COM20020_00 Datasheet, PDF (58/82 Pages) SMSC Corporation – Universal Local Area Network Controller with 2K x 8 On-Board RAM

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 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 are
needed (one from each leg of the crystal to
ground). No external resistor is required, since
the COM20020 contains an internal resistor. The
crystal must have an accuracy of 0.020% or
better. The oscillation frequency range is from 10
MHz to 20 MHz.
The crystal must have an accuracy of 0.010% or
better when the internal clock multiplier is turned
on. The oscillation frequency must be 20MHz
when the internal clock multiplier is turned on.
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.
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