An Overflow/Underflow-Free Fixed-Point Bit-Width Optimization Method for OS-ELM Digital Circuit

TL;DR

This paper introduces a novel bit-width optimization method for OS-ELM digital circuits that prevents overflow and underflow, ensuring reliable online learning on resource-limited IoT devices with minimal area overhead.

Contribution

It presents an overflow/underflow-free fixed-point bit-width optimization technique specifically for OS-ELM digital circuits, enhancing reliability in on-chip online learning.

Findings

Achieves overflow/underflow-free operation with 1.0x - 1.5x increased area

Ensures stable online training for resource-constrained IoT devices

Reduces unexpected circuit behavior due to numerical issues

Abstract

Currently there has been increasing demand for real-time training on resource-limited IoT devices such as smart sensors, which realizes standalone online adaptation for streaming data without data transfers to remote servers. OS-ELM (Online Sequential Extreme Learning Machine) has been one of promising neural-network-based online algorithms for on-chip learning because it can perform online training at low computational cost and is easy to implement as a digital circuit. Existing OS-ELM digital circuits employ fixed-point data format and the bit-widths are often manually tuned, however, this may cause overflow or underflow which can lead to unexpected behavior of the circuit. For on-chip learning systems, an overflow/underflow-free design has a great impact since online training is continuously performed and the intervals of intermediate variables will dynamically change as time goes by. In this paper, we propose an overflow/underflow-free bit-width optimization method for fixed-point digital circuits of OS-ELM. Experimental results show that our method realizes overflow/underflow-free OS-ELM digital circuits with 1.0x - 1.5x more area cost compared to the baseline simulation method where overflow or underflow can happen.