Simulation and complete lack of real time

Question

Hi,
I have installed multisim and put down a a very basic 555 timer where  I noticed everything is in slow motion.
So I hooked up a signal gen to a scope at 50 hz, the scope then draws the signal wave at less than 1hz?
My processor is almost on idle so I cannot forsee any lack of processing power.
What completly blows me away is that NO where in the manual is this made obvious, how can simulation of electronics be done at 1/10th of the real speed?
What even confuses me even more is that if I up the freq of the signal gen, I get  proper high speed results plotted on the scope, so this means it has nothing to do with CPU usuage.
When I plot 50hz I get this rediculous slow motion less than 1hz on the output of the scope.
please help explain this?
cheers
Gavin

Answer 1

I usually have the opposite situation. My low frequency signals are usually fairly fast and my high frequency signals simulate extremely slow.
Multism using the transient simulation (pressing the run button)  does not run in "real time". The time indicator at the very bottom of the screen indicates what I would refer to as circuit time. It is the exact time in the real world that it would take for a circuit to accomplish what it is doing even though it may take 1 minute of our clock time to simulate a 1 second circut event. The reason behind this is that it take time for all the spice commands and circuit mathmatics to be executed, interpreted, and then a solution arrived at and then all the outputs plotted on the instruments. It is this number crunching "on the fly" that makes the simulator run out of sync with real time.
One thing that can be used to speed things up is to look under SIMULATE>INTERACTIVE SIMULATOR SETTINGS then go down to where it says Set Maimum Timesteps (TMAX). Click the circle next to Maimum Time Step (TMAX) and in the the little box to the right change it from 1e-005 to 1e003.
With the obve setting you can approach real time with simple circuits, but with more complex circuits or high frequency circuits this could cause problems as the simulator doesn't have time to converge to a solution and you will get "time step too small" errors or the output will not look as you expected it to look.. This setting is basically the resolution setting of the simulator. The lower this setting is the less accuate the results but the simulation speed is faster. The higher this setting is the more accuare the results but the slower the simulation speed.
I hope I have helped somewhat and that this helps explain what you are seeing. 
Message Edited by lacy on 10-28-2007 08:30 AM
Kittmaster's Component Database
http://ni.kittmaster.com
Have a Nice Day