Juan Antonio Leñero-Bardallo

A 3.6

This paper presents a 128 × 128 dynamic vision sensor. Each pixel detects temporal changes in the local illumination. A minimum illumination temporal contrast of 10% can be detected. A compact preamplification stage has been introduced that allows to improve the minimum detectable contrast over previous designs, while at the same time reducing the pixel area by 1/3. The pixel responds to illumination changes in less than 3.6 μs. The ability of the sensor to capture very fast moving objects, rotating at 10 K revolutions per second, has been verified experimentally. A frame-based sensor capable to achieve this, would require at least 100 K frames per second.

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Address Event Representation (AER) is an emergent technology for assembling modular multiblock bio-inspired sensory and processing systems. Visual sensors (retinae) are among the first AER modules to be reported since the introduction of the technology. Spatial-contrast AER retinae are of special interest since they provide highly compressed data flow without reducing the relevant information required for performing recognition. The reported AER contrast retinae perform a contrast computation based on the ratio between a pixels local light intensity and a spatially weighted average of its neighborhood. This resulted in compact circuits but with the penalty of all pixels generating output signals even if they sensed no contrast. In this paper, we present a spatial-contrast retina with a signed output: Contrast is computed as the relative difference (not the ratio) between a pixels local light and its surrounding spatial average and normalized with respect to ambient light. As a result, contrast is ambient light independent, includes a sign, and the output will be zero if there is no contrast. Furthermore, an adjustable thresholding mechanism has been included, such that pixels remain silent until they sense an absolute contrast above the adjustable threshold. The pixel contrast-computation circuit is based on Boahens biharmonic operator contrast circuit, which has been improved to include mismatch calibration and adaptive-current-based biasing. As a result, the contrast-computation circuit shows much less mismatch, is almost insensitive to ambient light illumination, and biasing is much less critical than in the original voltage biasing scheme. The retina includes an optional global reset mechanism for operation in ambient-light-independent Time-to-First-Spike contrast-computation mode. A 32 32 pixel test prototype has been fabricated in 0.35-m CMOS. Experimental results are provided.

s Latency Asynchronous Frame-Free Event-Driven Dynamic-Vision-Sensor

This article investigates the potential of the first ever prototype of a vision sensor that combines tricolor stacked photo diodes with the bio-inspired asynchronous pixel event communication protocol known as Address Event Representation (AER). The stacked photo diodes are implemented in a 22 × 22 pixel array in a standard STM 90 nm CMOS process. Dynamic range is larger than 60 dB and pixels fill factor is 28%. The pixels employ either simple pulse frequency modulation (PFM) or a Time-to-First-Spike (TFS) mode. A heuristic linear combination of the chips inherent pseudo colors serves to approximate RGB color representation. Furthermore, the sensor outputs can be processed to represent the radiation in the near infrared (NIR) band without employing external filters, and to color-encode direction of motion due to an asymmetry in the update rates of the different diode layers.

A Five-Decade Dynamic-Range Ambient-Light-Independent Calibrated Signed-Spatial-Contrast AER Retina With 0.1-ms Latency and Optional Time-to-First-Spike Mode

Low current applications, like neuromorphic circuits, where operating currents can be as low as a few nanoamperes or less, suffer from huge transistor mismatches, resulting in around or less than 1-bit precisions. A neuromorphic programmable-kernel 2-D convolution chip has been reported where each pixel included two compact calibrated digital-to-analog converters (DACs) of 5-bit resolution, for currents down to picoamperes. Those DACs were based on MOS ladder structures, which although compact require 3N + 1 unit transistors (N is the number of calibration bits). Here, we present a new calibration approach not based on ladders, but on individually calibratable current sources made with MOS transistors of digitally adjustable length, which require only N-sized transistors. The scheme includes a translinear circuit-based tuning scheme, which allows us to expand the operating range of the calibrated circuits with graceful precision degradation, over four decades of operating currents. Experimental results are provided for 5-bit resolution DACs operating at 20 nA using two different translinear tuning schemes. Maximum measured precision is 5.05 and 7.15 b, respectively, for the two DAC schemes.

