COM20020 Datasheet, PDF (9/72 Pages) SMSC Corporation – COM20020 3.3V ULANC Universal Local Area Network Controller with 2K x 8 On-Board RAM

Selecting Clock Frequencies Above 2.5 Mbps
To realize a 5 Mbps network, an external 40 MHz
clock must be input. However, since 40 MHz is
near the frequency of FM radio band, it is not
practical for use for noise emission reasons.
Therefore, higher frequency clocks are
generated from the 20 MHz crystal as selected
through two bits in the Setup2 register,
CKUP[1,0] as shown below. The selected clock
is supplied to the ARCNET controller.
CKUP1
0
0
1
1
CKUP0
0
1
0
1
CLOCK FREQUENCY (DATA RATE)
20 MHz (Up to 2.5Mbps) Default (Bypass)
40 MHz (Up to 5Mbps)
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 "sec (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.
NETWORK RECONFIGURATION
A significant advantage of the COM20020 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 COM20020 is turned on (creating a
new active node on the network), or if the
COM20020 has not received an INVITATION TO
TRANSMIT for 420mS, or if a software reset
occurs, the COM20020 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 COM20020 senses an idle line for
greater than 41"S, which occurs only when the
token Is lost, each COM20020 starts an internal
timeout equal to 73"s times the quantity 255
minus its own ID. The COM20020 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
COM20020 starts sending INVITATION TO
TRANSMIT with the Destination ID (DID) equal to
the currently stored NID. Within a given network,
only one COM20020 will timeout (the one with the
highest ID number). After sending the
INVITATION TO TRANSMIT, the COM20020
waits for activity on the line. If there is no activity
for 37.4"S, the COM20020 increments the NID
value and transmits another INVITATION TO
TRANSMIT using the NID equal to the DID. If
activity appears before the 37.4"S timeout
expires, the COM20020 releases control of the
line. During NETWORK RECONFIGURATION,
INVITATIONS TO TRANSMIT are sent to all NIDs
(1-255).
Each COM20020 on the network will finally have
saved a NID value equal to the ID of the
COM20020 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
9