DCT-RAM: A Driver-Free Process-In-Memory 8T SRAM Macro with Multi-Bit Charge-Domain Computation and Time-Domain Quantization

Abstract

Process-In-Memory (PIM) is a promising solution to alleviating the memory-wall bottleneck in memory-intensive applications like CNNs. Recent demonstrations of SRAM-based PIM designs, particularly those computing in the charge domain [1]–[5], have greatly improved the linearity of analog multiply-and-add computations (MAC) and quantization, and their robustness to process variations, making their inference accuracy approach that of digital hardware in practical computer vision benchmarks such as CIFAR-10. However, there remain several limitations towards large scale integration of PIM macros, especially the assumptions on the availability of powerful external reference voltage drivers and the lack of scaling friendly designs. More specifically, high-bandwidth analog buffers driving large output load are necessary to distribute the massive number of analog signals (e.g. DAC outputs) across the macro, without sacrificing signal fidelity and computing speed. [10] is one work that reports its DAC drivers occupying 11.4% of the macro area and incurring 94-pJ energy overhead in 28 nm, accounting for 68.5% of the total energy in a macro supporting $5\mathrm{b}$ activations and $8\mathrm{b}$ weight. Second, SAR ADCs are popular for the common 5–9 bit resolution range. High-speed power-hungry analog buffers are required in conventional SAR ADCs to drive the capacitive DACs (CDACs) to reference voltages, with short settling time and high accuracy. Given the hundreds of ADCs in each macro, the design complexity and overheads incurred by these drivers are dominant. Our simulated reference driver takes 2.9-pJ energy in 65 nm, which is comparable to an ADC (e.g. 3.56 $\text{pJ}$ in [12]). Third, it is challenging to fit any conventional $\geq 7\mathrm{b}$ SAR ADC into the narrow width of SRAM cells due to the bulky CDACs and layout matching requirements, ultimately limiting the computing parallelism and energy amortization.