IEEE
JOURNAL
OF
SOLID-STATE
CIRCUITS.
VOL. 29,
NO.
8,
AUGUST
1994
895
Sma -Pixel Cellula Neu al Ne wo ks
in
Analog Cu en -Mode CMOS Technology
S.
Espejo,
A.
Roddguez-Vazquez,
Membe ,
ZEEE,
R. Domhguez-Cas o,
J.
L.
Hue as,
and
E.
Shnchez-Sinencio
Abs ac -This pape p esen s a sys ema ic app oach o design
CMOS chips wi h concu en pic u e acquisi ion and p ocess-
ing capabili ies. These chips consis o egula a angemen s o
elemen a y
uni s,
called
sma pixels.
Ligh de ec ion is made
wi h e ical CMOS-BJT’s connec ed in a Da ling on s uc u e.
Pixel sma ness is achie ed by exploi ing he Cellula Neu al
Ne wo k pa adigm [l], [2], inco po a ing a each pixel loca ion
an analog compu ing cell which in e ac s wi h hose o nea by
pixels. We p opose a cu en -mode implemen a ion echnique
and gi e measu emen s om wo
16
x
16
p o o ypes in a single-
poly double-me al CMOS n-well 1.6-pm echnology. In addi ion
o he senso y and p ocessing ci cui y, bo h chips inco po a e
ligh -adap a ion
ci cui y o au oma ic con as adjus men . They
ob ain sma -pixel densi ies up o
89
uni s/mm2, wi h a powe
consump ion down o
105
pW/uni and image p ocessing imes
below
2
ps.
I.
INTRODUCTION
OMMON a chi ec u es o image-p ocessing sys ems use
C
a
on -end senso y plane wi h digi al-encoding o he
pixel alues, and se ial ansmission o hese digi al da a o
subsequen p ocessing using ei he ASIC’s o gene al-pu pose
compu e s. Con a y o his app oach,
sma -pixel
chips [3]
inco po a e an analog compu ing cell a each senso y poin ,
achie ing high speed and low a ea occupa ion in he combined
senso ylp ocessing unc ions by ully exploi ing
pa allelism.
The combined spa ial dis ibu ion o senso y and p ocessing
ci cui y elimina es he ime equi ed o da a ansmission
om he senso y o he p ocessing plane du ing he image
acquisi ion p ocess. In addi ion, in some image-p ocessing
applica ions, he ele an in o ma ion con ained in he ou pu
image can be desc ibed by
a
educed numbe o a iables,
allowing
a
as downloading o he esul s o subsequen
e alua ion.
CMOS echnologies o e unique ea u es o he design
o sma -pixel chips. On one hand, MOS ansis o ope a ion
unde no mal biasing in s ong in e sion is no d as ically
a ec ed by inciden ligh ; on he o he , pho osensi i e CMOS
de ices can be buil by exploi ing he many junc ion de ices
a ailable in CMOS echnologies [4]. Howe e , p e ious ap-
p oaches o CMOS design o sma -pixel chips lack gene ali y,
as
hey ely on implemen a ion me hods sui able o speci ic
Manusc ip ecei ed Decembe 1993; e ised Ap il
6,
1994.
S.
Espejo,
A.
Rod iguez-VBzquez, R. Domlnguez-Cas o, and
J.
L.
Hue as
a e wi h he Cen o Nacional de
Mic oelec 6nica-Uni e sidad
de Se illa,
Edi icio CICA, Cma ia
sn,
41012-Se illa, Spain.
E. Sinchez-Sinencio is wi h he Depa men
o
Elec ical Enginee ing,
Texas A&M Uni e si . College S a ion. TX
77843
USA.
applica ions. In some cases, he p ocessing- ask pe o med a
each pixel does no imply collec i e compu a ion [3], while
mos o he app oaches o “pixel-sma ness’’ a e based on
ac i e implemen a ions o esis i e-g id ne wo ks [5], [6].
The pa adigm o Cellula Neu al Ne wo ks (CNN) [l], [2]
is
a
e y sui able amewo k o
sys ema ic
design o pa allel
senso y-p ocessing chips. On one hand, CNN’s consis o
egula a angemen s o
cells-
- opologically iden ical o sma -
pixel chips. On he o he , hei cells a e only locally connec ed,
and hus, equi e simple ou ing. Also, he as body o li e a-
u e on CNN heo y and applica ions demons a es ou s anding
ea u es o his pa adigm o a ay-p ocessing [7]. In pa icula ,
esis i e g ids ha e ecen ly been demons a ed
as
a
pa icula
CNN class [8].
