# Digital Logic Circuits - Kaist Digital Logic Circuits 1 Introduction DIGITAL LOGIC CIRCUITS Logic Gates Boolean Algebra Map Specification Combinational Circuits Flip-Flops Sequential Circuits Memory Components Integrated Circuits Computer Organization Prof. H. Yoon Digital Logic Circuits Logic Gates 2 LOGIC GATES Digital Computers - Imply that the computer deals with digital information, i.e., it deals with the information that is represented by binary digits - Why BINARY ? instead of Decimal or other number system ? * Consider electronic signal 7 6 5 signal 4 3 range 2 1 0 1 0 binary * Consider the calculation cost - Add 0 1 0 0 1 1 1 10 Computer Organization octal 0 1 2 3 4 5 6 7 8

9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 1 2 3 4 5 6 7 8 9 10 2 3 4 5 6 7 8 9 1011 3 4 5 6 7 8 9 101112 4 5 6 7 8 9 10111213 5 6 7 8 9 1011121314 6 7 8 9 101112131415 7 8 9 10111213141516 8 9 1011121314151617 9 101112131415161718 Prof. H. Yoon Digital Logic Circuits Logic Gates 3 BASIC LOGIC BLOCK - GATE Binary Digital Input Signal . . . Gate Binary Digital Output Signal Types of Basic Logic Blocks - Combinational Logic Block Logic Blocks whose output logic value depends only on the input logic values - Sequential Logic Block Logic Blocks whose output logic value depends on the input values and the state (stored information) of the blocks Functions of Gates can be described by - Truth Table - Boolean Function - Karnaugh Map Computer Organization Prof. H. Yoon Digital Logic Circuits Logic Gates 4 COMBINATIONAL GATES Name AND

OR Symbol A B A B Function X X X=AB or X = AB X=A+B I A X X=A Buffer A X X=A NAND A B NOR XOR A B A Exclusive OR B XNOR A Exclusive NOR or Equivalence B Computer Organization X

X = (AB) X X = (A + B) X X=AB or X = AB + AB X X = (A B) or X = AB+ AB Truth Table A 0 0 1 1 A 0 0 1 1 A 0 0 1 1 A 0 0 1 1 A 0 0 1 1 A 0 0 1 1 B 0 1 0 1 B 0 1 0 1

A 0 1 A 0 1 B 0 1 0 1 B 0 1 0 1 B 0 1 0 1 B 0 1 0 1 X 0 0 0 1 X 0 1 1 1 X 1 0 X 0 1 X 1 1 1 0 X 1 0 0 0 X 0 1 1 0 X 1 0 0 1

Prof. H. Yoon Digital Logic Circuits 5 Boolean Algebra BOOLEAN ALGEBRA Boolean Algebra * Algebra with Binary(Boolean) Variable and Logic Operations * Boolean Algebra is useful in Analysis and Synthesis of Digital Logic Circuits - Input and Output signals can be represented by Boolean Variables, and - Function of the Digital Logic Circuits can be represented by Logic Operations, i.e., Boolean Function(s) - From a Boolean function, a logic diagram can be constructed using AND, OR, and I Truth Table * The most elementary specification of the function of a Digital Logic Circuit is the Truth Table - Table that describes the Output Values for all the combinations of the Input Values, called MINTERMS - n input variables 2n minterms Computer Organization Prof. H. Yoon Digital Logic Circuits Boolean Algebra 6 LOGIC CIRCUIT DESIGN x 0 0 0 0 1 1 1 1 Truth Table Boolean Function Logic Diagram Computer Organization y 0 0 1 1

0 0 1 1 z 0 1 0 1 0 1 0 1 F 0 1 0 0 1 1 1 1 F = x + yz x y F z Prof. H. Yoon Digital Logic Circuits Boolean Algebra 7 BASIC IDENTITIES OF BOOLEAN ALGEBRA  x + 0 = x  x + 1 = 1  x + x = x  x + x = 1  x + y = y + x  x + (y + z) = (x + y) + z  x(y + z) = xy +xz  (x + y) = xy  (x) = x  x 0 = 0  x 1 = x  x x = x  x X = 0  xy = yx  x(yz) = (xy)z  x + yz = (x + y)(x + z)  (xy) = x + y  and  : De Morgans Theorem Usefulness of this Table

