Specifications
62 Chapter 4—Simulation
Description of States
The High and Low states are the normal ones expected in a binary circuit,
but are not sufficient to realistically simulate circuit operation, so the High
Impedance, Don’t Know and Conflict states are added. There will always
be some cases where the simulation will not correctly mimic what would
appear in a real circuit, and some of these cases are discussed in following
sections. In particular, if a circuit takes advantage of some analog property
of a specific device—such as inputs that float high, known state at power–
up, input hysteresis, and so on—it is unlikely to simulate correctly.
High Impedance
This state (“Z” on a logic probe) is used for cases when no device output is
driving a given signal line. This may occur for an unconnected input, or for
a disabled “three–state” or “open–collector” type device. If a device input
is in the High Impedance state, it is treated as unknown for the purposes of
simulation, even though in a real circuit the device may assume a high or
low state, depending on the circuit technology used.
Don’t Know
The Don’t Know state (“X” on a logic probe) results when the simulator
cannot determine the output of a device. This may occur, for example,
when an input is unconnected or when the output from a previous device is
unknown. The Don’t Know signal will be propagated though the circuit,
showing the potential effects of that condition.
The Don’t Know state is used in LogicWorks in cases where the actual
result in a real circuit would depend on the circuit technology used, on ran-
dom chance, or on analog properties of the device not predictable using a
strictly digital simulation. For example, if the following ring oscillator cir-
cuit is created in LogicWorks, all signals will be permanently unknown—
since each depends on the previous one, which is also unknown. In actual
hardware, this circuit may oscillate, or may settle into an intermediate logic
level, which would not be defined in a digital circuit.
LW Reference.bk Page 62 Monday, December 15, 2003 5:59 PM