No expe imen al sma -pixel CNN chips ha e been epo ed
o da e. This pape ou lines
a
design app oach using Da ling on
pho o ansis o s and cu en -mode p ocessing ci cui y. I is
based on
a
modi ied e sion
o
he o iginal CNN model which
enables op imum speedlpowe and a ea occupa ion in VLSI
design
[9],
[IO]. The senso s include an au oma ic adjus men
ci cui y which ensu es p ope beha io unde di e en illu-
mina ion condi ions. Ou p oposals a e demons a ed ia wo
wo king sma -pixel chips, in
a
single-poly, 1.6-pm, n-well
CMOS echnology. In addi ion o hei op ical inpu , hese
chips exhibi much be e a ea and speed/powe igu es han
p e ious CNN implemen a ions
[
111,
[
121.
Sec ion I1 desc ibes some gene al aspec s o sma -pixel
chips, and Sec ion I11 ou lines he p oposed compu a ion algo-
i hm. Sec ions IV and V discuss he senso y and p ocessing
ci cui y, espec i ely, and he expe imen al p o o ypes a e
desc ibed in Sec ion VI.
11.
SMART-PIXEL
CHIPS
In his pape ,
pixel
deno es he elemen a y senso y uni
used o de ec poin wise ligh signals. These senso y uni s a e
ealized in CMOS echnology using any compa ible junc ion
de ice o gene a e
a
cu en whose alue is an inc easing
unc ion o he ligh in ensi y [3],
[
131,
[
141. The acquisi ion o
wo-dimensional
scenes equi es pixels a anged on o egula
g ids,
as
shown in Fig. 1. Each pixel in his senso y plane
gene a es
a
cu en
I,
which codi ies
a
co esponding poin o
he inpu image, whe e he index
c
(i.,j) indica es he pixel
a he i h ow and j h column on he g id and a ies o e he
whole g id domain
GD(c
E
GD).
Thus, he whole image is
IEEE
Log
Numbe 4402 142;
cap u ed in o
a
ma ix o cu en s
[Ic].
0018-9200/94$04.00
0
1994 IEEE
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896 IEEE JOURNAL
OF
SOLID-STATE CIRCUITS, VOL. 29.
NO.
8, AUGUST 1994
om o he
cells
J
Fig.
1.
Illus a ing he co e a chi ec u e
o
sma -pixel chips.
Fig. 1 illus a es he a chi ec u e o sma -pixel chips: each
uni (also called
sma -pixel
o
cell
)
senses
a
poin o he inpu
image and
in e ac s
wi h he o he uni s in he a angemen o
pe o m pa allel-p ocessing asks on he inpu cu en ma ix
Sma -pixel chips a e o s ong p ac ical in e es o pa e n
[IC].
ecogni ion p oblems, o de ec ea u es o he inpu image. Fo
example, Fig.
2
illus a es he ask o de ec ion o connec ed
componen s (DCC), which consis s o coun ing he numbe
o
Fig.
2.
Connec ed componen de ec ion in ou di e en di ec ions.
-
connec ed pieces encoun e ed by scanning an inpu image in
a
gi en di ec ion [15]. Pa e n ecogni ion can be ealized by
p ocessing he da a ob ained a e pe o ming his ask in he
di ec ions shown in Fig.
2
[16], [17]. This da a is con ained in
a ew ows and columns a he g id bo de s. In addi ion o hei
usage o p ep ocessing asks, sma -pixel chips a e also use ul
as s and-alone uni s o nonin ensi e compu a ion asks such
as hal oning
[
181,
mo ion de ec ion
[
191-[21], ange- inding
[3], e c.
111.
THE CNN PARALLEL PROCESSING PARADIGM
As
Fig.
1
illus a es, sma -pixel CNN chips consis o
egula a angemen s o
iden ical
uni s, each including
a
pho osenso
and an analog compu ing
cell.
Such an en i y
ans o ms he inpu image
[I,]
in o an ou pu ma ix
[y,]
ia
a
dynamic
p ocess o in e ac ions among he compu ing
cells. The dis inc i e ea u e o he CNN pa adigm is ha hese
in e ac ions a e
local,
limi ed o each cell o a educed se o
neighbo s, loca ed wi hin
a
dis ance
T
in he g id. In pa icula ,
he e is
a
wide ca alog o image p ocessing asks a ailable
o ne wo ks whe e pa ame e
T
(called
neighbo hood adius)
is uni y- e y appealing o
VLSI
implemen a ions because
connec ion among uni s
is
made by abu men , equi ing no
ex a ou ing.
The dynamic compu a ion p ocess o CNN's, as p oposed
in [1], in ol es h ee a iables pe cell:
(a)
cell
s a e:
xc( ),
which con eys cell ene gy in o ma ion as a unc ion o ime;
(b) cell
ou pu :
yc( ),
ob ained om he cell s a e ia
a
so -
limi e
piecewise-linea
ans o ma ion,
(1)
Yc
=
(G)
=
;(I.c
+
11
-
15,
-
11)
d awn in Fig. 3(a); and (c) cell
ex e nal-inpu :
U,.
