COM20020-5 Datasheet, PDF (40/58 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

of 128 or 4. The software may count the
number of times the EXCNAK bit goes high, and
once the final count is reached, the "Disable
Transmitter" command may be issued.
The New Next ID bit permits the software to
detect the withdrawal or addition of nodes to the
network.
The Tentative ID bit allows the user to build a
network map of those nodes existing on the
network. This feature is useful because it
minimizes the need for human intervention.
When a value placed in the Tentative ID
Register matches the Node ID of another node
on the network, the TENTID bit is set, telling the
software that this NODE ID already exists on the
network. The software should periodically place
values in the Tentative ID Register and monitor
the New Next ID bit to maintain an updated
network map.
OSCILLATOR
The COM20020-5 contains circuitry which, in
conjunction with an external parallel resonant
crystal or TTL clock, forms an oscillator.
If an external crystal is used, two capacitors and
one resistor are needed (one from each leg of
the crystal to ground). A 1KΩ resistor is
required, and the crystal must have an accuracy
of 0.020% or better.
The XTAL2 side of the crystal may be loaded
with a single 74HC-type buffer in order to
generate a clock for other devices.
The user may attach an external TTL clock,
rather than a crystal, to the XTAL1 signal. In
this case, a 390Ω pull-up resistor is required on
XTAL1, while XTAL2 should be left
unconnected.
Third Overtone such
as Ecliptek
EC400-40.0000
XTAL1
40 MHz
1k
XTAL2
47pF
47pF
COM20020-5
FIGURE 9 – OSCILLATOR CIRCUIT
40