Answers Database
F1.4,F1.5 Simulator: Possible System Oscillations; please wait or press stop button.
Record #4300
Product Family: Software
Product Line: Aldec
Product Part: Foundation Logic Simulator
Product Version: 1.4
Problem Title:
F1.4,F1.5 Simulator: Possible System Oscillations; please wait or press stop button.
Problem Description:
Urgency: Standard
General Description:
When running a Foundation Functional simulation, the simulation may terminate
with a window saying:
"Possible System Oscillations: Please wait or press stop button".
Solution 1:
This message has also been seen when attempting to perform timing simulation by opening the simulato
r via the 'Functional Simulation' phase button and switching the simulation mode to 'timing'. For t
iming simulation, be sure to open the simulator via the 'Timing Simulation' phase button.
Solution 2:
The Oscillations may be created by the user design.
This can happen when not all possible transitions are defined
or there exists multiple valid transitions for a single state.
An example is a state machine with multiple, true conditions,
each leading to a different action in one state - there is no
clear path for the state.
Solution 3:
The oscillations can be falsely reported by the simulator.
This occurs when a piece of logic has combinatorial feedback.
During Functional simulation, all gates have zero delay, therefore, the combinatorial
feedback can cause oscillations.
EX. Imagine a 2 input AND gate feeding an inverter. The output of the inverter feeds
back to 1 of the inputs of the AND gate.
The other input of the AND gate is a data line.
If the circuit starts with feedback = 1 and Data = 1, the feedback would then be 0.
Since the gate delay is zero, the circuit immediately begins oscillating, until the
data line goes to 0, forcing a constant feedback value of 1.
To resolve this issue the mode of the simulator must be changed from Functional
to Unit:
The Unit Delay mode is used for functional verification of designs that have
oscillating feedbacks, and cannot be simulated in the functional mode.
To select the Unit Delay mode, select the Unit option from the last box in
the Simulator toolbar. The Unit Delay simulation mode is based on unit
propagation delays, which means that all components have the same delays
from input to output. The value of this standard delay is factory-set to be
equal to the current simulation precision. The simulation precision can be
accessed through the Options - Preferences window (Simulation Tab).
For additional information, please review the Foundation On-line documentation.
Also, be sure to verify your design through Timing Simulation. The accurate delays
are then simulated and oscillation should not be a problem.
End of Record #4300 - Last Modified: 11/03/99 12:46 |
