  Standard deep learning algorithms are implemented using floating-point real
numbers. This presents an obstacle for implementing them on low-end devices
which may not have dedicated floating-point units (FPUs). As a result,
researchers in tinyML have considered machine learning algorithms that can
train and run a deep neural network (DNN) on a low-end device using integer
operations only. In this paper we propose PocketNN, a light and self-contained
proof-of-concept framework in pure C++ for the training and inference of DNNs
using only integers. Unlike other approaches, PocketNN directly operates on
integers without requiring any explicit quantization algorithms or customized
fixed-point formats. This was made possible by pocket activations, which are a
family of activation functions devised for integer-only DNNs, and an emerging
DNN training algorithm called direct feedback alignment (DFA). Unlike the
standard backpropagation (BP), DFA trains each layer independently, thus
avoiding integer overflow which is a key problem when using BP with
integer-only operations. We used PocketNN to train some DNNs on two well-known
datasets, MNIST and Fashion-MNIST. Our experiments show that the DNNs trained
with our PocketNN achieved 96.98% and 87.7% accuracies on MNIST and
Fashion-MNIST datasets, respectively. The accuracies are very close to the
equivalent DNNs trained using BP with floating-point real number operations,
such that accuracy degradations were just 1.02%p and 2.09%p, respectively.
Finally, our PocketNN has high compatibility and portability for low-end
devices as it is open source and implemented in pure C++ without any
dependencies.
