All-Analog Silicon Integration of Image Sensor and Neural Computing Engine for Image Classification
Benjamin Zambrano, Sebastiano Strangio, Tommaso Rizzo, Esteban Garzón, Marco Lanuzza, Giuseppe Iannaccone
Abstract
We have designed a fully-integrated analog CMOS cognitive image sensor based on a two-layer artificial neural network and targeted to low-resolution image classification. We have used a single poly 180 nm CMOS process technology, which includes process modules for realizing the building blocks of the CMOS image sensor. Our design includes all the analog sub-circuits required to perform the cognitive sensing task, from image sensing to output classification decision. The weights of the network are stored in single-poly floating-gate memory cells, using a single transistor per analog weight. This enables the classifier to be intrinsically reconfigurable, and to be trained for various classification problems, based on low-resolution images. As a case study, the classifier capability is tested using a low-resolution version of the MNIST dataset of handwritten digits. The circuit exhibits a classification accuracy of 87.8%, that is comparable to an equivalent software implementation operating in the digital domain with floating point data precision, with an average energy consumption of 6 nJ per inference, a latency of 22.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> and a throughput of up to 133.3 thousand inferences per second.