Tool switches

@boldport asked:

What are your #FPGA design space exploration techniques?

which he expands upon:

“Design space exploration” is the process of trying out different settings and design methods for achieving better performance. Sometimes the goals are met without any of it — the default settings of the tools are sufficient. When they’re not, what are your techniques to meet your performance goals?

Yet again, the 140 character constraint leaves me with things unspoken….

Working where I do in the automotive market means that it’s not good enough to miss timing by a few picoseconds and say “it’ll be fine, ship it”. If you miss timing, you /have/ to make it pass.

My experience with tool tweakery is that it gains you a 2-5% timing improvement – which can be enough to meet timing when you just missed.

The downside is that usually, when you go and change the design (due to the requirements changing yet again), you find yourself with a slightly different 10ps timing violation which maybe this time the tools can’t get around. Or maybe with a change one of the seed parameters, it will, after some trial runs.

So, I’ve given up on that approach as being too variable. It’s much harder to give estimates of when something will be ready when timing closure is a “tweak the knobs a number of times and see”.

What I do now is rework things until it meets timing easily. That way, it’s likely to stay that way.

Techniques include:

  • Pipelining – adding registers
  • Constraining unconstrained integers – occasionally, the synthesiser doesn’t figure out the range an integer variable or signal can take on, so needs telling. This is happening less and less as synthesis tools get cleverer.
  • Simplifying algorithms

This give me a much more predictable build process. It’s seen me fine, even for a nearly full Spartan3 device with some logic running at 160+MHz DDR.

Of course, if you are right up against the limits of the device speed and you’ve pipelined and constrained and everything else, then tweaking tool parameters is all you have left – anyone in that position has my sympathies!

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