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jimmy

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problem of changing accuracy during transient sim
Nov 28^th, 2004, 8:35pm

Hi all,

I wanted to change the tolerance during transient simulation, say, use a loose one for quick settling, then change to tight one for accuracy. I tried the following method proposed by someone on this forum before. Somtimes it works, sometimes not.

My circuit contains clocks. But I avoided to save data at the clocks' edges.

Can anyone give me some hint?

thank you,
jimmy

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You can use the combination of "writefinal" and "readic"
in the transient Options form to run the transient analysis
from 0-200ns and then from 200ns-400ns.

During the 0-200ns simulation, write the final transient
timepoint to a file called spectre_200.fc. Use this file
as the initial conditions for the transient analysis
initial transient solution during the 200ns-400ns simulation.

- In the Analog Simulation window, select Analyses->Choose.
- In the Choosing Analyses form, select tran and set the stop time to 200ns.
- Click on the Options button at the bottom of the form. The transient
Options form will pop up.
- In the "write final" field, enter spectre_200.fc and OK the forms.
- Run the simulation. The final transient timepoint is saved to the file
spectre_200.fc. You can use this file as the initial conditions for the
transient analysis initial transient solution during the 200ns-400ns
simulation.
- Next, in the transient analysis form set the stop time to 400ns.
- In the transient Options form, set
start = 200ns
readic = spectre_200.fc
write final = spectre_400.fc.
- OK the forms and run the simulation. The simulation will run from
200ns-400ns using spectre_200.fc as the initial condition file.

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August West

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Posts: 71

Re: problem of changing accuracy during transient
Reply #1 - Nov 29^th, 2004, 9:31am

You can also add a Verilog-A block that contains a timer function to your circuit. If t_start is the time you want to tighten tolerances and t_step is the timestep size you want after t_start, then use

@(timer(t_start,t_step)
;

in your model. This does nothing until t_start, then it constrains the simulator to take steps every t_step seconds. This particular approach does not tighten the KCL tolerances, just the time step. And it is appropriate if you want to perform a Fourier analysis if you use it to assure that the simulator places a time point at every Fourier sample point.

Alternatively you can use the $boundstep function, with something like

if ($abstime > t_start)
$boundstep(t_step)

This also reduces the timestep, but does not force the timepoints to be at any particular points. So it results in faster simulation but does not help the Fourier analysis as much.

Whatever method you use, be aware that any error made when using loose tolerances will decay away at a rate set by the longest time constant of the circuit. This fact generally undermines the use of these types of approches to speed up simulation.

-August

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jimmy

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Posts: 32

Re: problem of changing accuracy during transient
Reply #2 - Nov 30^th, 2004, 9:23pm

Hi August,

Thank you for your reply.

I am afraid that the method you suggested might make the time step too small when they don't need to somtimes. When I tight the tolerance, i noticed that tie time steps are not constant. someitme the timesteps are very small, sometimes not.

I found the problem in my simulation when I use the "two-step' method. The ahdl model of dff I used was the trouble maker. I just took the model from ahdlLib, didn't change anything.

The problem is : when I started the second transient simulation, the output of the dff was reset to 0 even though both its input and output are high at that momeht (on the other words, at the moment when the first simulation stopped). I didn't find any resetting command in the ahdl code. I am guessing the simulator jsut force the ahdl block output to 0 at beginning.

It was this undesired "resetting" make the simulation wrong.

Can you help me to figure out a solution for this issue?

thanks a lot,
jimmy

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Andrew Beckett

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me about Life...

Posts: 1742
Bracknell, UK

Re: problem of changing accuracy during transient
Reply #3 - Dec 1^st, 2004, 10:13pm

In the upcoming MMSIM60 release (due out in the next few weeks), spectre will have a true save and restart capability, which should correctly store the state at the stopping point. Currently some of the state is not written into the writefinal file, and so internal verilog-a variables are not restored when you restart from a checkpoint. That should get solved with the save/restart mechanism.

Andrew.

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jimmy Community Member Offline Posts: 32	Re: problem of changing accuracy during transient Reply #4 - Dec 2^nd, 2004, 11:13am Andrew, Thank you for your reply. Do you have solution to solve this problem without using the new release. We are using the university version of cadence, it is already behind. thank you so much, jimmy
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Andrew Beckett Senior Fellow Offline Life, don't talk to me about Life... Posts: 1742 Bracknell, UK	Re: problem of changing accuracy during transient Reply #5 - Dec 2^nd, 2004, 9:59pm 'fraid not. Sorry. Actually, one thought comes to mind. You could do it in AMS Designer, which has had a save/restart for some time... Andrew.
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jimmy Community Member Offline Posts: 32	Re: problem of changing accuracy during transient Reply #6 - Dec 3^rd, 2004, 10:38am Thankj you Andrew, Do I need to change anything in my circuit to run in AMS designer? jimmy
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Andrew Beckett Senior Fellow Offline Life, don't talk to me about Life... Posts: 1742 Bracknell, UK	Re: problem of changing accuracy during transient Reply #7 - Dec 5^th, 2004, 12:04pm You shouldn't need to change anything in the circuit. You may need to do some work to your design kit if it doesn't currently support AMS Designer, but that's not generally that hard (see the docs, particularly on the conversion toolbox). Andrew.
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