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Monday, 2 January 2012

The glitch that stole the FPGA's energy efficiency

Field-programmable gate arrays (FPGAs) are notorious for high power consumption. They are hard to power down in the same way as custom logic - so they have considerable static power consumption - and they use a lot more gates to achieve the same job with their greater flexibility.

However, a good proportion of an FPGA's power consumption is avoidable. A 2007 study carried out by researchers at the University of British Columbia and published in IEEE Transactions on VLSI Systems found that up to three quarters of the dynamic power consumption could be ascribed to glitches rather than actual functional state transitions for some types of circuit.

The heart of the problem lies with timing: early-arriving signals can drive outputs to the wrong state before the situation is 'corrected' by later signals and before the final state is ready for sampling at the next clock transition. When you consider the large die size of FPGAs relative to custom logic, it is not hard to see why delays can be so large between signals.

UBC's Julien Lamoureux and colleagues recommended the use of delay elements to align signals in time to reduce these glitch events.

At the International Symposium on Low Power Electronic Design earlier this year, Warren Shum and Jason Anderson of the University of Toronto proposed an alternative: making use of the don't care conditions used in logic synthesis to also filter out potential glitches:

"This process is performed after placement and routing, using timing simulation data to guide the algorithm...Since the placement and routing are maintained, this optimization has zero cost in terms of area and delay, and can be executed after timing closure is completed."

The alterations are made in the LUTs iteratively to create new truth tables that will reduce the number of glitch transitions during operation, borrowing some concepts from asynchronous design where glitches are considered actively dangerous rather than inconvenient. On benchmark circuits, the technique reduced glitch power by around 14 per cent on average and up to half in some cases.

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