Tip of the Week: Rapid DC convergence in large circuits

DC Op Point simulation time can be dramatically reduced by use of either of two Spectre functions, "readns" or "readforce".  These allow you to use your circuit's previous DC solution as the starting point for a new simulation.  When running repetitive sim's of the same circuit, the time savings can be dramatic, especially for larger circuits.

"Readns" uses the DC solution from your previous simulation as an initial "guess" for the DC solution for the present simulation.  From that starting point, it then solves for the DC operating point as usual.  Since this starting point is almost always much closer to the actual DC operating point than if the simulator had to start from scratch, the result is usually much more rapid convergence, even when you have made changes to your circuit since the last simulation.

"Readforce" is similar, except that it actually FORCES your simulation to keep exactly the same DC op point it used last time, even if things have changed in your circuit since then.  In this case, it only computes new operating points for any nodes you may have added since the last simulation.  The readforce option should be avoided except in rare cases, because if you forget you have it on you can end up having your circuit set up to old (erroneous) DC operating points even when the circuit has changed substantially.

To use readns, in the simulator window go to
ANALYSIS-->CHOOSE-->DC-->OPTIONS... and fill in the "readns" field with the
same filename as is present in the "write" field beneath it (the default filename is spectre.dc).  If you watch your CIW while running your next simulation, you will see that it takes many less tries (and less time) to achieve convergence than before you set this option.

In the rare event you wish to use readforce instead, the technique is the same, except you fill in the file name in the readforce blank instead, and you also must set the  "force" option button to "all" for the force to take effect.

IN RARE CASES, especially if you have lots of "ideal" blocks, readns or readforce can actually cause a convergence problem.  This can sometimes be fixed by increasing the "rforce" parameter, found in the regular analog simulator options, from the default value of 1 ohm to something higher, such as 100 ohms.  This parameter controls the effective driving resistance that is used by each of the ideal sources that are setting the initial conditions.

- Hugh