3-Layer CNN Chip for Focal-Plane Complex Dynamics with Adaptive Image Capture

2006 10 h In e na ional Wo kshop on Cellula Neu al Ne wo ks and Thei Applica ions, Is anbul, Tu key, 28-30 Augus 2006 3-Laye CNN Chip o Focal-Plane Complex Dynamics wi h Adap i e Image Cap u e C. M. Dominguez-Ma as, R. Ca mona-Galan, F. J. Sainchez-Fe naindez, A. Rod iguez-Vazquez Ins i u o de Mic oelec 6nica de Se illa-CNM- CSIC A . Reina Me cedes s/n, 41012 Se illa, Spain e-mail: [email p o ec ed] Abs ac This pape p esen s a CMOS implemen a ion o a laye ed CNN concu en wi h 32x32 pho osenso s wi h locally p og ammable in eg a ion ime o adap i e image cap u e. The ne wo k is a anged in wo laye s con aining eedback and con ol empla es, in e -laye connec ions and p og ammable a io o ime cons an s. The e a e also eed o wa d connec ions o a hi d laye , which is as e , and de o ed exclusi ely o combining he ou pu s o he o he wo. A mo e obus and linea mul iplie block has been employed o educe i egula analog wa e p opaga ion ough o asymme ic synapses. Global and local adap a ion ci cui s a e included on-chip. The p edic ed compu ing powe pe powe consump ion, 240MOPS/mW, is amongs he la ges epo ed, wha ende s his kind o de ices as especially adequa e o po able applica ions o a i icial ision. Index Te ms Vision chips, CNN, pa allel p ocessing. I. INTRODUCTION Fo he mos o us humans, ision is he dominan senso y modali y in he acquisi ion o in o ma ion om he en i onmen . Fo his o be possible, na u e has de eloped one o he mos e icien de ices in ended o adap i e image cap u e and eal- ime image p ocessing: he e ina [1]. Meanwhile, he s uggle o b ing a i icial ision o ai ly inaccessible places con inues. This is mainly ough o he di icul ies o handle ex ao dina y amoun o da a con ained in he isual s imuli wi h he help o con en ional mic op ocesso s. E en i such da a low can be managed, i is done a he expense o conside able physical p o ile and ene gy consump ion. This conce n migh no be a p oblem in machine ision applica ions in indus ial en i onmen s. Howe e , in applica ions like obo ic ision [2], senso ne wo ks o ambien in elligence [3] o e inal p os hesis o he blind [4], powe e icien compu a ion and he use o he simples and he leas ha dwa e possible a e manda o y. He e is whe e con en ional digi al p ocesso s, wi h a se ial p ocessing scheme, ail o mee he speci ica ions. As can be seen in Fig. 1, gene al pu pose p ocesso s a e no e y ene gy e icien . DSP's and ha dwa e accele a ed p ocesso s pe o m be e , bu he eal boos in pe o mance is ob ained by he This wo k was pa ially suppo ed by p ojec s TIC2003-09817-C02-01 o he Spanish MCyT, and N-00014-02-1-0884 o he ONR. F. J. Sanchez is suppo ed by a g an o he Spanish MEC. adap a ion o he a chi ec u e o he na u e o he s imuli. This is qui e common in biological senso y o gans, ha exploi he high le el o pa allelism p esen in agg ega es o neu al cells. In o de o ealize an e icien VLSI implemen a ion o a ay p ocessing, analog and mixed-signal ci cui s ep esen a good al e na i e. The numbe o ope a ions pe second in analog chips has been calcula ed assuming peak pe o mance is du ing a con olu ion, wha ende s he o mula in [7]: (Nadd + Np od)Ncells Z'Con (Nbi s + 1) ln 2 (1) The e o e, he numbe o OPS is he a io be ween he o al numbe o addi ions and p oduc s ealized in pa allel in he chip, and he ime i akes o he chip o se le o he inal esul o he con olu ion wi hin he equi ed accu acy. Back o Fig. 1, using analog ci cui s a he elemen a y p ocessing uni s a oids A/D con e sion a he pixel le el, and, o mode a e accu acy equi emen s, hey occupy less a ea and consume less powe han hei digi al coun e pa s. In he ollowing sec ions he CACE2 ision chip, he de ails o he elemen a y p ocessing uni , and he ex ended ea u es o he chip a e explained. II. CACE2 SYSTEM DESCRIPTION The CACE2 sys em a chi ec u e is in ended o be implemen ed in single chip, cons i u ing a comple e ision Fig. 1. Compu ing powe pe mW o di e en p ocesso s. 1-4244-0640-41061$20.00 ©2006 IEEE i-/ a 0.01 sys em on a chip (VSoC). I s ope a ion will be con olled by an embedded mic op ocesso , he CACE2 MCU (Fig. 2). A his poin , his a chi ec u e has been implemen ed in 2 chips: image cap u e and ea ly ision asks a e ealized by a specialized pe iphe al, he CACE2 APAP, which is he chip epo ed he e, and an FPGA con aining he CACE2 MCU. The APAP consis s in an analog/mixed-signal pa allel a ay p ocesso o 32x32 cells. Each cell is equipped wi h p og ammable spa io- empo al dynamics, local suppo o analog and logic in-pixel a i hme ic ope a ions, local analog and logic memo ies and a pho osenso wi h ex ensions o adap i e image cap u e (Fig. 3). I ollows he a chi ec u e o he CNN-UM [9]: an analog p og ammable a ay con olled by signals common o all o hem and s o ed in i s in e nal swi ch con igu a ion egis e s (SCR's). F om he poin o iew o he ne wo k opology, each cell inco po a es wo nodes o a CNN, belonging o laye s o di e en ime cons an s, and a hi d node o combining hei ou pu s. This ne wo k suppo s complex dynamic phenomena exp essed by a se o coupled eac ion-di usion equa ions [10]. In his i s p o o ype o he sys em, he CACE2 MCU has been implemen ed in a FPGA, oge he wi h he necessa y pe iphe als o boo ing up he sys em, p og am and da a s o age and communica ion. The SCR's o he APAP a e alloca ed wi hin he add ess space o he MCU. The p og ammabili y and con ol o he ne wo k dynamics, he calib a ion and biasing o he analog and mixed-signal building blocks, he adap i e image cap u e mechanisms, a e con olled by he MCU ia he signals s o ed in he SCR's. Access o hem is di ec ed by he add ess bus (A-bus). The access mode, ei he eading o w i ing, is indica ed by he con ol bus (C-bus). The da a bus (D-bus) is employed o sending o ecei ing pa ame e s. Each SCR mus be p ope ly upda ed o manage he in a- and in e -cell connec i i y o he p ocessing elemen s, he sequences o signals ha con ol he a ay ope a ion and he codes o he in e nally gene a ed analog e e ences employed o mixed-signal ci cui s ope a ion. The a ay p ocesso coun s also wi h a seconda y da a bus (8-bi s wide) employed o image I/0. This bus is di ec ly connec ed o he sys em memo y ia an access con olle (DMA) which is also ac i a ed by he MCU when equi ed by he so wa e p og am. III. CNN PROCESSING UNIT The ype o signal p ocessing ealized by he CACE2 APAP is based on he dynamic e olu ion o a 3x32x32 CNN. This beha io is desc ibed in e ms o he inpu (Uk), s a e (Xk) and ou pu (Yk) a iables. Each laye , k, o he a ay ollows he e olu ion law exp essed by: xk() )]+I[Ak, Oy +Bk, Ou,]+z d n k (2) The symbol 0 s ands o he linea con olu ion be ween he eedback and eed o wa d empla es, wi h he ou pu and inpu ma ices o laye , n, whe e n can be 1, 2 o 3: [Ak, (gyn](i,j) = E A (l,m)y,(i+ 1, j+m) 1=- m=- [Bk, (J u, ](i j) E YB, (1, m)u (i + 1 j + m) 1=- m=- (3) whe e is he neighbou hood adius. In his pa icula implemen a ion, c1l and 'C2 a e compa able while 'C3 is much smalle han he o he s. I he ull-signal- ange CNN model is employed [11], he ou pu and s a e a iables can be iden i ied. In his condi ions, each ma ix elemen in Eq. (3) is Fig. 2. Func ional diag am o he CACE2 sys em om he con ol uni o/ om he a ay 1/C ~~~~1i ~ ~ ~ ~ L Laye -2 F l;j Fig. 3. Concep ual diag am o he basic cell. ob ained om he mul iplica ion o he s a e (o inpu ) a iable by a p og ammable weigh . These ope a o s, esponsible o mul iplying he s a e (o inpu ) a iable by a p og ammable weigh , a e e med synapses o synap ic blocks in his con ex . They a e basically ou quad an s mul iplie s in which linea i y wi h he s a e (o inpu ) a iable and a symme ic cha ac e is ic a e s ongly desi ed. The e ec o a ying he p og ammed weigh s is o modi y he ne wo k dynamics, and hus, changing he ype o p ocessing ealized by he a ay. Con inuing wi h he e olu ion law, he e is a losses e m: mO [xkQ(ix) -1] + m i Xk (i,j)>1l g[xkQ(, j)] =lim m6xk (i, j) i Xk (i,Ij) < 1 (4) mO [xk (ix,j) + 1] -m4 i XkA(i,j)< and he ac i a ion unc ion, o gene a e he ou pu : YkQ,I ) [xkQ,i )] = 'i1 (Sj +mX i )m] (5) 2~ ~ ~~~klI m]}C In bo h equa