C3PU: Cross-Coupling Capacitor Processing Unit Using Analog-Mixed Signal In-Memory Computing for AI Inference

TL;DR

This paper introduces C3PU, an analog-mixed signal in-memory computing unit for AI inference that performs multiply-and-accumulate operations with high energy efficiency and small area, verified through simulations and neural network application.

Contribution

The paper presents a novel capacitive-based in-memory computing unit (C3PU) supporting MAC operations with reduced energy and area, validated by simulation and neural network testing.

Findings

C3PU consumes 66.4 fJ/MAC at 0.3 V with 5.7% error.

Achieves 3.4x lower energy and 3.6x smaller area than digital MAC.

Neural network application yields 90% accuracy on iris classification.

Abstract

This paper presents a novel cross-coupling capacitor processing unit (C3PU) that supports analog-mixed signal in memory computing to perform multiply-and-accumulate (MAC) operations. The C3PU consists of a capacitive unit, a CMOS transistor, and a voltage-to-time converter (VTC). The capacitive unit serves as a computational element that holds the multiplier operand and performs multiplication once the multiplicand is applied at the terminal. The multiplicand is the input voltage that is converted to a pulse width signal using a low power VTC. The transistor transfers this multiplication where a voltage level is generated. A demonstrator of 5x4 C3PU array that is capable of implementing 4 MAC units is presented. The design has been verified using Monte Carlo simulation in 65 nm technology. The 5x4 C3PU consumed energy of 66.4 fJ/MAC at 0.3 V voltage supply with an error of 5.7%. The proposed unit achieves lower energy and occupies a smaller area by 3.4x and 3.6x, respectively, with similar error value when compared to a digital-based 8x4-bit fixed point MAC unit. The C3PU has been utilized through an iris fower classification utilizing an artificial neural network which achieved a 90% classification accuracy compared to ideal accuracy of 96.67% using MATLAB.