P ocessing
i sel
is
go e ned by a se o coupled nonlinea di e en ial
equa ions, one pe cell. We use equa ions ha di e om hose
o iginally p oposed by Chua-Yang
[
13, and which enable he
op imiza ion o he speedlpowe a io and a ea occupa ion o
VLSI
CNN chips. The p oposed equa ions a e gi en by [9],
[lo]:
dz,
d
-
=-
+
{A,dYd( )
+
&dud)
dE
A',
(c)
Vc
E
GD
(2)
whe e
g(
.)
is a nonlinea dissipa i e e m de ined
as,
m(5,
+
1)
-
1
2,
<
-1
g(z,)
=
.c
o he wise (3)
{
m(5,
-
1)
+
1
5,
>
1
whe e
m
>
1
is a pa ame e o he model. Func ion
g(.)
is
d awn in Fig. 3(b). Summa ions in
(2)
ex end o e he
neigh-
bo hood
o he c h cell, deno ed by
NT(c),
which con ains
adjacen cells loca ed wi hin a dis ance
T
in he g id, and
includes cell
c
i sel .
P ocessing asks pe o med by CNN's a e de e mined by
he
con e gence
o
(2)
o bina y
(y,
=
*l,Vc
E
GD)
equilib ium s a es ollowing he ansien ini ialized by
[xc(
O)],
d i en by
[U,],
and unde he bounda y condi ions imposed
by cells a he ne bo de . Depending on he applica ion, he
cu en
1,
gene a ed a each cell's pho osenso is used as
ini ial alue o he s a e a iable
zc(0)
o as ex e nal inpu
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ESPEJO
e
al.:
SMART-PIXEL
CELLULAR
NEURAL
NETWORKS
IN
ANALOG CURRENT-MODE
CMOS
TECHNOLOGY
(b)
Fig.
3.
CNN cell nonlinea i ies. (a) Ou pu nonlinea i y;
(b)
dissipa i e e m.
U,.
In he la e case, he ini ial s a es a e usually se o
a
cons an alue. The ou come o he ask depends on pa ame e s
Bcd,Acd
and
D,
o
(2),
called
con ol, eedback,
and
o se
pa ame e s, espec i ely, and on he bounda y condi ions.
The con ol and eedback pa ame e s can be a anged in o
ma ices, which p o ide
a
pic o ial iew o he in e ac ions
wi hin each cell’s neighbo hood. Fo uni o m ne wo ks hese
ma ices a e in a ian h oughou he g id domain- hey a e
empla es.
The unc ionali y o uni o m CNN’s is de e mined
by i s con ol,
B,
and eedback,
A,
empla e ma ices, and i s
o se pa ame e ,
D.
Fo illus a ion pu poses, Table
I
sum-
ma izes he empla es used o some signi ican p ep ocessing
asks.
To gua an ee co ec ope a ion o sma -pixel CNN chips,
an impo an ma hema ical issue is o de e mine condi ions o
he empla e pa ame e s ha yield con e gence o he ou pu
ma ix
[yC]
o bina y s a es o any inpu . Such
a
ma hema ical
analysis o he model p oposed in his pape , gi en by (2)
and (3), is ou o his pape ’s scope and has been epo ed
elsewhe e [9] o any
m
2
1.
Ou ci cui s use he pa icula
~
891
TABLE
I
SOME
CNN
TEMPLATES
Applica ion
A
B
D
Noise
Fil e ing
[!
1
PI
[!
!
?I
0
PI
000
010
Hole Filling [28]
Con ex Come s
Ex ac ion [2]
Bo de s
Ex ac ion [2]
Connec ed
Componen
De ec ion [15]
Shadow C ea ion
1291
-11.1
-11.1
-114
2
-114
-11-2
-114
2
-114
-114
-114
-114
-1
-11.1
-3
-1141
-1
14
-1
1.1
-2
0
0
case
m
-+
00,
in which he nonlinea dissipa i e e m o ces
he s a e a iable
zc
o emain wi hin he in e al
[-l>
11.
Consequen ly,
zc( )
=
yc( ),
and he implemen a ion o he
nonlinea ope a o in (1) is no equi ed.
IV.
SENSORY
CIRCUITRY
A.
Pho osenso s
The simples pho osensi i e de ices o CMOS n-well ech-
nologies a e e e se-biased
pho odiodes,
o med ei he di ec ly
be ween n+-di usion and subs a e [3] o be ween well and
subs a e [13]. Cu en le el o bo h de ices is an inc easing
unc ion o he junc ion a ea. In pa icula , we ha e measu ed
cu en s up o
20
nA o well-subs a e pho odiodes wi h well
a ea o 100
x
100pm2, in
a
1.6-pm single-poly echnology,
unde en i onmen al labo a o y ligh ing. This cu en le el
inc eases signi ican ly using
a
e ical CMOS-BJT
as
pho o-
senso . Fig.