ions, mL can be 0 o 1 o ha d o sigmoidal ype nonlinea i y, espec i ely. The physical ealiza ion o he elemen a y p ocessing uni o he CNN s a s wi h he selec ion o he app op ia e o ma o he ep esen a ion o he signals. On one side, ol ages can be easily deli e ed o neighbou ing a eas by connec ing wi es o high-impedance nodes. The e o e, inpu , ou pu and s a e a iables a e chosen o be ep esn d by he ma ices o ol ages Vu, Vh and Ve, espec i ely. On he o he side, signal addi ion can be easily ealized in he o m o cu en s wi ed oge he o a i ual g ound. Hence, he summands in he second membe o Eq. (2) should be ep esen ed by cu en s. And hen, his sum o cu en s will be in eg a ed in he s a e capaci o o ob ain he ins an aneous alue o he s a e a iable ol age: dV,k ( ) Ck d ,g[Vk( )]±Z[GA4kn®(DVyn, ±GB,kn(®VU l±I +1Z d n~~~~~~~~~~~~~~~~~~ (6) As can be seen, he elemen s o he eedback and eed o wa d empla es, Ak4i,j) and Bk4ij), a e now p og ammable linea ansconduc ances, GA,k(i,j) and GB,kn(i,j), ha mul iplied by inpu and ou pu ol ages ende he neighbou hood con ibu ions in he o m o cu en s. Thus, he synap ic block is a ansconduc o whose ou pu cu en is p opo ional, in he ideal case, o he p oduc o he s a e (o inpu ) a iable and he weigh . The double ans o ma ion implici in Eq. (6), V-I and hen I-V, allows o a compac ealiza ion o he p ocessing node, achie ing highe cell densi ies, meaning an a ay size o p ac ical in e es and, besides, a ole able ill ac o . The accu acy o hese e ms is e y impo an o accomplish a co ec ope a ion o he ne wo k, since he synapse o se s, as well as e e y misma ch on ideally symme ic weigh s, a e in eg a ed in he s a e capaci o . P ecisely, in he implemen a ion o ou -quad an mul iplie s, one o he common di icul ies is o main ain he symme y wi h espec o he o igin o he weigh s. A misma ch in weigh s ha ing he same absolu e alue bu opposi e signs can modi y he dynamic ou es o he cells in he ne wo k, ending in displaced equilib ium poin s, and hus, dis o ing he p esc ibed p ocessing. The main linea i y conce ns a e ound in he V-I con e sion, as linea cu en in eg a ion, and hus I-V ans o ma ion, can be p o ided by a ailable highly linea double-poly capaci o s. In his design, we ha e employed a linea ized OTA in o de o gene a e he uni a y cu en con ibu ion. Though he elemen a y ansconduc o achie ing V-I con e sion has a la ge numbe o ansis o s han he single- ansis o synapse in [12], ad an ages in he linea i y wi h he s a e (o inpu ) a iable and symme y o he V-I cha ac e is ic jus i y i s use. In addi ion, he suppo ing ci cui y can be simpli ied esul ing in a mo e obus implemen a ion inally wi hou any a ea penal y. The schema ics in Fig. 4 ep esen he co e o he elemen a y dynamic p ocesso . Ope a ing in closed loop (when he swi ch con olled by 'Loop' is on), i implemen s he e olu ion law desc ibed by Eq. (6). The weigh ed V-I con e sion o he s a e ol age is ca ied a se e al s ages. The single- o-di e en ial V- o-I con e sion is ealized by a linea ized ansconduc o (le -side o he schema ics), hese cu en signals a e eplica ed and scaled by se e al p og ammable cu en mi o s o gene a e he con ibu ions owa ds he neighbo s and i sel (a he cen e and igh sides) and he cu en signals om he neighbo s and sel - eedback a e added and in eg a ed in he s a e capaci o , when eedback loop is closed (by he block a he cen e ). The ansconduc o esponsible o ans o ming he s a e capaci o ol age V, in o a di e en ial cu en is a sou ce degene a ed di e en ial pai wi h diode-connec ed loads. I is based on a linea ized OTA [13]. The ope a ion o his ci cui Fig. 4. Schema ic o he linea ized OTA and synap ic blocks alone is inhe en ly symme ic i wo king in ully-di e en ial mode, ep esen ing an enhancemen om wha ha e been achie ed by p e ious implemen a ions. This symme y hough is b oken by using a single-ended inpu ol age, bu s ill he esul ing V-I cha ac e is ic main ains symme y le els beyond hose o o he implemen a ions. The bene i s o a di e en ial ep esen a ion we e no signi ican o be wo h handling wi h double capaci