A load-balancing readout method for large event-based PWM imaging arrays

This paper describes concept and implementation of an asynchronous, column-parallel readout method for event-based pulse-width-modulation (PWM) image sensors. These time-based imaging devices transmit exposure information in the form of asynchronous spike-events (AER) via an arbitrated asynchronous data bus that is common to all pixels. Event-collisions on the bus distort the time information and lead to errors in the instantaneous illumination measurement of the concerned pixel. The effect becomes manifest when imaging uniform, homogenous (parts of) scenes. One method to balance the load on the communication channel is to avoid a global reset signal. The proposed concept spreads the reset times according to the varying local illumination in the array after a global starting point, in fact realizing an asynchronous, column-parallel, light-dependent "rolling-shutter" mode. This novel concept has been realized in the design of a spiking asynchronous, time-based QVGA image sensor. We present theoretical considerations and preliminary measurement results from the chip fabricated in a standard 0.18μm CMOS process.