the most significant bit of the "Enable Receive to Page fnn" command
to a logic "0".
4.5
There are three timeouts associated with the COM20020I 3V operation. The values of these timeouts are
controlled by bits 3 and 4 of the Configuration Register and bit 5 of the Setup 1 Register.
Response Time
The Response Time determines the maximum propagation delay allowed between any two nodes, and should be chosen
to be larger than the round trip propagation delay between the two furthest nodes on the network plus the maximum turn
around time (the time it takes a particular COM20020I 3V to start sending a message in response to a received message)
which is approximately 6.4 μS. The round trip propagation delay is a function of the transmission media and network
topology. For a typical system using RG62 coax in a baseband system, a one way cable propagation delay of 15.5 μS
translates to a distance of about 2 miles. The flow chart in Figure 1 uses a value of 37.4uS (15.5 + 15.5 + 6.4) to
determine if any node will respond.
SMSC COM20020I 3.3V Rev.E
5Mbps ARCNET (ANSI 878.1) Controller with 2K x 8 On-Chip RAM
Network Reconfiguration
Broadcast Messages
Extended Timeout Function
DATASHEET
Page 12
Revision 09-11-06
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