The Design Of Pixel Circuits In Time-based AER Asynchronous Vision Sensor

As the CMOS (Complementary Metal Oxide Semiconductor) Image Sensor (CIS)progresses toward high-speed, high-resolution and high-dynamic-range, it is widelyused in a variety of image captures. The traditional CIS with frame-based architecturehas the advantage of small area, high fill factor and mature technology. However, it alsohas some inherent defects, such as redundant information, low time accuracy, lowdynamic range, which limited the applications. Based on the study of biological vision,the concept of silicon retina is proposed, which is a biological visual system achievedby the VLSI (Very Large Scale Integration). The time-based AER (Address EventRepresentation) asynchronous CMOS image sensor is a realization method of bio-inspired vison sensor, which has the characteristics of high dynamic range, low dataredundancy and high time accuracy.First, the basic structure and working principle of change detection unit andexposure measurement unit are studied, and the behavioral level model is establishedand simulated. Second, based on the study of the source of time error in asynchronouspixel structure, a complete mathematical model is established, and several main factorswhich will affect the time error are analyzed, result in a new method of time errorcorrection is proposed. Third, in time-based Pulse Width Modulation (PWM) pixel withthe Correlated Double Sampling (CDS), a time error mathematical model is establishedfor the purpose of studying the influence of nonlinear response and noises on imaging.By analyzing the noises, which include fixed pattern noise (FPN), reset noise, shot noise,et al, the influence of the reference voltage on imaging the errors introduced bynonlinearity and certain noises are deduced. Finally, the change detection circuit,exposure measurement circuit, correction circuit, arbitration circuit and read out circuitis designed, simulated and verified. In addition, an8×8pixel array is simulated andverified.The pixel array which we designed in the paper can realize the change detectonand exposure measurement. The simulation results of Matlab show that, in theasynchronous CIS with time error correction, when the illumination is103~105lx, theimaging error introduced by time error is less than0.31%, which would be0.4%~28%without time error correction. So the proposed structure can reduce the impacts of the time error on the image accuracy, especially in the condition of high light intensitytogether with large rate of collision events. The circuit simulation results of an8×8pixelarray show that, the minumim detectable relative change of the change detect circuit is12%, and the exposure measurement circuit can realize the light measurement withCDS PWM. It proves that the method proposed can perform real-time optical signalcapturing, processing and readout; as well as be applied to the flied of high-speed andhigh dynaic range image and vision.