- Simplification of the Boolean function - Derivation of equivalent Boolean functions to obtain logic diagrams utilizing different logic gates -- Ordinarily ANDs, ORs, and Inverters -- But a certain different form of Boolean function may be convenient to obtain circuits with NANDs or NORs Applications of De Morgans Theorem xy = (x + y) I, AND NOR Computer Organization x+ y= (xy) I, OR NAND Prof. H. Yoon Digital Logic Circuits Boolean Algebra 8 EQUIVALENT CIRCUITS Many different logic diagrams are possible for a given Function F = ABC + ABC + AC = AB(C + C) + AC = AB 1 + AC = AB + AC (1) ....... (1)  ... (2)   .... (3) A B C F (2) A B F C (3) A B F C Computer Organization Prof. H. Yoon Digital Logic Circuits

9 Boolean Algebra COMPLEMENT OF FUNCTIONS A Boolean function of a digital logic circuit is represented by only using logical variables and AND, OR, and Invert operators. Complement of a Boolean function - Replace all the variables and subexpressions in the parentheses appearing in the function expression with their respective complements A,B,...,Z,a,b,...,z A,B,...,Z,a,b,...,z (p + q) (p + q) - Replace all the operators with their respective complementary operators AND OR OR AND - Basically, extensive applications of the De Morgans theorem (x1 + x2 + ... + xn ) x1x2... xn (x1x2 ... xn)' x1' + x2' +...+ xn' Computer Organization Prof. H. Yoon Digital Logic Circuits Map Simplification 10 SIMPLIFICATION Boolean Function Truth Table Many different expressions exist Unique Simplification from Boolean function - Finding an equivalent expression that is least expensive to implement - For a simple function, it is possible to obtain a simple expression for low cost implementation - But, with complex functions, it is a very difficult task Karnaugh Map (K-map) is a simple procedure for simplifying Boolean expressions. Truth Table Karnaugh Map Boolean function Computer Organization Simplified Boolean Function Prof. H. Yoon Digital Logic Circuits

Map Simplification 11 KARNAUGH MAP Karnaugh Map for an n-input digital logic circuit (n-variable sum-of-products form of Boolean Function, or Truth Table) is - Rectangle divided into 2n cells - Each cell is associated with a Minterm - An output(function) value for each input value associated with a mintern is written in the cell representing the minterm 1-cell, 0-cell Each Minterm is identified by a decimal number whose binary representation is identical to the binary interpretation of the input values of the minterm. x 0 1 x 0 0 1 1 x 0 1 F 1 0 y 0 1 0 1 F 0 1 1 1 Computer Organization 0 Identification of the cell 1 y x 0 1 0 0 1 1 2 3 Karnaugh Map value x

of F 0 0 1 1 F(x) = (0) 1-cell y0 1 x 0 0 1 1 1 1 F(x,y) = (1,2) Prof. H. Yoon Digital Logic Circuits Map Simplification 12 KARNAUGH MAP x 0 0 0 0 1 1 1 1 y 0 0 1 1 0 0 1 1 z 0 1 0 1 0 1 0 1 F 0 1 1 0 1 0 0 0 u 0 0 0

0 0 0 0 0 1 1 1 1 1 1 1 1 v 0 0 0 0 1 1 1 1 0 0 0 0 1 1 1 1 w 0 0 1 1 0 0 1 1 0 0 1 1 0 0 1 1 x 0 1 0 1 0 1 0 1 0 1 0 1

0 1 0 1 y yz x 00 01 11 10 0 0 1 3 2 x 1 4 5 7 6 z F 0 1 0 1 0 0 1 0 1 1 0 1 0 0 1 0 yz x 00 01 11 10 0 0 1 0 1 1 1 0 0 0 F(x,y,z) = (1,2,4) w wx uv 00 01 11 10 00 0 1 3 2 v 01 4 5 7 6 12 13 15 14 u 11 10 8 9 11 10 x wx uv 00 01 11 10 00 0 1 1 0 01 0 0 0 1 11 0 0 0 1 10 1 1 1 0 F(u,v,w,x) = (1,3,6,8,9,11,14) Computer Organization Prof. H. Yoon Digital Logic Circuits Map Simplification 13 MAP SIMPLIFICATION - 2 ADJACENT CELLS Rule: xy +xy = x(y+y) = x

