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More cores, less power

Reducing SoC power consumption through multiprocessor design. By Chris Rowen.

Whilst transistor density continues to increase, the ability of chip packaging systems to dissipate power is reaching its limits. This widening gap means chip design will be driven increasingly by power considerations.
It is possible to reduce power by moving high performance functions away from the main processor into hardwired logic functions. Unfortunately, hardwired functions require significant time to design and verify and are easily obsoleted by changes in standards. Programmable processor based solutions can implement these functions quickly and future proof designs.
Modern SoCs typically employ a control processor plus other processors for functions that used to be designed in hardwired logic. Whether it’s an additional processor bringing audio functions to a chip or an array of small processors to route network traffic, application specific tailored processors are replacing blocks of logic because they are as energy efficient as logic blocks, but easier to design and adapt.
Processors based on a modern configurable architecture have significant power advantages over traditional risc architectures. Their superior baseline architecture and the absence of accumulated ‘feature baggage’ gives these newer processors a power advantage of 2 to 3x.
Whilst the ability to configure a processor’s instruction set is commonly associated with adding features, this technology also allows unnecessary features to be pruned, saving code, area, power and energy. Application specific processors can reduce the energy requirements for data intensive computation by an order of magnitude or more, compared to general purpose risc processor cores.
Application specific instruction set extensions pack the same work into fewer cycles, allowing the SoC to run more slowly and at a lower operating voltage. Extending the processor does increase power dissipation per clock cycle, but this is usually far smaller than the increase in application performance. The power savings will improve, in line with the cycle count reduction.

Graham Pitcher

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