4(a)
shows
a
concep ual layou and c oss-sec ion
o his de ice, whose cu en is app oxima ely p opo ional
o he a ea o he wellhubs a e junc ion,
A,
in he igu e.
Cu en gene a ed by his de ice is
p
+
1
imes la ge han
ha
o
a
pho odiode wi h he same well a ea
whe e
IT
deno es he pho o ansis o cu en ,
Il
is he
co esponding cu en o he well-diode, and is he ansis o
cu en -gain; measu ed
in
o his echnology is 37.7
0.8,
basically independen o ansis o geome y [22].
We ha e measu ed cu en s up o 430 30 nA (unde no mal
labo a o y illumina ion) o pho o ansis o s wi h passi a ed
well a ea o
60
x
60pm2.
Consequen ly, and since cu en
and well a ea a e app oxima ely linea ly ela ed, i ex apola es
cu en le els
o
20
nA o minimum a ea de ices (13.6
x
13.6 pm2)-needed o inc eased densi y sma -pixel chips.
Howe e , o some c i ical asks [7] hese le els p o ided by
minimum pho osenso s may no be la ge enough o gua an ee
he
ma ching le el equi ed by he signal p ocessing ci cui y,
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898
IEEE
JOURNAL
OF
SOLID-STATE
CIRCUITS,
VOL.
29,
NO.
8,
AUGUST
1994
I'
I
i
0
0
Fig.
4.
igu a ion
o
wo e ical p-n-p ansis o s.
(a)
CMOS compa ible e ical p-n-p an sis o .
(1
J)
Da
ling on con-
hus equi ing some ampli ica ion. Simples s a egies use
ei he la ge wells o cascaded cu en ampli ie s- e y cos ly
in e ms o a ea occupa ion and, o he la e , inaccu a e. In-
s ead, we use
an
addi ional e ical BJT o achie e Da ling on
ampli ica ion by
a
ac o o
p
+
1,
wi h p ac ically no a ea
o e head. Fig. 4(b) shows he concep ual layou and c oss-
sec ion o his Da ling on pho o ansis o . Cu en o his
de ice is
1,
N
(P
+
1)1T
N
(P
+
1)21w.
(5)
while i s a ea occupa ion is sca cely inc eased by ha o
a
minimum-size e ical BJT. Measu emen s wi h
A,
in
Fig. 4(b) equal o
60
x
60pm2
esul in cu en s up o
18
*
2pA.
Fig.
5(a)
shows he
ou pu
cha ac e is ic measu ed om
a
Da ling on pho o ansis o wi h
A,
=
60
x
60pm2
unde
cons an en i onmen al illumina ion (b igh -cu en ), while
Fig. 5(b) shows he esul ob ained when he en i onmen ligh
is g adually educed o comple e da kness. Da k-cu en was
215+
10
PA, which means ha he b igh - o-da k cu en - ange
is close o
100
dB o en i onmen al labo a o y illumina ion.
The same ange is obse ed o
a
single p-n-p de ice, while
simple pho odiodes yield abou
80
dB. Al hough hese esul s
a e op imis ic in he sense ha in eal images he e will be
(b)
Fig.
5.
Measu ed ou pu cha ac e is ics
o a
Da ling on pho o ansis o wi h
A +
=
GO
x
GO
pm2:
(a)
unde cons an en i onmen illumina ion;
(b)
e ec
o g adual educ ion o illumina ion du ing he sweep o
I
;.E.
no comple ely da k a eas, he b igh - o-da k cu en - a ios
measu ed p o ide
a
wide enough ange o da a acquisi-
ion. The ampli ica ion o he Da ling on s uc u e p o ides
a su icien cu en le el e en i de ice a ea is subs an ially
dec eased.
B.
Au oze o S a egy
Al hough pho osenso s p oduce unidi ec ional cu en low,
double- ail signals a e easily ob ained by bias-shi ing,
as
shown in Fig.
6(a).
Cu en sou ce
ITH
se s he ze o-le el o
he double- ail signal. To gua an ee good con as , i s alue
should be se somewhe e be ween he maximum and he
minimum ligh -induced cu en s among all pho osenso s. I
ligh ing condi ions o all possible inpu scenes a e uni o m
and known
a
p io i,
ITH
can be se o
a
ixed alue. In
a
mo e
gene al case whe e he chip mus handle scenes wi h di e en
ligh ing condi ions, some kind o au o-ze o s a egy mus be
de ised o gene a e
ITH
app oxima ely equal o he a e age
o he pho osenso cu en s o e he whole a ay.
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ESPEJO e
al.:
SMART-PIXEL CELLULAR NEURAL
NETWORKS
IN
ANALOG CURRENT-MODE CMOS TECHNOLOGY
899
(b)
Fig.
6.
cu en
ou pu ;
(b) au o-ze o ci cui y.
Th eshold ci cui y o pho osenso s.
