Hi,
I am curious about the big difference between the simulation speed for two similar circuits. Would you please help me?
I have been redesigning a circuit using 0.25um CMOS technology. The original circuit was implemented using 0.35um CMOS technology.
The design has two oscillators, one very fast (~1GHz), the other slow (~1MHz for the original design, ~4MHz for the new design). So it takes so long time to run the transient simulation for both design.
But I am curious that the simulation time for the new design is much longer than that for the original design though both circuits has similar structure and spec.
It took near 300,000 seconds to run a 500us transient simulation using SPICE option method=gear. But it took more than 450,000 seconds to run just a 50us transient simulation using default integration method (trapzoid). As well known, transient simuation is much slower if using gear method.
So it makes me deep confused.
The detail statistics is shown below.
For the original design.
analysis time # points tot. iter conv.iter
transient 291621.51 500001 81511395 22966268 rev=******
For the new design.
analysis time # points tot. iter conv.iter
transient 409348.59 5001 45405049 12645293 rev=******
From the above data, the convergent iteration of the new design is about half of that of the original design. But remember it is a 500us transient simulation for the original design, and just a 50us transient simulation for the new design. That means the average internal time step is about 1/5 of that for the orignal design.
What makes the simulation for the new design interated so much times? And its internal time step smaller?
I notice one difference between the SPICE models for 0.35um and 0.25um. The SPICE model for 0.35um has a non-zero HDIF parameter(hdif=3.5e-7), but HDIF=0.0 for the 0.25um SPICE model. I suspect that will deteriorate the discontinuity of the 0.25um SPICE model. Thus SPICE tool need smaller internal time step and much more iteration.
I am not an expert in SPICE algorithm and MOS model. So I need your help!
What do you think the reason? Do you have any advise to speed the simulation?
Thanks
Yawei