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er Gy = Gat P3e3
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AzA, A, Ao
Bs Ba 8, BoA bel asteler coolly aarp lots alend -15 BINARY PARALLEL ADDER
‘inary parallel adder is a digital circuit that adds two binary numbers in parallel form and
snluces the arithmetic sum of those numbers in parallel form, It consists of full adders connected
‘tachain, with the output carry from each full-adder connected to the input carry of the next full-
tke in the chain,
Figure 7.20 shows the interconnection of four full-adder (FA) circuits to provide a 4-bit
| adder. The augend bits of A and addend bits of B are designated by subscript numbers
Sonright to left, with subscript 1 denoting the lower-order bit. The carries are connected in a
ia iough the full-adders. The input carry to the adder is C,, and the output carry is C,. The S
‘Qus generate the required sum bits. When the 4-bit full-adder circuit is enclosed within an IC
thas four terminals for the augend bits, four terminals for the addend bits, four terminals
 
 
ior & Sy Ss Ss:
Figure 7.20 Logic diagram of a 4-bit binary parallel adder.1 err OP EE
iba slab an havea)COMBINATIONAL LOGIC DESIGN 299
ABCD aulder citcuit must be able to opera
ye ceil MUST e able to do the folowing
1. Add two 4-bit BCD code groups, using straight binary add
2, Determine, ifthe sum of this addition is greater the yon
(decimal 6) to this sum and gene
a ewe BCD ae, TetbY Using a4-bit binary parallel adder such as the T4LSB3 IC.
Ce Treaty cee SOUPSAsA,A,A, and B,B,B,B, ae aplied to ait parallel
pide adder will output SS, where S, is actually C,, the carry-out of the MSB bits
The sum output S,8,8,8,S, can range anywhere from 00000 to 10010 (when both the BCD
gs Tous are 1001 = 9). The circuitry for a BCD adder must include the logic needed to detect
o that the ‘correction can be added in, Those cases, where
  
‘ceordance with the above steps, In other
 
n
, han 1001 (decimal 9); if itis, add 0110
Ye @ carry to the next decimal position.
 
 
 
   
ger the sum is greater than 01001,
Gesmis greater than 1001 are listed in Table 7.1
 
 
*
a
 
HHH ccocec]
ece--e Woo
IH oo--oo-+
__Letus define a logic output X that will go HIGH only when the sum is greater than 01001
for the cases in Table 7.1). If we examine these cases, we see that X will be HIGH for either of
(sum greater than 15)
__2 Whenever $, = 1 and either S, or S, or both are 1 (sums 10 to 15)
__ This condition can be expressed as ip
X=S, 45,5, +S)
X = 1, it is necessary to add the correction factor 0110 to the sum bits, and to
igure 7.28 shows the complete circuitry for a BCD adder, including the logic
n for X.300 FUNDAMENTALS OF DIGITAL CIRCUITS
second BCD adder, the carry-out of the second BCD adder is connected as the ¢;
BCD adder and so on. ATY-in of yy
8, 8 5 8, thing
    
 
 
 
 
 
 
next BCD adder
 
 
 
