Geoffrey_Coram
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Suppose you have a counter that is a variable, and thus hidden state, but you use (* instrument_module *). Now, for 9 periods, your module puts out a sinusoidal output, and it looks like it's periodic. But maybe on the 10th, the counter resets and the output is zero. If the rest of the circuit is simple, then PSS may appear to converge in 5 iterations (eg, 5 fixed-point iterations, which are essentially periods of plain old transient analysis, except that the simulator checks if the voltages and currents (but not internal variables!) have the same value at the end of a period as they did at the start.
But if the circuit is a little complicated and takes more than 9 iterations, all of a sudden your module does something completely different, and the simulator gets confused.
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