(a)
Bias-shi ing o double- ail
A simple, ye con enien , au o-ze o s a egy uses ou
ex a ansis o s a each senso . Fig. 6(b) shows he schema ic
o
a
senso including he au o-ze o ci cui y. All p-channel
ansis o s ha e equal size; he same applies o n-channel
ansis o s. The low-impedance node labelled
SUM
is
a
global
node, common o all pixels. No e ha he cu en
I,
a he c h
pho osenso is eplica ed wice. One o he eplicas in e aces
he p ocessing ci cui y, while he o he is oo ed o he global-
node
SUM,
and agg ega ed o he emaining senso cu en s.
Thus, calcula ion o he cu en
ITH
h ough ansis o
MTH
ob ains he ollowing
whe e
gOp
is he ou pu conduc ance o he p-channel ansis o ,
gmn
is he ansconduc ance o he n-channel ansis o , and
N
he numbe o pixels. Fo simplici y (6) assumes equal
ansconduc ances and conduc ances o all pixels. The i s
ac o in (6) e lec s he cu en di ision pe o med a node
SUM,
while he second co esponds o he gain
o
he mi o
o med by he pa allel combina ion o he
Msuh
ansis o s
and
MTH.
Assuming
gmn
>>
go,,
(6) gi es
ITH
equal o he
a e age o he pho osenso cu en s, and he ligh - h eshold is
au oma ically adjus ed o he a e age illumina ion.
Fig.
7.
sma pixel.
Concep ual block diag am
o
he p ocessing ci cui y
o
a CNN
V.
PROCESSING
CIRCUITRY
A.
Basic Ci cui Building Blocks
Fig.
7
is
a
block diag am o he p ocessing ci cui y o he
c h uni in
a
sma -pixel CNN chip, acco ding o
(2).
This
igu e shows
a
co e in eg a o wi h nonlinea losses and
an
ou pu s uc u e o gene a e weigh ed eplicas o he c h inpu
U,
and s a e
x,,
o ansmission o he neighbo sma -pixels.
The in eg a o is d i en by weigh ed eplicas o he inpu and
s a e signals o he sma pixels in he neighbo hood
N,(c),
plus an o se e m, ob aining he ollowing signal o d i e he
co e in eg a o
Jc( )
=
D,
+
{Acdxd( )
+
Bcd~d}.
(7)
Cu en -mode p o ides
a
con enien choice o ealize he
p ocessing ci cui y
o
sma -pixel CNN's. On one hand, i
enables di ec in e ace wi h he senso s, whose ou pu s a e
cu en s. On he o he , cu en summa ion a he in eg a o
inpu node is di ec ly achie ed by ou ing signals o
a
common
node. Finally, analog ope a o s in ol ed in Fig.
7
(weigh ed-
eplica ion, in eg a ion, and limi a ion)
a e
ealized by simple
cu en mi o ci cui s.
Fig.
8(a)
ealizes he co e in eg a o . Inpu cu en
J,*( )
is
an unno malized e sion o
Jc( )
in
(7),
wi h no maliza ion
ac o
IQ:J:(~)
=
IQJc( ).
Ou pu cu en
z ( )
is he
co esponding unno malized e sion o
x,
( ).
The pa allel
combina ion o he diode-connec ed inpu ansis o
A41
and
capaci o
C
yields a ime cons an
=
GIs,,
whe e
gm
is
he ansconduc ance pa ame e o
MI.
On he o he hand, no e
ha cu en
x:
canno swing beyond he alues o he cu en
sou ces which d i e he common ou pu node
o
ansis o s
and M3-meaning ha
IxZI
<
IQ.
Thus, analysis o his
ci cui esul s in:
dE
A',
(c)
as
equi ed o ealize
(2),
and whe e
g(.)
is he unc ion de ined
in
(3)
wi h
m
i
CO.
In p ac ice
7
does no emain cons an ,
bu a ies wi h inpu cu en le el. Howe e , mos p ocessing
asks ole a e his a ia ion wi h no deg ada ion o he ne wo k
unc ionali y
[9].
Fig. 8(b) shows
a
ci cui o ealize he ou pu s uc u e
o
Fig.
7
om ol age
V,,
and using he basic cu en mi o
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900
IEEE JOURNAL
OF
SOLID-STATE CIRCUITS,
VOL.
29,
NO.
8,
AUGUST
1994
""'I6
;d(
6 Cd'IQ
I
1
(b)
Fig.
8.
sma
pixels.
(a)
Co e
in eg a o ;
(b)
ou pu s uc u e.
Cu en -mode ci cui
blocks
o
he
p ocessing ci cui y
o
CNN
p inciple o weigh ed eplica ion [23]. No e ha Fig. 8(b)
con ains wo di e en subs uc u es o co e each possible sign
o he weigh
Acd.
Posi i e weigh s a e ob ained using a single
ou pu ansis o whose geome y ac o is
lAcdl
imes ha o
ansis o
MI.
