COM20020-5 Datasheet, PDF (38/58 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

Register should be written before the Node ID
Register. Once the Node ID Register is written
to, the COM20020-5 reads the value and
executes two write cycles to the RAM buffer.
Address 0 is written with the data D1H and
address 1 is written with the Node ID. The data
pattern D1H was chosen arbitrarily, and is
meant to provide assurance of proper
microsequencer operation.
INITIALIZATION SEQUENCE
Bus Determination
When the COM20020-5 is powered on the
internal registers may be written to. Writing to
and reading from an odd address location from
the COM20020-5's address space causes the
COM20020-5 to determine the appropriate bus
interface. Since writing a non-zero value to the
Node ID Register wakes up the core, the Setup
Register should be written to before the Node ID
Register. Until a non-zero value is placed into
the NID Register, no microcode is executed, no
tokens are passed by this node, and no
reconfigurations are generated by this node.
Once a non-zero value is placed in the register,
the core wakes up, but the node will not attempt
to join the network until the TX Enable bit of the
Configuration Register is set.
Before setting the TX Enable bit, the software
may make some determinations. The software
may first observe the Receive Activity and the
Token Seen bits of the Diagnostic Status
Register to verify the health of the receiver and
the network.
Next, the uniqueness of the Node ID value
placed in the Node ID Register is determined.
The TX Enable bit should still be a logic "0" until
it is ensured that the Node ID is unique. If this
node ID already exists, the Duplicate ID bit of
the Diagnostic Status Register is set after a
maximum of 420mS (or 840mS if the ET1 and
ET2 bits are other than 1,1). To determine if
another node on the network already has this
ID, the COM20020-5 compares the value in the
Node ID Register with the DID's of the token,
and determines whether there is a response to
it. Once the Diagnostic Status Register is read,
the DUPID bit is cleared. The user may then
attempt a new ID value, wait 840mS before
checking the Duplicate ID bit, and repeat the
process until a unique Node ID is found. At this
point, the TX Enable bit may be set to allow the
node to join the network. Once the node joins
the network, a reconfiguration occurs, as usual,
thus setting the MYRECON bit of the Diagnostic
Status Register.
The Tentative ID Register may be used to build
a network map of all the nodes on the network,
even once the COM20020-5 has joined the
network. Once a value is placed in the
Tentative ID Register, the COM20020-5 looks
for a response to a token whose DID matches
the Tentative ID Register. The software can
record this information and continue placing
Tentative ID values into the register to continue
building the network map. A complete network
map is only valid until nodes are added to or
deleted from the network. Note that a node
cannot detect the existence of the next logical
node on the network when using the Tentative
ID. To determine the next logical node, the
software should read the Next ID Register.
IMPROVED DIAGNOSTICS
The COM20020-5 allows the user to better
manage the operation of the network through
the use of the internal Diagnostic Status
Register.
A high level on the My Reconfiguration
(MYRECON) bit indicates that the Token
Reception Timer of this node expired, causing a
reconfiguration by this node. After the
Reconfiguration (RECON) bit of the Status
Register interrupts the microcontroller, the
interrupt service routine will typically read the
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