Specifications

Signal Simulation Characteristics 63
For the purposes of simulation, all circuits must have some provision for
initialization to a known state. In most cases, circuits can be initialized by
using the Clear Unknowns command or by setting the initial value
attribute, described in “Setting Initial Values” on page 75. Alternatively,
circuitry can be added to allow a reset to be done, as in the following mod-
ification to the ring oscillator:
A problem arises in simulating circuits with multiple open collector
devices—such as a bus line, illustrated here:
In this circuit, the upper device has an unconnected input at IN1 and there-
fore outputs a Don’t Know value. The lower device has a low input and
therefore outputs a low value. In order to correctly resolve this situation the
simulator needs to distinguish between a Don’t Know output from a normal
“totem–pole” type output and a Don’t Know from an open–collector,
open–drain, or other single–drive output. In this case, the upper device will
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