Thus, a cu en
A,,x;
is sou ced o he ou pu
node. Nega i e weigh s equi e an addi ional cu en mi o
wi h uni y weigh o sign in e sion.
B.
Some Ci cui Design
Issues
The ollowing is a b ie commen o dominan nonideali ies
encoun e ed in he p ac ical implemen a ion o sma -pixel
CNN chips and associa ed ci cui s.
I)
Cu en Gain E o :
A majo sou ce o e o is he ini e
a io o he inpu conduc ance
yi,
o he ou pu conduc ance
yo
o he cu en mi o s, which causes cu en gain e o due
o spu ious cu en di ision. I is especially signi ican a he
inpu node o he in eg a o , whe e he gain e o
,
is gi en
app oxinia ely by [9]:
egula ed mi o s, o a combina ion
o
bo h
mus
be used
[23].
In
pa icula ,
analysis
shows ha he
cascode
mi o
o
Fig. 9(a) ob ains alues o
g0/gi L
se e al o de s
o
magni ude
lowe han ha o single mi o s, wi h smalle a ea occupa-
ion. Chips epo ed in Sec ion
VI
a e ealized using hese
mi o s, and sized o handle he whole inpu cu en ange
wi h minimum dis o ion and smalles possible de ices. Fo
mi o s biased by a cu en
IQ,
we ob ain he ollowing sizing
equa ions
whe e
k,
=
pC0,/2.
V,,
is he h eshold ol age, and
Vc.4~
is he cascode ol age, which can be gene a ed as shown in
Fig. 9(b). We assume he same geome ies
W,
and
Ln
o
all n-channel ansis o s in he cascode mi o .
W
alues
o la ge cu en s (associa ed
o
weigh ed eplica ion) a e
calcula ed by imposing he cons ain ha all ansis o s ha e
equal cu en densi y. Al e na i ely, o a gi en aspec a io
W,/L,,
(10)
es ablishes a bound o he maximum bias
cu en o he de ice.
2)
Misma ch e o ; a ea, powe and eliabili y:
T ansis o
geome y a ios, s a ic gain e o due o nonnull
go/gzn,
and
powe dissipa ion inc ease wi h
IQ.
Hence, a bias cu en as
small as possible should be chosen. The issue is o iden i y he
minimum easible ail cu en alue. A lowes limi is ce ainly
es ablished by leakage, which in ou case is inc eased by ligh
e ec s. Howe e , a mo e es ic i e bound exis s due o MOS
ansis o misma ch and Ea ly ol age
(V,)
deg ada ion wi h
channel leng h.
Misma ch is p oduced mainly by a ia ions o
V,
and
/3
=
pCo,W/L,
whose s anda d de ia ions
~(VT)
and
o(@)/p
o de ices wi h equal layou show a componen in e sely
p opo ional o he squa e oo o he channel a ea, and
ano he p opo ional o he dis ance be ween de ices [24].
Howe e , in he echnology used and o ansis o pai s close
han abou 2.5 mm, he dis ance-dependen componen is
negligible o de ices wi h channel a ea o less han
100
piii2
[24]-la ge han he alues ob ained using
(10)
o bias
cu en below
-50
pA
and channel leng hs o
3.2
/mi.
Lowe
channel leng hs ha e no been conside ed o se e al easons,
like sho -channel e ec s, ea ly- ol age deg ada ion, and in-
c eased misma ch e ec s due o he associa ed low channel
a eas.
In
addi ion, lowe ansis o geome ies do no esul in
app eciable a ea educ ions due o he minimum con ac size
(4
pm
wi h su ounding me al and di usion in he echnology
used).
Ano he impo an conside a ion is ha o a gi en
o(V'+)
and
g(/?)//?,
he a io
a(I)/I
in
MOS
ansis o s ope a ing in
s ong in e sion and a e pinch-o has an in e se dependency
wi h
iigs
-
V,.
This means ha once geome ies ha e been se
o achie e accep able misma ch le els, bias cu en canno be
EM
(N+
I)*
(9)
Sin
whe e
N
deno es he numbe o mi o s d i ing his node-up
o
18
o empla es wi h no ze o en ies on a ec angula
g id ne wi h uni y neighbo hood pa ame e . Fo imp o ed
go/gin
igu es wi h sho channel de ices, cascode mi o s,
dec eased oo a below he bound gi en by
(lo),
since his
would p oduce a low
ugs
ol age a he bias poin , wi h he
co esponding la ge
o(I)/I.
Hence, misma ch conside a ions
es ablish bounds o bo h minimum a ea and powe ends.
3)
Ligh e ec s on he p ocessing ci cui y:
Op ical image
acquisi ion o ces he p ocessing ci cui y o be exposed o
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ESPEJO e al.: SMART-PIXEL CELLULAR NEURAL NETWORKS IN ANALOG CURRENT-MODE CMOS TECHNOLOGY
YO1
-
W
L
-
.....
