Schedule Versus Specifications

How many power reduction opportunities are left on the table due to tight schedules in today’s SoCs?

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With power being paramount in SoCs today, I was surprised to hear the amount of time spent on power reduction exercises can be only a few days.

According to William Ruby at Ansys/Apache, how much time engineers spend on power reduction activities depends on how sensitive the design is to power and whether they are still trying to meet the power spec or — based on the early power estimates – if they’re at their target or below. “If you are meeting your target then you may say, ‘Well, I don’t really need to do much, I’ll just run through synthesis, place & route and finish my design.’ but that doesn’t usually happen.”

Of course for mobile designs it matters what the power targets are, and there is always more effort placed on trying to get the power lower and lower. In this case, engineering teams can be spending quite a bit of time in their schedule to reduce power. In fact, he pointed out it becomes, at least in one customer case he has seen, they now have power targets on a per block basis and the RTL engineers are responsible for meeting those power targets on a per block basis. Bottom line: their RTL is not done until the power target is met.

But then there are other teams that may only budget two weeks in the project schedule to do what they would call the ‘power scrub’ or ‘power cleanup’ type of exercise. “They may say within those two weeks we will implement the RTL changes suggested by the tool as much as we can and for those that we don’t have the time for we can file them away and when we do the next revision of the chip maybe we will come back and address those. So again it’s the classic project management problem – it’s schedule versus specifications.”

I suppose I expected to hear that power is observed throughout the design flow, but the fact is, accuracy about power only comes at the RTL stage…at least for now. Technology enhancements to bring more visibility to power at a higher level of abstraction will be interesting to see.

How much time do you spend on power reduction exercises?