Bio-Inspired Asynchronous Pixel Event Tricolor Vision Sensor

This paper presents a novel event-based vision sensor with two operation modes: intensity mode and spatial contrast detection. They can be combined with two different readout approaches: pulse density modulation and time-to-first spike. The sensor is conceived to be a node of an smart camera network made up of several independent an autonomous nodes that send information to a central one. The user can toggle the operation and the readout modes with two control bits. The sensor has low latency (below 1 ms under average illumination conditions), low power consumption (19 mA), and reduced data flow, when detecting spatial contrast. A new approach to compute the spatial contrast based on inter-pixel event communication less prone to mismatch effects than diffusive networks is proposed. The sensor was fabricated in the standard AMS4M2P 0.35-um process. A detailed system-level description and experimental results are provided.

A Calibration Technique for Very Low Current and Compact Tunable Neuromorphic Cells: Application to 5-bit 20-nA DACs

This paper investigates the potential of an image sensor that combines event-based asynchronous outputs with conventional integration of photocurrents. Pixels voltages can be read out following a traditional approach with a source follower and analog-to-digital converter. Furthermore, pixels have circuitry to implement Pulse Density Modulation (PDM) sending out pulses with a frequency that is proportional to the photocurrent. Both read-out approaches operate simultaneously. Their information is combined to render high dynamic range images. In this paper, we explain the new vision sensor concept and we develop a theoretical analysis of the expected performance in standard AMS 0.18μm HV technology. Moreover, we provide a description of the vision sensor architecture and its main blocks.

A Bio-Inspired Vision Sensor With Dual Operation and Readout Modes

A new bio-inspired pixel concept is proposed. It can compute the spatial contrast and provide intensity images. The pixel sends spikes to communicate with its neighbors, computing the spatial contrast. Its expected fixed-pattern noise within neuromorphic arrays is 1% without using calibration. Its fill factor is 8.5%. Furthermore, the pixel has two different readout modes: pulse density modulation and time to first spike. The user can toggle any time between operation and readout modes, depending on the desired image quality and the bandwidth and power consumption requirements. The pixel has been implemented in AMS4M2P 0.35-μm CMOS technology. Experimental results are provided.

A high dynamic range image sensor with linear response based on asynchronous event detection

This article investigates the potential of a bio-inspired vision sensor with pixels that detect transients between three primary colors. The in-pixel color processing is inspired by the retinal color opponency that are found in mammalian retinas. Color transitions in a pixel are represented by voltage spikes, which are akin to a neurons action potential. These spikes are conveyed off-chip by the Address Event Representation (AER) protocol. To achieve sensitivity to three different color spectra within the visual spectrum, each pixel has three stacked photodiodes at different depths in the silicon substrate. The sensor has been fabricated in the standard TSMC 90 nm CMOS technology. A post-processing method to decode events into color transitions has been proposed and implemented as a custom interface to display real-time color changes in the visual scene. Experimental results are provided. Color transitions can be detected at high speed (up to 2.7 kHz). The sensor has a dynamic range of 58 dB and a power consumption of 22.5 mW. This type of sensor can be of use in industrial, robotics, automotive and other applications where essential information is contained in transient emissions shifts within the visual spectrum.

A Dual-Operation-Mode Bio-Inspired Pixel

Reported AER (Address Event Representation) contrast retinae perform a contrast computation based on the ratio between a pixels local light intensity and a spatially weighted average of its neighbourhood. This results in compact circuits, but with the penalty of all pixels generating output signals even if they sense no contrast. In this paper we present a spatial contrast retina with bipolar output: contrast is computed as the relative normalized difference (not the ratio) between a pixels local light and its weighted spatial average, normalized to average light. As a result, contrast includes a sign, is ambient light independent, and the output will be zero if there is no contrast. Furthermore, an adjustable thresholding mechanism has been included, such that pixels remain silent until they sense an absolute contrast above the adjustable threshold. The pixel contrast computation circuit is based on Boahens Biharmonic operator contrast circuit, which has been improved to include mismatch calibration and adaptive current based biasing. As a result, the contrast computation circuit shows much less mismatch, is almost insensitive to ambient light illumination, and biasing is much less critical than in the original voltage biasing scheme. The retina also includes an optional TFS (Time-to-First-Spike) integration mode. A full AER retina version has been fabricated and tested. In the present paper we provide preliminary experimental results.

A Bio-Inspired AER Temporal Tri-Color Differentiator Pixel Array

We demonstrate the first array of asynchronous event pixels that react to temporal color contrast of three different color spectra. The three different spectra are transduced into photo currents by stacked photo diodes. Temporal changes of the contrast of these three spectra are quantified as pulse density modulated signals and conveyed off-chip by the Address Event Representation (AER) protocol. The 16 × 16 pixel array has been fabricated in the standard TSMC 90nm CMOS process.