. . .
....
lin
io
+
W
L
-
-
W
L
-
W’
L
W
L
-
‘W
-
L
W
L
-
(b)
Fig.
9.
and e e ence ci cui y,
(b)
cascode ol age gene a ion.
CMOS bias-shi ed mi o s and biasing de ices. (a) Cascode mi o
ligh , which esul s in an inc ease o he leakage cu en s
a he e e se-biased subs a e-di usion junc ions. Uni a y
bias cu en s mus be su icien ly la ge in o de o neglec
his e ec . Also, MOS h eshold ol age depends on ligh
in ensi y, inc easing he misma ch e ec on cu en mi o s
and sou ces. Cu en mi o ansis o s a e commonly placed
nea by, and hence ligh -in ensi y g adien s ha e a educed
e ec . On he con a y, cu en sou ces in di e en cells,
biased by common global ol ages, can exhibi la ge
dispe sions. The ole ance o a pa icula applica ion o
a ia ions in he uni a y bias cu en mus be e alua ed
in gene al, and local e e ences should be used when
equi ed.
Fig.
10.
Mic opho og aph
o
he
DCC p o o ype.
VI.
EXPERIMENTAL
RESULTS
A.
16
x
16
DCC
P o o ype
The ollowing measu emen s we e aken om a
16
x
16 sma -pixel CNN chip in ended o ho izon al connec ed
componen de ec ion
[15]
(see Fig. 2)-a basic p ep ocessing
s ep o pa e n ecogni ion. Fig.
10
shows a mic opho og aph
o he p o o ype, which in addi ion o he sma -pixel a -
ay con ains bounda y cells, ou pu bu e s, bias s ages, and
some
digi al con ol ci cui y o he ou pu image download-
ing p ocess. The dimensions o he co e a ay a e 1890
x
1530
pm2,
and i s powe dissipa ion is 27 mW. The o al chip
dimensions, including he bonding pads, a e 2480
x
2500
p n2,
wi h a o al powe dissipa ion o 42 mW and a o al o 24 pins.
Fig.
11
shows he schema ic and layou o one elemen a y
uni . Uni dimensions a e
118
x
96pm2,
which include he
senso and associa ed egula ion ci cui y
(-30%
o he a ea),
he p ocessing ci cui y, an addi ional cu en eplica ion o
ou pu e alua ion, and all equi ed ou ing (cells
a e
connec ed
o each o he by abu men ). Senso is ealized wi h wo
minimum-size p-n-p de ices in a Da ling on con igu a ion, o
p oduce a b igh -cu en unde labo a o y ligh ing o abou
lpA,
la ge enough o he ma ching equi emen s o his
applica ion. Cascode s uc u es a e used o bo h cu en
sou ces and mi o s, and excep o con ol swi ches, all
MOS
ansis o s ha e
W
=
4p n
and
L
=
3.2pm.
Powe
dissipa ion wi h a
5
V
supply and unde en i onmen al ligh
in he labo a o y is lO5bW pe cell, uni a y cu en being
We ha e ob ained 100% success (ou o
30
ials) o
ull de ice le el Mon eca lo simula ion
o
his chip. These
Mon eca lo simula ions a e based on he expec ed a ia ions
o he h eshold ol ages
VTo
and he la ge signal anscon-
duc ance
p
(body e ec pa ame e
y
in luences only he
cascode ansis o s). Global biasing ol ages a e used o
cu en e e ence gene a ion, and bias s ages a e included in
he simula ion. Dispe sion due o misma ch among ansis o s
o di e en cu en sou ces did no p oduce c i ical esul s.
Thus, global biasing is a ai app oach o his applica ion.
Fig. 12(a) illus a es he chip measu emen se up and
Fig. 12(b) shows i e inpu images (le column) and he
IQ
=
2pA.
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902
IEEE
JOURNAL
OF
SOLID-STATE CIRCUITS,
VOL.
29,
NO.
8,
AUGUST
1994
SET!
VPB!VCP!VCN! Objec D.U.T.
(Film Nega i e)
(Chip)
Wo k
S a ion
Digi al
De ice
Tes ing
Equipmen
Regulable Focusing
Ligh
Sou ce
(a)
VTH!
START!
(a)
io
6
/ai*
9
.*'(
a
111"
(b)
Fig.
11.
elemen a y uni o
DCC
p o o ype.
(a)
Schema ic
( e e
o ex
o
dimensions) and (b) layou o
measu ed ou pu images ( igh column). The p o o ype
was exhaus i ely es ed wi h 1200 inpu images. Fig. 12(c)
con ains he ou pu wa e o ms obse ed om he cells in
a pa icula ow o he a ay du ing a p ocessing example.
The inpu pixels a e displayed a he le side column,
while he ou pu ones a e a he igh . The signals display
he measu ed ansien e olu ion o he ou pu o he cells
in he ow. Measu ed con e gence ime is
1.6~~.
