Xetal-Pro: An ultra-low energy and high throughput SIMD processor

This paper presents Xetal-Pro SIMD processor, which is based on Xetal-II, one of the most computational-efficient (in terms of GOPS/Watt) processors available today. Xetal-Pro supports ultra wide V_DD scaling from nominal supply to the sub-threshold region. Although aggressive V_DD scaling causes severe throughput degradation, this can be compensated by the nature of massive parallelism in the Xetal family. The predecessor of Xetal-Pro, Xetal-II, includes a large on-chip frame memory (FM), which cannot operate reliably at ultra low voltage. Therefore we investigate both different FM realizations and memory organization alternatives. We propose a hybrid memory architecture which reduces the non-local memory traffic and enables further V_DD scaling. Compared to Xetal-II operating at nominal voltage, we could gain more than 10× energy reduction while still delivering a sufficiently high throughput of 0.69 GOPS (counting multiply and add operations only). This work gives a new insight to the design of ultra-low energy SIMD processors, which are suitable for portable streaming applications.