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Published byBeverly Hodge Modified over 9 years ago
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Analog VLSI Neural Circuits CS599 – computational architectures in biological vision
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Charge-Coupled Devices Uniform array of sensors Very little on-board processing Very inexpensive
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CMOS devices More onboard processing Even cheaper! Example: ICM532B from www.ic- media.com: single-chip solution includes photoreceptor array, various gain control and color adjustment mechanisms, image compression and USB interface. Just add a lens and provide power!www.ic- media.com
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The challenge Digital processing is power hungry Analog processing is much more energy efficient But … so much variability in the gain of transistors obtained when fabricating highly integrated (VLSI) chips that analog computations seem impossible: nearly each analog amplifier on the chip should be associated with control pins, analog memories, etc to correct for fabrication variability. Hopeless situation?
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A VLSI MOS transistor
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An analog chip layout: the wish
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An actual chip: the cold reality
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Biological motivation Well, there is also a lot of variability in size and shape of neurons from a same class But the brain still manages to produce somewhat accurate computations What’s the trick? online adaptability to counteract morphological and electrical mismatches among elementary components.
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Remember? Electron Micrograph of a Real Neuron
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Mahowald & Mead’s Silicon Retina Smoothing network: allows system to adapt to various light levels.
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Andreou and Boahen's silicon retina See http://www.iee.et.tu-dresden.de/iee/eb/ analog/papers/mirror/visionchips/vision_chips/ andreou_retina.html
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Diffusive network dQn/dt is the current supplied by the network to node n, and D is the diffusion constant of the network, which depends on the transistor parameters, and the voltage Vc.
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Full network Two layers of the diffusive network: upper corresponds to horizontal cells in retina and lower to cones. Horizontal N- channel transistors model chemical synapses. The function of the network can be approximated by the biharmonic equation where g and h are proportional to the diffusivity of the upper and lower smoothing layers, respectively.
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Full network
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VLSI sensor with retinal organization
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Carver Mead: the floating gate www.cs.washington.edu/homes/hsud/fg_workshop.html
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Spatial layout
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Electron tunneling
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Hot electron injection
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Spatial layout
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A learning synapse circuit
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