Place Coding in Analog VLSI: A Neuromorphic Approach to Computation

217 Pages·1998·8.13 MB·English

by  Oliver Landolt (auth.)

PLACE CODING IN ANALOG VLSI
PLACE CODING IN
ANALOGVLSI
A Neuromorphic Approach to
Computation
by
OLIVER LANDOLT
CSEMSA
SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.
A C.I.P. Catalogue record for this book is available from the Library of Congress.
ISBN 978-1-4613-7610-1 ISBN 978-1-4615-5701-2 (eBook)
DOI 10.1007/978-1-4615-5701-2
Printed on acid-free paper
AII Rights Reserved
© 1998 Springer Science+Business Media Dordrecht
Originally published by Kluwer Academic Publishers in 1998
Softcover reprint of the hardcover 1st edition 1998
No part of the material protected by this copyright notice may be reproduced or
utilized in any form or by any means, electronic or mechanical,
including photocopying, recording or by any information storage and
retrieval system, without written permission from the copyright owner.
To Joelle
Contents
Preface ix
Foreword xi
PART I: CONCEPTS
Introduction 3
Discrete place coding 11
Continuous place coding 29
PARTTI: CIRCUITS
Fuzzy rule chip 71
Incremental oculo-motor control chip 105
Saccadic oculo-motor control chip 133
PART III: PERSPECTIVES
Extensions 173
Conclusion 197
References 205
Index 209
Preface
The central topic of this book is an uncommon way to represent numbers
in a computational system. Instead of relying on a single voltage or current
as in conventional analog circuits, or on a pattern of binary signals as in
digital circuits, numbers are encoded by the location of a small spot of
activity in a cellular array. This representation convention is called place
coding. The underlying principle is strongly related to the way information
is encoded in topological maps, which are widespread neural structures
found in natural brains. It is also related to existing technical concepts,
especially in the field of fuzzy logic, and some types of artificial neural
networks. Potential advantages of place coding for hardware design include
the possibility of implementing a wide variety of functions of possibly
several variables by application of a straightforward procedure. Another
interesting feature of place coding is that computational accuracy and
immunity to many kinds of perturbations depend on the number of cells in
the map, and can be improved to theoretically arbitrary levels by increasing
map size. This feature implies a reduction of the noise-limited absolute
minimum power consumption of analog circuits based on place-coding,
compared to conventional analog circuits.
In a theoretical part of this book, a formal definition of place coding
applicable to engineering is given. It expresses quantitatively the
relationship between an activity spot on a map and the value it represents. In
addition, a procedure is described for the synthesis of circuits creating a
specific relation between two or more maps. This procedure is justified on
the basis of the elementary mathematical concepts of sets and relations (both
classical and fuzzy). In a second part of this book, three analog integrated
circuits based on place coding are described, which demonstrate the
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x Place coding in analog VLSI
applicability of this approach in hardware design. Two of them are based on
original current-mode functional blocks exploiting recently discovered
properties of the MOS transistor. Measurement results are presented for the
three chips. A third part of the book introduces possible extensions of the
concept of place coding, and ways to apply it to difficult computational
problems.
The present book results from contributions of many people and a few
organizations which I am deeply grateful to. Above all, I wish to thank Eric
Vittoz for sharing his deep understanding and experience of semiconductor
devices and analog circuit design, and for supervising this work. Many
colleague engineers have also contributed to this work, including
Alessandro Mortara, Philippe Venier, Friedrich Heitger, Patrick Debergh,
Xavier Arreguit, Pierre Marchal, Steve Gyger, Pascal Pilloud, Eduardo
Franzi, Pierre-Fran~ois Ruedi, David Ruffieux, Peter Masa, Pascal Heim,
William Beaudot, Mark Wezelenburg and Laurent Loeffel. Special thanks
are due to Rita Studer for her friendly secretarial talents, and to Michel
Perdrix for taking chip photographs. The work described herein has been
carried out at the company CSEM SA in the frame of internal research
projects funded by Swiss government resources. The author thanks CSEM
for letting him incorporate these research results into his doctoral thesis [1],
which this book is based on.
Oliver D. Landolt
CSEM, Neuchtltel, Switzerland
Foreword
Microelectronics is a superb technology. Considering its steady
explosive growth, the number of transistors per chip can reasonably be
expected to exceed several billion in the early 2000s. This dramatic potential
for large complexity threatens all present approaches in circuit architectures,
design methodologies and design tools with obsolescence. Moreover, the
search for maximum processing speed and the generalized need for reducing
power dissipation and supply voltage tend to blur the traditional distinction
between digital and analog circuits. Built-in distributed redundancy will be
needed to produce reliable billion-transistor circuits at low cost. Solving
new tasks of perception or evaluation will require massively parallel
architectures to collectively process large sets of continuously varying data,
which can be incomplete, fuzzy or partially incoherent. To address all these
issues, designers of integrated circuits need to be creative in proposing and
exploring new approaches.
This book intends to contribute to this challenge. It introduces a new
concept, inspired from neurobiology, for representing and for processing
data. It identifies and explores the properties of this representation, and
shows that it combines the advantages of analog and digital techniques. A
formalism is proposed and a methodology is developed for the VLSI
implementation of any arbitrary non-linear function. The resulting
processing hardware is essentially a large but dense passive network of links
and cells operating in current mode, which is easily amenable to automatic
synthesis. The validity of the concept is supported by several examples of
working VLSI chips. Reading is made easy and pleasant by the intuitive
explanations which are added to the rigorous demonstrations.
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xu Place coding in analog VLSI
This book introduces new viewpoints and new ideas which might have a
real practical impact on the future of microelectronic systems. A very
refreshing technical reading which should inspire creative electronic
engineers for further exploration.
Prof Eric A. Vittoz
CSEM, Neuchatel, Switzerland
April 1998
PART I
CONCEPTS