 
— 4-bit parallel adder (74LS83)
hs °
Correction
adder
Figure 7.28 Logic diagram of a BCD adder using two 4-bit adders and a correction-detector circuit.116 CODE CONVERTERS
qhe availability of a large variety of codes for the same discrete elements of information results in
sheuse of different codes by different digital systems. It is sometimes necessary to use the output
of one system as the input to another. A conversion circuit must be inserted between the two
gjstems if each uses different codes for the same information. Thus a code converter is a logic
circuit whose inputs are bit patterns representing numbers (or characters) in one code and whose
outputs are the corresponding representations in a different code. It makes two systems compatible
even though each uses a different binary code. Code converters are usually multiple output circuits.
Toconvert from binary code A to binary code B, the input lines must supply the bit combination
of elements as specified by code A and the output lines must generate the corresponding bit
combination of code B. A combinational circuit performs this transformation by means of logic
gates.
For example, a binary-to-Gray code converter has four binary input lines B,, B,, B,, and B,,
and four Gray code output lines G,, G,, G,, and G,. When the input is 0010, for instance, the
output should be 0011 and so forth. To design a code converter, we use a code table treating it as a
tnuth table to express each output as a Boolean algebraic function of all the inputs.
In this example of binary-to-gray code conversion, we can treat the binary to the Gray code
lables four truth tables to derive expressions for G,, Gy, G,, and G, . Each of these four expressions
Would, in general, contain all the four input variables B,, B,, B,, and B,. Thus, this code converter
'sactually equivalent to four logic circuits, one for each of the truth tables.
The logic expressions derived for the code converter can be simplified using the usual
‘schniques, including ‘don’t cares’ if present. Even if the input is an unweighted code, the same
cell Numbering method which we used earlier can be used, but the cell numbers must correspond
‘0 the input combinations as if they were an 8-4-2-1 weighted code. For example, in Excess-3 to
Conversion, number ABCD = 0110, which represents 3,, is assigned the cell number 6 and
"tthe cell number 3. Be careful to determine which input combinations, if any, will never occur
can be treated as don’t cares. Of course, it is the input bit patterns, and not the output bit
Patterns that determine don’t cares.
came erated circuits (ICs) are available to convert data from one form to another. Binary-to-BCD.
a TSions are most often encountered in connection with computer applications. Numerical data
mitted in BCD form from input devices must be converted to binary, so that arithmetic306 FUNDAMENTALS OF DIGITAL ciRCUITS
operations can be performed on it, The binary aes
cD for transmission to output devices. Therel
the major components ofthe computer system iS°" TT
tables may be stored in the ROM. In some systen
itself, through execution of a specially designes Dl en
opposed to the hardware conversion performed by log!
Gray Code Converter
ster circuit is a 4-bit binary and the output is a
sof 4-bit binary input and all of them are vali
ponding Gray code are shown in the conversion
serve that the expressions for the outputs
ts of arithmetic operations must be converted ,,
* conversions are often accomplished by ysing
her than special converter circuits. Conversion
versions are accomplished by the computey
fam. This is called software conversion, a,
7.16.1. Design of a 4-bit Binary-to~
The input to the 4-bit binary-to-Gray code conve
4-bit Gray code. There are 16 possible combination:
Hence no don’t cares. The 4-bit binary and the corres}
table (Figure 7.32a). From the conversion table, we obs
y Gy, Gy, and G, are as follows:
 
G,= E48, 9, 10, 11, 12, 13, 14, 15)
G,=Em64, 5, 6, 7, 8, 9, 10, 11)
G,=Em(2, 3, 4, 5, 10, 11, 12, 13)
G,=Em(1, 2,5, 6, 9, 10, 13, 14)
‘The K-maps for G,, G,, G,, and G, and their minimization are shown in Figure 7.32b,
The minimal expressions for the outputs obtained from the K-map are:
G, B,
G,=B,B, +B,B, =B, ®B,
G= B. B, + BB, =B,@B,
G,=B,B, +B,B, =B, ®B,
‘2 viet Bs paises ‘can be achieved by using three X-OR gates as shown in the logic diagram
 