o a ea and a complex signal ou ing. The implemen a ion o he weigh s is based on geome ical ela ions be ween ansis o s. This has he ad an age o being less in luenced by p ocess pa ame e a ia ions bo h in e - and in a-die. I has also he d awback o only pe mi ing he use o a disc e e se o weigh alues, namely -4, -2, -1, 0, 1, 2 and 4. Opposi e-sign con ibu ions a e ob ained by c ossing he wi es con eying he cu en s o he collec ing nodes, hus, achie ing by a chi ec u e a symme ic ope a ion. Finally, he sum o all he cu en s coming om he neighbo hood is injec ed in o he a ge s a e capaci o . Bu 4lo16A 4 ,YW=-4., 2 W=2 IWI0 2 W '. WI;,.,- -2 W=2 W44 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 V S a e a iable ol age (Vin-Ve ) Fig. 5. Ou pu cu en s. s a e ol age o he OTA-based synapse. be o e ha , di e en ial o single-ended cu en con e sion is ealized wi h he help o a cu en mi o . I is impo an o men ion ha he achie able ou pu esis ances using sel - biased o ex e nally biased Cascode cu en mi o s a e no su icien o ensu e he necessa y independence om he ou pu ol age. In o he wo ds, he esul ing e o in he copied cu en , because o he ini e ou pu esis ance o he mi o , was beyond he p edic ed e o s due o pa ame e s misma ch. The e o e, gain boos ing o he Cascode de ices is needed o educe his e ec . The accu acy o he cu en eplica ion in his mi o is c ucial o achie ing he equi ed linea i y and symme y in he V-I cha ac e is ic. Also, we ha e employed 0.5/2.0 ansis o s o he cu en mi o s, ensu ing enough ma ching. As a esul , he ou pu cu en has a high linea i y. The ansconduc ance ela i e e o in la ge signal is kep below 0.7%. Conce ning he symme y o he cha ac e is ic, he di e ence o he ou pu cu en s co esponding o weigh s wi h he same absolu e alue bu opposi e sign is ze o on a e age because o se cancella ion, he s anda d de ia ion, ob ained by Mon e Ca lo simula ion, being 2% o he absolu e alue o he indi idual cu en s. Compa ed o p e ious implemen a ions, in e ms o linea i y o he V-I cha ac e is ic o he mul iplie s, his ci cui pe o ms one o de o magni ude be e o compa able a ea and powe consump ion. This is no always equi ed o he co ec ope a ion, i. e. con e gence o he ne wo k dynamics o he co ec equilib ium poin s, bu decisi e o linea di usion. Func ional ope a ion o he comple e cell, including he local memo ies and he swi ching ee ha allows communica ion wi h he ou side o he a ay, has been e i ied by simula ion. A small ne wo k composed o 3x3 cells ac ually a 5x5 ne wo k i he bounda y cells a e conside ed has been p og ammed o implemen di e en image p ocessing empla es. Fig. 6(a) displays he s a e a iables o he cells o he 3x3 ne wo k when p og ammed o ealize connec ed componen de ec ion in he ho izon al di ec ion. F om he simula ions, i can be seen ha he ime cons an o he cells is designed o be unde I OOns. IV. EXTENDED CHIP FEATURES A. T ue esis i e g id One o he mos use ul ools o ocal-plane image p ocessing is he di usi e p opaga ion o he pixel alues. This is achie ed by a Gaussian lowpass il e , which o a disc e ized image g id, can be ealized by he con olu ion o he o iginal image wi h a spa ial mask. The e olu ion law implemen ed a e e y node o he CNN can suppo his di usi e dynamics by app op ia ely se ing he co ec in e connec ion weigh s in he eedback empla e. The main d awback o implemen ing his ope a o and o he s using symme ic weigh s, in a VLSI s uc u e designed o ully- p og ammable CNN dynamics is misma ch in gene a ing cu en con ibu ions. Because o he local compu a ion o he con ibu ions o he neighbo hood, he amoun o cu en being injec ed om cell C(i,j) in o neighbo ing cell C(i+lj), o ins ance, does no ma ch in absolu e alue he cu en being injec ed om cell C(i+lj) back in o he s a e capaci o o cell C(ij). The consequence o his is easy o de i e, he supposedly symme ic di usion is con e ed in o an un uled p opaga ion o he pixel alues. Special ca e has been pu in coun e ac ing he e ec s o misma ch by design. This is, esizing he ansis o s in o de o a oid excessi e de ia ion om he nominal