Approximated 2-Bit Adders for Parallel In-Memristor Computing With a Novel Sum-of-Product Architecture

Conventional computing methods struggle with the exponentially increasing demand for computational power, caused by applications including image processing and machine learning (ML). Novel computing paradigms such as in-memory computing (IMC) and approximate computing (AxC) provide promising solutions to this problem. Due to their low energy consumption and inherent ability to store data in a nonvolatile fashion, memristors are an increasingly popular choice in these fields. There is a wide range of logic forms compatible with memristive IMC, each offering different advantages. We present a novel mixed-logic solution that utilizes properties of the sum-of-product (SOP) representation and propose a full-adder circuit that works efficiently in 2-bit units. To further improve the speed, area usage, and energy consumption, we propose two additional approximate (Ax) 2-bit adders that exhibit inherent parallelization capabilities. We apply the proposed adders in selected image processing applications, where our Ax approach reduces the energy consumption by $\mathrm {31~\!\%}$ – $\mathrm {40~\!\%}$ and improves the speed by $\mathrm {50~\!\%}$ . To demonstrate the potential gains of our approximations in more complex applications, we applied them in ML. Our experiments indicate that with up to $6/16$ Ax adders, there is no accuracy degradation when applied in a convolutional neural network (CNN) that is evaluated on MNIST. Our approach can save up to 125.6 mJ of energy and 505 million steps compared to our exact approach.