 
4-bit binary #olGray
Cua ET ML aa acer
5% BB G&G GG
00 0 0 oo
Dat rns
00 10 is 4D tasleinad
Oe ela abel ceed 10 meet eh
fol! ig Tp (det bad)
OW His” Lomi iba Fone ial et
gt ted Oc tke lade Daas
oo
NM ot Hovedg> ie Cre at
Oia Ojo Naish stmetont0; tu
tl ald 9 ea
1
teeter) rae ee
slop Dcultboinos peltuig pe
o
Lode anoatoprd
(a) Conversion table
Fi ir
'gure 7-92 4-bit binary-to-Gray code converter (Contd,)COMBINATIONAL LOGIC DESIGN 307
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
| a
of ie 10 L1} Li} ) D-6
G,=B, 2B G,=B.28,
Kemap for Ge (kee | OTRREEG eae
Figure 7.32. 4-bit binary-to-Gray code converter.
1182. Design of a 4-bit Gray-to-Binary Code Converter
The input tothe 4-bit Gray-to-binary code converter circuit is a 4-bit Gray code and the output is
‘tit binary, There are 16 possible combinations of 4-bit Gray input and all of them are valid.
ee no don't cares, The 4-bit input Gray code and the corresponding output binary numbers are
‘town in the conversion table of Figure 7.332. From the conversion table we observe that the
“pressions for the outputs B,, B,, B, and B, are:
B,=EZm(12, 13, 15, 14, 10, 11, 9,8) == m0B, 9, 10, 11, 12, 13, 14, 15)
B,=Em(6,7,5, 4, 10, 11, 9,8) = m(4, 5, 6, 7,8, 9, 10, 11)
B,=Em(3, 2, 5,4, 15, 14,9, 8)= 2 m(2, 3,4, 5, 8,9, 14, 15)
B,=EZm(1, 2,7, 4, 13, 14, 11, 8)=2mU1, 2,4, 7,8 11, 13, 14)
Drawing the K-maps for By, By, By and B, in terms of G,, Gy, Gy and G, as shown in
we 7.335 and simplifying them, the minimal expressions for the outputs are as follows:
= G,
3=G,6,+6,G,=G,®G,
3.6.6.6, +6,6,G, + 6,6,6, + 6,6,
=G,G, 8 G,) + G(G, @G,) = 6, ® G; ® Ga = Bs @ Ge
3,=G,6,6,6, +6,G,6,G, +G,6,6,6, +G,6,6,6,+6,G,6,6, ___
+G,G,G,G, + G,G,G,G,+ G,G,6,G,308 FUNDAMENTALS OF DIGITAL CIRCUITS
 
G(G OG,) + GGG OG)
@G,) + 6,G,G, ® 5) + Od
=G6,6,G, eecr ‘
=(G,86,\G, ®G;) + (Gr GG, NG, ® G3)
G, ©G,®G, ®G,
=B, OG,
cae a logic circuit can be drawn as shown in Figure 7.33,
Based on the above expressions,
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
_eerGay
& 6 G&G 8 % Be B
0.00 0 0 0 0 0
0007 09 0 Of
eee gh a 0
ee ee oo 7 1 G,—»——_ &
o 140 0 1 0 O
idee ex eee eee Ome )tea0b eat Ly
010 t 0 1 1 0 By
0100 01% 18 Gs
sialon Obeaestee 01100
+ 40% 4001
+ 41 4 4 0 1 0 8,
+ 410 4 01 1 Ge
+ 0 10 41 0 0
7
+ 004 417 1 0 8,
Hee vet ons MMe ee REET EEE &,
(@) Conversion fable (6) Logic diagram
GG, GG;
GAX_0 oft 10 GeX_0__01_11_10
7—] 3a] Weel asooa
| 00
ya 3-79
gu Ce a
aa 9
a "
33} 7] Fg
calla | —3| 7} 9
ttt to Ga a 4)
B.=G,
* =G, 06.
K-map fc pen aGs
G6, uP EBs K-map for By
G,GX_00_o1 es
77 Gev_%0 01 11_ 10
09] 73a
00] i 1
0 yy
of A a
a
7
id or
o/ Co
7 z 7
wat tat
8, =G,@G6,6G, ni
K-map for By G,86,26, 26,
(©) K-maps Kmap for B,
Figure 7.33 4-bit Gray-to-binary code convertera
COMBINATIONAL LOGIC DESIGN 309
pesign of a 4-bit Binary-to-BCD Code Converter
a 4bit binary. There are 16 possible
digits). the output has to be an 8-bit one; but since een ot il allbea 9
mbinations of inputs the output can be treated as a bit one The eo pesca ie i
ble in Figure 7.34a, From the conversion table, we ier eee
   
 
for? conversion tal
i ouput ate 38 follows;
ft A=Em(10, 11, 12, 13, 14, 15)
B=Em@,9)
C=EmG, 5, 6,7, 14, 15)
D=Em(2, 3,6, 7, 12, 13)
 
E=m(1, 3,5, 7,9, 11, 13, 15)
prawing the K-maps for the outputs and minimizing them as shown in Figure 7.34c the
al expressions for the BCD outputs A, B,C, D, and E in terms of the 4-bit binary inputs By
dB, are as follows:
A=B,B,+B,B,
B=B,B,B,
| C=B,B,+B,B,
D=B,B,B, + B,B,
E=B,
logic diagram can be drawn based on the above minimal expressions.
sisi
ByBy amt
 
