  We present PyCARL, a PyNN-based common Python programming interface for
hardware-software co-simulation of spiking neural network (SNN). Through
PyCARL, we make the following two key contributions. First, we provide an
interface of PyNN to CARLsim, a computationally-efficient, GPU-accelerated and
biophysically-detailed SNN simulator. PyCARL facilitates joint development of
machine learning models and code sharing between CARLsim and PyNN users,
promoting an integrated and larger neuromorphic community. Second, we integrate
cycle-accurate models of state-of-the-art neuromorphic hardware such as
TrueNorth, Loihi, and DynapSE in PyCARL, to accurately model hardware latencies
that delay spikes between communicating neurons and degrade performance. PyCARL
allows users to analyze and optimize the performance difference between
software-only simulation and hardware-software co-simulation of their machine
learning models. We show that system designers can also use PyCARL to perform
design-space exploration early in the product development stage, facilitating
faster time-to-deployment of neuromorphic products. We evaluate the memory
usage and simulation time of PyCARL using functionality tests, synthetic SNNs,
and realistic applications. Our results demonstrate that for large SNNs, PyCARL
does not lead to any significant overhead compared to CARLsim. We also use
PyCARL to analyze these SNNs for a state-of-the-art neuromorphic hardware and
demonstrate a significant performance deviation from software-only simulations.
PyCARL allows to evaluate and minimize such differences early during model
development.
