AXSERBUS: A quality-configurable approximate serial bus for energy-efficient sensing

Mobile, wearable, and implantable devices integrate an increasing number and variety of sensors such as microphones, image sensors, and accelerometers. These devices spend substantial amounts of time reading the sensors within them, thereby incurring significant energy dissipation over off-chip serial interconnects. This paper proposes AXSERBUS, a quality-configurable approximate serial bus that exploits the locality of sensory data and the error resiliency of sensing applications to reduce energy dissipation. AXSERBUS significantly reduces signal transitions by encoding the differentials of sensory data in three encoding modes, depending on the magnitude of the differentials: very small differentials are zeroed out, incurring no energy dissipation; intermediate differentials are encoded using special low-transition count patterns; and for high differentials, the absolute value (not the differential) of the data is transmitted. Compared to previous schemes, the proposed multi-level encoding results in more data being encoded as low-energy patterns. In addition, in the intermediate differential encoding mode, the differentials are encoded in an approximate manner, and the approximation bounds are proportional to the magnitude of the differentials. Since small differentials are more frequent than large differentials in sensory data, the proposed encoding scheme also minimizes quality degradation. We demonstrate that AXSERBUS achieves improved energy vs. quality tradeoffs compared to previous schemes. In the context of an optical character recognition (OCR) application, AXSERBUS achieves 79.4% reduction in dynamic power dissipation, while maintaining accuracy above 95%.