Adjacent cells - binary identifications are different in one bit minterms associated with the adjacent cells have one variable complemented each other Cells (1,0) and (1,1) are adjacent Minterms for (1,0) and (1,1) are x y --> x=1, y=0 x y --> x=1, y=1 F = xy+ xy can be reduced to F = x From the map y x 0 1 0 0 0 1 1 1 2 adjacent cells xy and xy merge them to a larger cell x F(x,y) = (2,3) = xy+ xy =x Computer Organization Prof. H. Yoon Digital Logic Circuits Map Simplification 14 MAP SIMPLIFICATION - MORE THAN 2 CELLS uvwx + uvwx + uvwx + uvwx = uvw(x+x) + uvw(x+x) = uvw + uvw = uv(w+w) = uv uv wx wx ux w w uv uv 1 1 1 1 1 1 1 1 vw 1 1 1 1 v v 1 1 1 1 u u 1 1 1 1 uw x x vx

uvwx+uvwx+uvwx+uvwx+uvwx+uvwx+uvwx+uvwx = uvw(x+x) + uvw(x+x) + uvw(x+x) + uvw(x+x) = u(v+v)w + u(v+v)w = (u+u)w = w wx w w V uv uv 1 1 1 1 1 1 w 1 1 v v 1 1 u u 1 1 1 1 u 1 1 1 1 1 1 x x Computer Organization Prof. H. Yoon Digital Logic Circuits Map Simplification 15 MAP SIMPLIFICATION wx uv 00 00 1 01 0 11 0 10 0 01 1 0 1 1 11 10 0 1 0 0 1 0 0 0 (0,1), (0,2), (0,4), (0,8) Adjacent Cells of 1 Adjacent Cells of 0 (1,0), (1,3), (1,5), (1,9) ... ...

Adjacent Cells of 15 (15,7), (15,11), (15,13), (15,14) w u 1 0 0 0 1 0 1 1 0 1 0 0 1 0 0 0 v x F(u,v,w,x) = (0,1,2,9,13,15) Merge (0,1) and (0,2) --> uvw + uvx Merge (1,9) --> vwx Merge (9,13) --> uwx Merge (13,15) --> uvx F = uvw + uvx + vwx + uwx + uvx But (9,13) is covered by (1,9) and (13,15) F = uvx + vwx + uvx Computer Organization Prof. H. Yoon Digital Logic Circuits Map Simplification 16 IMPLEMENTATION OF K-MAPS - Sum-of-Products Form Logic function represented by a Karnaugh map can be implemented in the form of I-AND-OR A cell or a collection of the adjacent 1-cells can be realized by an AND gate, with some inversion of the input variables. y x y z 1 1

x 1 z F(x,y,z) = (0,2,6) x y z x y z x y z Computer Organization x y z x y z F x z 1 y z 1 1 x z F y z I AND OR Prof. H. Yoon Digital Logic Circuits Map Simplification 17 IMPLEMENTATION OF K-MAPS - Product-of-Sums Form Logic function represented by a Karnaugh map can be implemented in the form of I-OR-AND If we implement a Karnaugh map using 0-cells, the complement of F, i.e., F, can be obtained. Thus, by complementing F using DeMorgans

theorem F can be obtained y F(x,y,z) = (0,2,6) x x y 1 0 0 1 0 0 0 1 z z F = xy + z F = (xy)z = (x + y)z x y F z I Computer Organization OR AND Prof. H. Yoon Digital Logic Circuits 18 IMPLEMENTATION OF K-MAPS - Dont-Care Conditions - Map Simplification In some logic circuits, the output responses for some input conditions are dont care whether they are 1 or 0. In K-maps, dont-care conditions are represented by ds in the corresponding cells. Dont-care conditions are useful in minimizing the logic functions using K-map. - Can be considered either 1 or 0 - Thus increases the chances of merging cells into the larger cells --> Reduce the number of variables in the product terms y x x y z Computer Organization 1 d d d z x

1 1 yz F Prof. H. Yoon Digital Logic Circuits Combinational Logic Circuits 19 COMBINATIONAL LOGIC CIRCUITS x 0 0 1 1 Half Adder Full Adder x y 0 0 0 0 0 1 0 1 1 0 1 0 1 1 1 1 cn-1 cn 0 0 1 0 0 0 1 1 0 0 1 1 0 1 1 1 x y cn-1 Computer Organization y 0 1 0 1

c 0 0 0 1 s 0 1 1 0 1 0 0 1 y 0 1 x 1 0 s = xy + xy =x y y 0 0 x 0 1 c = xy s 0 1 1 0 y x x y c s y 0 0 0 1 c n-1 1 1 0 1 cn x 0 1 0 1 s 1 0 c

