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Simulators >> Circuit Simulators >> Instance length does not fit the given lmax-lmin
https://designers-guide.org/forum/YaBB.pl?num=1213779607

Message started by qiushidaren on Jun 18^th, 2008, 2:00am

Title: Instance length does not fit the given lmax-lmin
Post by qiushidaren on Jun 18^th, 2008, 2:00am

Hi all,

I need to use some PMOSFETs which have a length of 40u, but in the .scs file provided by TSMC, the length can not be larger than 20u, so I modify the lmax=20u to 50u in the file. But, when I simulate the circuit in Spectre it still says: “IXX.MXXX: Instance length or width does not fit the given lmax-lmin or wmax-wmin range for any model in the ‘pch’ group”, what should I do to solve this problem? Thank you in advance.

Best regards,
Terry

Title: Re: Instance length does not fit the given lmax-lmin
Post by ACWWong on Jun 23^rd, 2008, 4:42pm

Hi qiushidaren,

Generally its not good practice to hack foundary model files, especially for MOS models based on BSIM fitting. The maximum limits for the MOS length is set so the model is only applicable for devices within the limits, with no guarantees as to model performance/match to silicon beyond these limits.
Anyway If you want a 40um length, why not use 2 series 20um length devices. There have been a few discussions on this topic in this forum, so do a quick search for more details...
e.g http://www.designers-guide.org/Forum/YaBB.pl?num=1162979112/30#30

cheers
aw

Title: Re: Instance length does not fit the given lmax-lmin
Post by Geoffrey_Coram on Jul 1^st, 2008, 5:35am

Adjusting lmax should have worked ... assuming you got the right model card. Are you sure you got the PMOSFET model card (not nmos) and the right "type" (standard-vt, high-vt, etc.)? Also, obviously, you should check the wmin/wmax values.

And ACWWong is right about no guarantee that the model card will accurately represent silicon behavior beyond the foundry's lmax.

Title: Re: Instance length does not fit the given lmax-lmin
Post by Miconos on Jul 1^st, 2008, 5:56am

Another possibility you have is to use the length of the transistor as a parameter and to sweep it.
I thinh that way you avoid both the echematic entry and the simulator limitations.

Title: Re: Instance length does not fit the given lmax-lmin
Post by qiushidaren on Jul 20^th, 2008, 7:58pm

Geoffrey_Coram wrote on Jul 1^st, 2008, 5:35am:

Yes, I got the right model and right type, and I changed both the lmin/lmax and the wmin/wmax values, but it still said that, I don't know why, maybe there are some other values that are relative with the wmin/wmax and lmin/lmax parameter?

Terry

Title: Re: Instance length does not fit the given lmax-lmin
Post by qiushidaren on Jul 20^th, 2008, 7:59pm

Miconos wrote on Jul 1^st, 2008, 5:56am:

Another possibility you have is to use the length of the transistor as a parameter and to sweep it.
I thinh that way you avoid both the echematic entry and the simulator limitations.

Hi Miconos,

Then what will happen?

Title: Re: Instance length does not fit the given lmax-lmin
Post by Geoffrey_Coram on Jul 21^st, 2008, 9:07am

qiushidaren wrote on Jul 20^th, 2008, 7:58pm:

How do you know you got the right model and type? Why did you change wmin/wmax if it was length that was out of range? I would have just changed lmax to 1 meter. (If you set lmax=40u, it could be that "DL" makes the device have a slightly longer channel so it doesn't fit the bin.)

Can you cut out a single instance of the model from the library file and run it? How complicated is the library -- I've seen some with "sections" and "includes" that make it quite difficult to find what's actually being simulated (eg, maybe you changed lmax in the fast corner rather than nominal).