COM20020_00 Datasheet, PDF (56/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

appropriate bus interface. When the COM20020
is powered on the internal registers may be written
to. Since writing a non-zero value to the Node ID
Register wakes up the core, the Setup1 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
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
420mS 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 has joined the network.
Once a value is placed in the Tentative ID
Register, the COM20020 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 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
MYRECON bit of the Diagnostic Status Register.
Reading the Diagnostic Status Register resets the
MYRECON bit. Successive occurrences of a
logic "1" on the MYRECON bit indicates that a
problem exists with this node. At that point, the
transmitter should be disabled so that the entire
network is not held down while the node is being
evaluated.
The Duplicate ID (DUPID) bit is used before the
node joins the network to ensure that another
node with the same ID does not exist on the
network. Once it is determined that the ID in the
Node ID Register is unique, the software should
write a logic "1" to bit 5 of the Configuration
Register to enable the basic transmit function.
This allows the node to join the network.
The Receive Activity (RCVACT) bit of the
Diagnostic Status Register will be set to a logic "1"
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