n-1 1 0 cn = xy + xcn-1+ ycn-1 = xy + (x y)cn-1 s = xycn-1+xycn-1+xycn-1+xycn-1 = x y cn-1 = (x y) cn-1 S cn Prof. H. Yoon Digital Logic Circuits 20 Combinational Logic Circuits COMBINATIONAL LOGIC CIRCUITS Other Combinational Circuits Multiplexer Encoder Decoder Parity Checker Parity Generator etc Computer Organization Prof. H. Yoon Digital Logic Circuits Combinational Logic Circuits 21 MULTIPLEXER 4-to-1 Multiplexer Select S1 S 0 0 0 0 1 1 0 1 1 Output Y I0 I1 I2 I3 I0 I1 I2 Y

I3 S0 S1 Computer Organization Prof. H. Yoon Digital Logic Circuits Combinational Logic Circuits 22 ENCODER/DECODER Octal-to-Binary Encoder D1 D2 A0 D3 D4 D5 D6 D7 A2 A1 2-to-4 Decoder E 0 0 0 0 1 A1 0 0 1 1 d A0 0 1 0 1 d D0 0 1 1 1 1 Computer Organization

D0 D1 1 0 1 1 1 D2 1 1 0 1 1 D3 1 1 1 0 1 A0 D1 D2 A1 E D3 Prof. H. Yoon Digital Logic Circuits Flip Flops 23 FLIP FLOPS Characteristics - 2 stable states - Memory capability - Operation is specified by a Characteristic Table 1 0 0 1 0 1 1 0-state 0 1-state In order to be used in the computer circuits, state of the flip flop should

have input terminals and output terminals so that it can be set to a certain state, and its state can be read externally. R S Computer Organization Q Q S 0 0 1 1 R 0 1 0 1 Q(t+1) Q(t) 0 1 indeterminate (forbidden) Prof. H. Yoon Digital Logic Circuits Flip Flops 24 CLOCKED FLIP FLOPS In a large digital system with many flip flops, operations of individual flip flops are required to be synchronized to a clock pulse. Otherwise, the operations of the system may be unpredictable. R Q c (clock) Q S Clock pulse allows the flip flop to change state only when there is a clock pulse appearing at the c terminal. We call above flip flop a Clocked RS Latch, and symbolically as S c R Q S

Q Q c R Q operates when clock is high Computer Organization operates when clock is low Prof. H. Yoon Digital Logic Circuits Flip Flops 25 RS-LATCH WITH PRESET AND CLEAR INPUTS P(preset) R Q c (clock) S Q clr(clear) Computer Organization S P Q c R clr Q S P Q c R clr Q S P Q c R clr Q S P Q c R clr Q Prof. H. Yoon Digital Logic Circuits Flip Flops 26 D-LATCH D-Latch

Forbidden input values are forced not to occur by using an inverter between the inputs Q E (enable) E Q D(data) D 0 1 Computer Organization D Q(t+1) 0 1 D E Q Q Q Q Prof. H. Yoon Digital Logic Circuits 27 Flip Flops EDGE-TRIGGERED FLIP FLOPS Characteristics - State transition occurs at the rising edge or falling edge of the clock pulse Latches respond to the input only during these periods Edge-triggered Flip Flops (positive) respond to the input only at this time Computer Organization Prof. H. Yoon Digital Logic Circuits Flip Flops 28 POSITIVE EDGE-TRIGGERED D-Flip Flop D

S1 Q1 SR1 C1 R1 Q1' Q S2 Q2 SR2 C2 R2 Q' Q2' D Q D-FF C Q' C SR1 inactive SR2 active SR2 inactive SR1 active JK-Flip Flop J K C S1 Q1 SR1 C1 R1 Q1' SR2 inactive SR1 active S2 Q2 SR2 C2 R2 Q2'

Q Q' J Q C K Q' T-Flip Flop: JK-Flip Flop whose J and K inputs are tied together to make T input. Toggles whenever there is a pulse on T input. Computer Organization Prof. H. Yoon Digital Logic Circuits Flip Flops 29 CLOCK PERIOD Clock period determines how fast the digital circuit operates. How can we determine the clock period ? Usually, digital circuits are sequential circuits which has some flip flops FF FF ... FF C . . . FF FF Delay Combinational Logic Circuit . . . Combinational Logic Circuit Combinational logic Delay td FF FF Setup Time FF Hold Time ts,th clock period T = td + ts + th Computer Organization Prof. H. Yoon

