Wednesday, March 20, 2019

RC Variation

RC variation is also considered as corners for the setup and hold checks. RC variation can happen because of fabrication process and the width of metal layer can vary from the desired one.

Critical corners for Setup and Hold check

We always check our chip to work in worst scenarios. We should be very pessimistic about setup and hold checks, so we consider worst case scenarios first.

Setup violation can be caused if data is coming very slow. So the condition when process is slow, voltage is minimum and temperature is maximum is the worst case for setup check.
Hold violation is caused if data comes faster. So process should be faster,voltage should be maximum and temperature should be minimum.

Now if setup and hold are checked in worst corners, then the chip should work in every scenario. Still we check them in typical corners because we need to analyze power consumption. Refer following table for the worst case scenarios for setup and hold.



In 90-nm technology and above, a timing path is predominantly governed by cell delays. And that’s the reasons only below mentioned 2 RC interconnect corners are sufficient for all the timing analysis.
1. Cbest (Also known as Cmin) – minimizes C, maximizes R
2. Cworst (Also known as Cmax) – maximizes C, minimizes R

However below 90nm node, the contribution of interconnect delay in a timing path become significant and the Coupling Cap component (Cc) in net delay can significantly alter slack values at an endpoint of a timing path. So, RC corners have to be split up as per the contribution of each component Ground Capacitance (Cg) and Coupling Capacitance (Cc). So on top of the 2 conventional RC corners Cmax and Cmin, foundry came up with 2 more RC corners.

1. RC best (also known as XTALK corner) - Cc is max , Cg x R is min
2. RC worst (also known as Delay corner) - Cc is min ,Cg x R is max

So we can say that there are overall 5 parasitic corners.
1. Cbest
2. Cworst
3. RCbest
4. RCworst
5. Typical

Few definitions/information for every corner:
1.C-best:
It has minimum capacitance. So also known as Cmin corner.
Interconnect Resistance is larger than the Typical corner.
This corner results in smallest delay for paths with short nets and can be used for min-path-analysis.

2.C-worst:
Includes corners which results maximum Capacitance. So also known as Cmax corner.
Interconnect resistance is smaller than at typical corner.
This corners results in largest delay for paths with shorts nets and can be used for max-path-analysis.

3.RC-best:
Includes corners which minimize interconnect RC product. So also known as RC-min corner.
Typically corresponds to smaller etch which increases the trace width. This results in smallest resistance but corresponds to larger than typical capacitance.
Corner has smallest path delay for paths with long interconnects and can be used for min-path-analysis.

4.RC-worst:
Includes corners which maximize interconnect RC product. So also known as RC-max corner.
Typically corresponds to larger etch which reduces the trace width. This results in largest resistance but corresponds to smaller than typical capacitance.
Corner has largest path delay for paths with long interconnects and can be used for max-path-analysis.

5.Typical:
This refers to nominal value of interconnect Resistance and Capacitance.

So there are 2 types of parasitic
-C-based (Capacitance dominates short wire)
-RC-based (R dominates for long wires)
In C-based C means worst and best case capacitance but in RC-based RC means worst and best case R with adjustment in C towards worst or best but keeping the process planar.
Based on the experience it was found that C-based extraction provides worst and best case over RC for internal timing paths because Capacitance dominates short wire. However for large design, inter-block timing paths were often worst with RC worst parasitic since R dominates for long wires.

Note: No corner guarantees min or max delay for an arbitrary transistor driving an arbitrary wire topology

With the help of below picture, we can understand easily :


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