27.5 A multi-granularity FPGA with hierarchical interconnects for efficient and flexible mobile computing

Following the rapid expansion of mobile computing in the past decade, mobile system-on-a-chip (SoC) designs have off-loaded most compute-intensive tasks to dedicated accelerators to improve energy efficiency. An increasing number of accelerators in power-limited SoCs results in large regions of “dark silicon.” Such accelerators lack flexibility, thus any design change requires a SoC re-spin, significantly impacting cost and timeline. To address the need for efficiency and flexibility, this work presents a multi-granularity FPGA suitable for mobile computing. Occupying 20.5mm² in 40nm CMOS, the chip incorporates 2,760 fine-grained configurable logic blocks (CLBs) with 11,040 6-input look-up-tables (LUTs) for random logic, basic arithmetic, shift registers, and distributed memories, 42 medium-grained 48b DSP processors for MAC and SIMD operations, 16 32K×1b to 512×72b reconfigurable block RAMs, and 2 coarsegrained kernels: a 64-8192-point fast Fourier transform (FFT) processor and a 16-core universal DSP (UDSP) for software-defined radio (SDR). Using a mixradix hierarchical interconnect, the chip achieves a 4× interconnect area reduction over commercial FPGAs for comparable connectivity, reducing overall area and leakage by 2.5×, and delivering a 10-50% lower active power. With coarse-grained kernels, the chip´s energy efficiency reaches within 4-5× of ASIC designs.