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Multiple Operating-Point Detection
Biasing is the method for establishing predetermined voltages and/or currents at various points of a circuit to set an appropriate operating-point. When biased as designed, the circuit performs as expected. However, multiple alternate operating-points, stable or meta-stable, may exist which render the circuit useless when self-biased with respect to an unintended operating-point. Such situations cannot in general be predicted by inspection of the circuit topology.
SMASH 5.12 instead proposes a capability for multiple operating-point search to overcome these limitations. It can automatically search for and find multiple (in most cases, all) operating-points of a circuit.
The operating-point, also known as "bias point" or quiescent point, is the set of DC voltages and/or currents which, when applied to the circuit, represents a non-ambiguous starting state when applying some input sequence. It is defined by the value of each net voltage and the state of each device for which the circuit is at a “rest state” or steady state.
Several operating-points can exist in analog, logic, or mixed signal circuits much like the various summits and troughs on a mountain range. The saddle points represent meta-stable operating-points.
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Description of the benefits
Finding DC operating-points is the first and maybe the most determinant design task for both the circuit designer and the SoC Integrator. In most simulators, the DC operating point is found by using Newton–Raphson based heuristics. Such heuristics have two shortcomings that are now overcome:
- Firstly, convergence cannot be guaranteed unless the initial starting point (to be specified by the designer) is sufficiently close to the actual solution. Unfortunately, the designer either may not know the solution or cannot give a sufficiently accurate initial guess. Furthermore, even if the designer could give an accurate initial guess of the required operating-point to cause the circuit to operate as desired, nothing guarantees that self-biasing of the circuit will place it in the desired operating-point!
- Secondly, a single operating-point can be found during each analysis. Neither existence nor even location of other operating points is determined once the heuristics have converged to a particular operating-point.
With the operating-point analysis, SMASH proceeds through a complete search, using all heuristics in sequence, and gives the first stable result that is found. With multiple operating-point analysis, SMASH helps the designer to determine whether operating points are multiple and to identify meaningful ones in order to simplify eradicating unwanted operating-points which render the circuit useless when self-biased in such operating conditions!
< SMASH Options
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