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.
CKUP1
0
0
1
1
CKUP0
0
1
0
1
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.
CLOCK FREQUENCY (DATA RATE)
20 MHz (Up to 2.5Mbps) Default (Bypass)
40 MHz (Up to 5Mbps)
Reserved
Reserved
RECONFIGURE BURST consisting of eight
marks and one space repeated 765 times. The
This clock multiplier is powered-down (bypassed)
purpose of this burst is to terminate all activity on
on default. After changing the CKUP1 and
the network. Since this burst is longer than any
CKUP0 bits, the ARCNET core operation is
other type of transmission, the burst will interfere
stopped and the internal PLL in the clock
with the next INVITATION TO
generator is awakened and it starts to generate
the 40 MHz. The lock out time of the internal PLL
TRANSMIT, destroy the token and keep any other
is 8uSec typically. After more than 8
μsec
(this
node from assuming control of the line.
wait time is defined as 1 msec in this data sheet),
it is necessary to write command data '18H' to
When any COM20020 senses an idle line for
the command register to re-start the ARCNET
greater than 41μS, which occurs only when the
core operation. This clock generator is called
token Is lost, each COM20020 starts an internal
“clock multiplier”.
timeout equal to 73μs times the quantity 255
Changing the CKUP1 and CKUP0 bits must be
minus its own ID. The COM20020 starts network
one time or less after releasing hardware reset.
reconfiguration by sending an invitation to transmit
first to itself and then to all other nodes by
The EF bit in the SETUP2 register must be set
decrementing the destination Node ID. If the
when the data rate is over 5 Mbps.
timeout expires with no line activity, the
COM20020 starts sending INVITATION TO
NETWORK RECONFIGURATION
TRANSMIT with the Destination ID (DID) equal to
the currently stored NID. Within a given network,
A significant advantage of the COM20020 is its
only one COM20020 will timeout (the one with the
ability to adapt to changes on the network.
highest ID number). After sending the
Whenever a new node is activated or deactivated,
INVITATION TO TRANSMIT, the COM20020
a NETWORK RECONFIGURATION is performed.
waits for activity on the line. If there is no activity
When a new COM20020 is turned on (creating a
for 37.4μS, the COM20020 increments the NID
new active node on the network), or if the
value and transmits another INVITATION TO
COM20020 has not received an INVITATION TO
TRANSMIT using the NID equal to the DID. If
TRANSMIT for 420mS, or if a software reset
activity appears before the 37.4μS timeout
occurs, the COM20020 causes a NETWORK
expires, the COM20020 releases control of the
RECONFIGURATION
by
sending
a
line. During NETWORK RECONFIGURATION,
9
SMSC [ SMSC CORPORATION ]
