Difference between ERC and sof check

[sudeeps]

Hello,

This question is asked in most interviews and I'm looking for a solid answer to this :

What is the difference between an ERC error and a soft check error ? Why in LVS we have these two as different checks?

Please help.

Thanks

 

[erikl]

What is the difference between an ERC error and a soft check error ? Why in LVS we have these two as different checks?

ERC is an independent check program, can be run on schematic already, i.e. long before layout, DRC & LVS, hence usually is not included in the LVS set.

It is a program which checks the violation of electrical rules, which the provider of the rules' set thinks are necessary to be checked. Any PDK provider may have different perceptions of which checks are necessary, hence ERC rules' sets from different providers may differ.

A soft check may be included into the ERC or may be performed by another program. AFAIK, soft check errors rather are considered as warnings, as e.g. the warning of parallel outputs, which surely is not allowed in non-intended cases, but very often is intended and isn't an error in such cases.

 

[sudeeps]

Can someone list out some soft check and ERC errors?
Thanks.

 

[DharmaSlice]

ERC errors may relate to something the is an electrical fault, like no path to power or ground within your layout. An example of this would be an NWELL which is not tied off to VDD. This can happen if someone places an NWELL guard ring and forgets to connect it to VDD.

-floating NWELL
-source and drain connected to power and ground
-floating gate
-floating DNWELL
-floating PWELL
-NWELL pickup connected to ground


It also shows up for 'hot-wells' , when the NWELL structure is connected to something other that a known power net , like in the case of a pmos input pair.

-

Soft-connects are very different however, this shows a connection between two nets or grounds. An example of this would be two grounds connected together like AVSS_analog and VSS_digital. Normally a soft-connect would show up if the nets are connected through the SUBSTRATE. They should have been isolated via DNWELL but for some reason they are not.

Only a quick explanation, hope that helps.

 

[sudeeps]

Thanks a ton