TTOL device to Interface analog and digital

QSPICE
1

Many of you may already realized this, but I just got myself clear until recent study and take a chance to share that. Qspice uses adaptive step size algorithms, and for example to use if(x,y,z) as a comparator implementation, you may find its output looks trapezoidal instead of square in a long run. Normally, I add .option MAXSTEP to limit maximum time step in simulation, but also significantly increase simulation time.

I just realized that, to utilize a device with TTOL (i.e. Switch, ¥-Device) can help. As TTOL allows one to determine how accurately the switch time should be found, if the comparator output flip event trigger switch action for a device with TTOL feature, extra time step can be added after switch action to achieve an instance response waveform.

Here is an example to compare with and without a buffer after B-source with if(x,y,z) as a comparator.
I just remind myself in many simulators, analog and digital requires an interface device, just like this simulation case. The only difference is that, originally, I didn’t consider this as a digital circuit as if(x,y,z) is not necessary be a 0/1 output but can be a formula or other stuff.

So, I am not sure if any other better method, but at least this can help to balance time step and simulation time.

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That is very handy to know - thanks for sharing!
I wonder if we can add TTOL to a DLL block in order to force similar behavior?

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Hi, Emrys.

Unless my tests have led me seriously astray, yes, you can use the Trunc() function to force the next timestep. It works like this:

At entry, the Trunc() t parameter is the “default” next simulation timepoint. The *steptime points to “infinity.” The difference between the Trunc() t parameter and the prior simulation timepoint (possibly saved from the t parameter in the evaluation function) is the steptime that QSpice is suggesting. Return a value less than that in *steptime and QSpice will set the next simulation timepoint to that value added to the prior sim timepoint.

I hope that’s clear. I need to post my test program that demonstrates this – it’s ugly but I’ll try to get that done tomorrow.

–robert

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That is pretty clear to me - I’ll have to play around with it when I have some time. I built an LM2611 Cuk converter model in QSpice not too long ago that this would probably help improve. Currently, I’m just running a pulse source as a clock to force calculations at particular intervals.

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I used your exact same technique in my simulation before.
I observed that voltage and current source with a Timectrl instance param, which can affect simulation step when approaching change.

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Can i use .options TTOL=1n to globally set all the “Y” devices ?

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Hello KSKelvin,
Thank you for sharing this!
I’ve got a huge time reduction for similar result as can be seen in the attached picture.

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@ChrisHew I don’t see a globally option for TTOL so far.
@mflorin Good to hear that. I think TTOL is one of the key making Qspice much faster.