Ou pu
image downloading equi es
8ps,
using a
2
MHz
digi al
clock equency o he se ial downloading p ocess. Ci cui
ope a ion emains co ec , wi h no speed deg ada ion, i he
ol age supply is educed om he nominal
5
V
down o 2.7
V.
This is ano he posi i e consequence o using cu en -mode
echniques.
1w
I
I
B.
16
x
16
Radon T ans o m P o o ype
This p o o ype pe o ms he Radon T ans o m [25] o
16
x
16
pixels inpu images. This chip accep s elec ical, as well as
op ical, inpu . The p ocessing ci cui y is based on a modi ied
e sion o (2), whe e ime has been disc e ized, and he
nonlinea i y is ha d
x,(n
+
1)
ime
1,
o
D,
+
{Acd~d(n)
+
Bcdud}
>
0. (C)
d€N,(c)
Fig. 12. (a) Measu emen se up, (b) i e inpu images and he ou pu
measu ed
om
DCC,
and
(c)
measu ed ansien esponse
on
a
ow
o
cells.
-
1,
o he wise
(1
1)
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ESPEJO
e
al.:
SMART-PIXEL
CELLULAR NEURAL
NETWORKS
IN
ANALOG
CURRENT-MODE
CMOS
TECHNOLOGY
903
FOT!LOAD!LOAD!
VTH!
CI!
c2!
Fig.
14.
Radon T ans o m p o o ype.
Fi e inpu images and he co esponding ou pu measu ed
om
he
(b)
Fig.
13.
Radon T ans o m p o o ype.
(a) Schema ic o elemen a y uni and (b) mic opho og aph o he
Also, his applica ion equi es signal-dependen weigh s [26].
In pa icula , he weigh s o he con ibu ions going om a
pa icula cell
c
o i s neighbo s depend on
5,.
The comple e
se
o
CNN coe icien s can be desc ibed using unidimensional
empla es as ollows
i
(z,
2
0)
A
=
{
[l
0
01, i
(zc
<
0)
[0
0
11,
B=[OOO]
D=O
(12)
which e lec he scaling ac o s applied o he con ibu ions
o a pa icula cell o i s neighbo s.
Fig. 13(a) shows a simpli ied schema ic o a cell, which uses
pass ansis o s o ealize he delay equi ed in
(1
1)
and a high-
esolu ion cu en compa a o [27] o he ha d nonlinea i y.
The design echnique and he algo i hm used in his ci cui y is
desc ibed in de ail in [9]. Fig. 13(b) shows a mic opho og aph
o he p o o ype. Cell dimensions
a e
121
pm
x
124
pm,
and
he powe dissipa ed by each cell is
1
mW-signi ican ly la ge
han o he DCC due o he ci cui y used o implemen he
ha d nonlinea i y.
The sys em con ains a numbe o blocks loca ed in he
pe iphe y o he cell a ay, like ou pu bu e s, bias s ages,
and digi al con ol ci cui y dedica ed o he uploading and
downloading p ocesses. This addi ional ci cui y, oge he wi h
he bonding pads, esul in a o al sys em a ea o
2670pm
x
2680
pm,
and a o al sys em dissipa ion o 330 mW. The chip
equi es a o al o 43 pins. This numbe is signi ican ly highe
han ha o he p e ious p o o ype due o 16 inpu pads used
o elec ical inpu image uploading.
Using a
2
MHz digi al clock equency, image p ocessing
ime is
8
ps.
The se ial downloading p ocess also equi es
8
ps.
As an example, Fig. 14 shows i e inpu images (le column)
and he co esponding ou pu images measu ed om he chip
( igh column). The comple e es o he p o o ype in ol ed
1200 images.
VII.
CONCLUSIONS
Summa izing, his pape has ou lined a basic model and
some design issues ela ed o a me hodology o design CNN
sma -pixel chips in digi al CMOS p ocesses, and has p e-
sen ed measu emen s om wo wo king p o o ypes in a 1.6-
pm n-well CMOS echnology. One calcula es he numbe o
connec ed pieces (DCC) o an inpu image in he ho izon al
di ec ion, and he o he e alua es he Radon T ans o m o
an inpu image. The DCC chip ob ains a densi y o
-89
sma -pixels pe mm2 (each including senso y, egula ion and
p ocessing ci cui y), wi h a powe consump ion o 105
pLw
pe sma pixel and image p ocessing imes below
2ps.
A ea
and speed igu es o he RT chip
a e
simila . Al hough powe
dissipa ion is la ge o his p o o ype, his can be co ec ed
wi h a ca e ul design o he cu en compa a o [27].
As compa ed o p e ious CNN implemen a ions, he
p oposed echnique makes he equi ed syne gy be ween
sensing and p ocessing, and signi ican ly imp o es a ea and
speed powe igu es. In pa icula , when compa ed o p e ious
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