  Neuromorphic systems typically employ current-mode circuits that model neural
dynamics and produce output currents that range from few pico-Amperes to
hundreds of micro-Amperes. On-line real-time monitoring of the signals produced
by these circuits is crucial, for prototyping and debugging purposes, as well
as for analyzing and understanding the network dynamics and computational
properties. To this end, we propose a compact on-chip auto-scaling Current to
Frequency Converter (CFC) for real-time monitoring of analog currents in
mixed-signal/analog neuromorphic electronic systems. The proposed CFC is a
self-timed asynchronous circuit that has a wide dynamic input range of up to 6
decades, ranging from pico-Amps to micro-Amps, with high current measurement
sensitivity. To produce a linear output frequency response, while properly
covering the wide dynamic input range, the circuit automatically detects the
scale of the input current and adjusts the scale of its output firing rate
accordingly. Here we describe the proposed circuit and present experimental
results measured from multiple instances of the circuit, implemented using a
standard 180 nm CMOS process, and interfaced to silicon neuron and synapse
circuits for real-time current monitoring. We demonstrate how the circuit is
suitable for measuring neural dynamics by showing the converted response
properties of the chip silicon neurons and synapses as they are stimulated by
input spikes.