‘4-bit binary BCD output.
Decimal 5p, BB, A B CODE
jsaq4107 070K OF gO ,10,,'0).0110
1 Dy 07 0 face mgs, On 1
it JOO at OF, 4 0m OL Ot (0.
ght vattigne oC 48 Al itroY OF OTT ft
Aso iog 0 gies Oss Otaueen (ORs esti on?
Be Ones eo 50. t
6 0, chs i Occ On Ob bs 2
7 0 S138 aie 1 opto stat
8 4+ \Oig OW Os eOlmnt 40x 0'7O
9 jade Darth gig Cute Oke. |
10 nt TOP rove eon OO
i TEOMA fMFOHTO® OF 1 3
2 1 100 it
: ‘alee eaPoaioaetmegero ts: 4 SOP circuit c
4 te, fe aOR 1 0150 e
15 1 (dual dats Oo1 5
(0) Block diagram
(a) Conversion table
Figure 7.34 4-bit binary-to-BCD code converter (Contd.)‘ALS OF DIGITAL CIRCUITS
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
8,8, BB,
Mee. pou mo Ot 10) BN OO Ott
eee easier ol | a
00
clean a
on of Ot Tey
vata 2} —73]] 9
1 "1 fal
=o ag |e ai} a
10| [4] 10
A=B,B, + B,B, B=8,8,8, C=B,B,+8,8,
K-map for A K-map for B K-map for C
8.8, BB,
BB\_0 or 1110 BB\_00 or 11 __ 10
eae geal a
00 1/7 00 ce
rs 7 a
on i|4 ot 1
a] 5a ce
ie " +i 4
oa ee aA 99
po 10 1] 4
D=8,8,8,+5,8, E=8,
KemapferD 6 kcanaps Kemap for E
Figure 7.34 4-bit binary-to-BCD code converter,7.18 COMPARATORS
A comparator is a logic circuit used to compare the magnitudes of two binary numbers. Depending
on the design, it may either simply provide an output that is active (goes HIGH for example) whe
the two numbers are equal, or additionally provide outputs that signify which of the numbers
greater when equality does not hold.
The X-NOR gate (coincidence gate) is a basic comparator, because its output is a1 only ii
‘wo input bits are equal, ie. the output is 1 iff and only if the input bits coincide.COMBINATIONAL LOGIC DESIGN 327
binary numbers are e F
nots eee cual ifand only if all their corresponding bits coincide. For example,
woe aN A By This, fs uo and BBB By are equal, if and only if, A, = B,, A. .
428, and Ag= Bo, Ths, equality hols when A, coincides with B,, A, coincides with B,, A,
Ades with Band Ay coincides with B,, The implementation of this logic ee
EQUALITY = (A, © :
/ B (As OBA, OBA, OB, (A, OB,)
 sraightforward. It is obvious that this cieuite a
5s areata a can be expanded or compressed to accommodate
‘The block diagram of a 1-bit comparator which c: fc f
target numbers is shown in Figure 7.5 is which can be used as a module for comparison of
     
 
 
L
E
6
Figure 7.55 Block diagram of a 1-bit comparator,
A
bit
‘comparator
B
 
Magnitude Comparator
 
718.1 1-1
‘The logic for a 1-bit magnitude comparator: Let the 1-bit numbers be A= A, and B= By.
IfA,= 1 and By = 0, then A> B
Therefore,
A>B:G=
 
IfA)=0 and By= 1, then A  7
 
 
 
 
 
 
 
 
 
 
 
a eee
Oi? Oe pate Beato)
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Figure 7.56 1-bit comparator.-Latthe wo 2-bitmumbers BEA=A,Ay and Bp, 9
 
 
   
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(A> B):G=A,B, + (A, ©B)A,B, + (4 © BA, 0 BAB,
+ (A, OBA, OB, XA, © B,)A,!COMBINATIONAL LOGIC DESIGN 329
similarly, the logic expression for A < B can be written as
(A B, A < B, and A=B.
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