Text of Analog VLSI Neural Circuits CS599 – computational architectures in biological vision
Analog VLSI Neural Circuits CS599 computational architectures in biological vision
Charge-Coupled Devices Uniform array of sensors Very little on-board processing Very inexpensive
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
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?
A VLSI MOS transistor
An analog chip layout: the wish
An actual chip: the cold reality
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 Whats the trick? online adaptability to counteract morphological and electrical mismatches among elementary components.
Remember? Electron Micrograph of a Real Neuron
Mahowald & Meads Silicon Retina Smoothing network: allows system to adapt to various light levels.
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
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.
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.
VLSI sensor with retinal organization
Carver Mead: the floating gate www.cs.washington.edu/homes/hsud/fg_workshop.html