Posts Tagged ‘constrains’

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Why Not Just Over-Constrain My Design?

June 25, 2008

This is a question often raised by beginners when trying to squeeze performance from their designs.
So, why over-constraining a design does not necessarily improve performance. The truth is that I don’t really know. I assume it is connected to some internal variables and measuring algorithms inside the synthesis tool and the fact that they give up trying to improve the performance because they reached a certain local minimum in some n-variable space (really!).

But empirically, I (and many others) have found out that you can not get the best performance by just over-constraining your design in an unrealistic manner. It has to be somehow closely related to the actual maximum speed that can be reached. The graph below sums up this problem pretty neatly.

As seen above, there is a certain min-max range for the performance frequency that can be reached and its peak is not the result of the highest frequency constrained!
The flat region on the left of the figure is the speed reached without any optimization, that is, right after mapping your HDL into gates. As we move towards the right, we see actual speed improvement as we constrain for higher speeds. Then a peak is reached and constraining for higher speeds results in poorer performance.

I worked relatively less with FPGAs in my carrier but I have seen this phenomenon there as well. Take it to your attention.

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