in he gene a ion o he uni a y cu en con ibu ions. Apa om his, he p o o ype chip includes a ue esis i e g id concu en wi h he CNN a ay. Each cell con ains wo esis o s, made o high- esis i i y poly-Si, ha can be lhime (§) (a) connec ed o he s a e capaci o in o de o o m a ec angula esis i e g id. The ime cons an o his g id is be ween 0.2- 1.0 s, and i is no co ela ed o he CNN ime cons an , nei he can be con olled by he use . The ope a ion o he esis i e ne wo k is illus a ed in Fig. 6(b). He e he 3x3 a ay o cells is p og ammed o e ol e wi h null empla es, using only he g id o poly esis o s. In less han 2j s, he s a e ol ages o all cells con e ge o he global a e age. This is no as , and i becomes wo se when he size o he ne wo k inc eases, bu i is con enien om he poin o iew o he con ol o he algo i hm o coun wi h a di usion mechanism ha uns slowe han he swi ch con igu a ion upda ing signals. B. Adap i e image cap u e Adap i e image cap u e in he CACE2 APAP is based in he local and global con ol o he pho osenso s' gain. Ope a ing in pho ocu en in eg a ion mode, he ol age ep esen ing he alue o he pixel depends on he in eg a ion ime, i. e. o he same powe o he inciden ligh o e he senso su ace, a la ge in eg a ion ime will allow he same pho ogene a ed cu en o discha ge he sensing capaci ance o a longe ime, esul ing in a la ge ol age excu sion o m he ese alue. In his chip, each pixel has a ese ansis o go e ned ei he by a global signal au oma ic adap a ion o he in eg a ion ime is o o by a compa a o d i en by a local ese con ol ol age and a global ime-e ol ing e e ence. The local suppo o his compa ison is explained in [14]. I s main unc ion is o adap he local gain o he pho osenso . As hey a e in eg a ing senso s, his gain adjus men is achie ed ia he adap a ion o he local in eg a ion ime acco ding o a locally de i ed ol age le el. In o de o do ha , he global e e ence will be an in e se ol age amp ha is deli e ed o e e y senso in he a ay. When he in e se amp c osses wi h nega i e slope he lime (s) (b) Fig. 6. Simula ion o he e olu ion o a educed (3x3) a ay wi h local memo y upda e, o se cancella ion and ese p ocesses. local h eshold, he in eg a ion o he pho ocu en s a s. In his way, he da ke he pixel hen he la ge in eg a ion ime ha will be alloca ed o ha pixel o he nex cap u e he algo i hm elies in he co ela ion be ween he alues o he same pixel in di e en ames in a sequence. Co espondingly, he b igh e he pixel, he less ime i will ha e in he nex cap u e. Conce ning he global adap a ion mechanism, he in e se amp is cen e ed in a p edic ed a e age in eg a ion ime alue. The e o e, i he p e ious image cap u e esul ed in an o e -exposed pic u e, he a e age ol age will be below he middle poin o he pixels' ol age ange. I he p e ious image is unde -exposed, he a e age ol age will be abo e his poin . The algo i hm p og ammed in o he chip co ec s he ime ex en o he amp acco dingly in o de o ha e smalle exposu es o b igh ness sa u a ed images and la ge exposu es o ex emely da k pic u es. This is achie ed by compa ing he a e age ol age o he pixels wi h uppe and lowe h esholds. I he esul ing a e age alls be ween hese h esholds, he only co ec ions in oduced a e due o local adap a ion. I he ol age alls abo e/below he uppe /lowe limi , a digi al ci cui igge ed by hese compa a o s, co ec s he equency di ision ealized on o he sys ems mas e clock, employed o gene a e he in e se amp, in he p ope sense. This ends in a wide /na owe amp shape un il he a e age pixel ol age alls be ween he wo h esholds. V. CHIP DATA AND CONCLUSIONS The p o o ype chip has been designed and ab ica ed in a CMOS 0.35p m. The die size is 7.6mm x 7.6mm. Fig. 7 displays a mic opho og aph o he chip. Table I shows a su ey o chip da a. These ea u es a e p edic ed om he simula ion esul s. The chip in now unde es , in o de o con i m he expec ed pe o mance. REFERENCES [1] D. H. Hubel, Eye, B ain and Vision. Scien i ic Ame ican Lib a y, No. 22. W. H. F eeman and Co., New Yo k, 1995. [2] T. Makimo o, T. T. 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