Digital Logic Circuits Sequential Circuits 30 DESIGN EXAMPLE Design Procedure: Specification State Diagram State Table Excitation Table Karnaugh Map Circuit Diagram Example: 2-bit Counter -> 2 FF's x=0 x=1 x=0 x=1 01 11 x=1 x=1 x=0 B 1 x d d A A d d Ja Ja = Bx 00 x=0 10 B d d d d x 1 Ka Ka = Bx Computer Organization current state A B 0 0 0 0 0 1 0 1 1 0 1 0 1 1 1 1

B B d d 1 d x d 1 x d 1 A 1 d A d d Jb Kb Jb = x Kb = x next input state x A B 0 0 0 1 0 1 0 0 1 1 1 0 0 1 0 1 1 1 0 1 1 1 0 0 x FF inputs Ja Ka Jb Kb 0 d 0 d 0 d 1 d 0 d d 0 1 d d 1 d 0 0 d d 0 1 d d 0 d 0 d 1 d 1 J Q C K Q' A J Q C K Q' B

clock Prof. H. Yoon Digital Logic Circuits Sequential Circuits 31 SEQUENTIAL CIRCUITS - Registers A0 A1 Q A2 Q D C A3 Q D C Q D C D C Clock I0 Shift Registers Serial Input I1 D Q C D Q C I2 I3 D Q C D Q C Serial Output Clock

Bidirectional Shift Register with Parallel Load A0 A1 A2 A3 Q Q Q Q D C 4x1 MUX Clock S0S1 SeriaI I0 Computer Organization Input D C D C D C 4x1 MUX 4x1 MUX I1 4x1 MUX I2 Serial I 3 Input Prof. H. Yoon Digital Logic Circuits Sequential Circuits 32 SEQUENTIUAL CIRCUITS - Counters

A0 A1 A2 A3 Q Q Q Q J K J K J K J K Clock Counter Enable Output Carry Computer Organization Prof. H. Yoon Digital Logic Circuits Memory Components 33 MEMORY COMPONENTS 0 Logical Organization words (byte, or n bytes) Random Access Memory N-1 - Each word has a unique address - Access to a word requires the same time independent of the location of the word - Organization

n data input lines k address lines Read 2k Words (n bits/word) Write n data output lines Computer Organization Prof. H. Yoon Digital Logic Circuits Memory Components 34 READ ONLY MEMORY(ROM) Characteristics - Perform read operation only, write operation is not possible - Information stored in a ROM is made permanent during production, and cannot be changed - Organization k address input lines m x n ROM (m=2k) n data output lines Information on the data output line depends only on the information on the address input lines. --> Combinational Logic Circuit address X0=AB + BC X1=ABC + ABC X2=BC + ABC X3=ABC + AB X4=AB Canonical minterms Computer Organization X0=ABC + ABC + ABC X1=ABC + ABC X2=ABC + ABC + ABC X3=ABC + ABC + ABC X4=ABC + ABC ABC 000 001 010 011 100 101 110 111 Output X0 X1 X2 X3 X4

1 1 0 0 0 1 0 0 0 1 1 0 0 0 0 0 0 0 0 1 1 0 0 1 0 0 1 0 1 1 0 0 0 0 0 0 0 0 1 1 Prof. H. Yoon Digital Logic Circuits 35 Memory Components TYPES OF ROM ROM - Store information (function) during production - Mask is used in the production process - Unalterable - Low cost for large quantity production --> used in the final products PROM (Programmable ROM)

- Store info electrically using PROM programmer at the users site - Unalterable - Higher cost than ROM -> used in the system development phase -> Can be used in small quantity system EPROM (Erasable PROM) - Store info electrically using PROM programmer at the users site - Stored info is erasable (alterable) using UV light (electrically in some devices) and rewriteable - Higher cost than PROM but reusable --> used in the system development phase. Not used in the system production due to erasability Computer Organization Prof. H. Yoon Digital Logic Circuits 36 Memory Components INTEGRATED CIRCUITS Classification by the Circuit Density SSI MSI LSI VLSI - several (less than 10) independent gates 10 to 200 gates; Perform elementary digital functions; Decoder, adder, register, parity checker, etc 200 to few thousand gates; Digital subsystem Processor, memory, etc Thousands of gates; Digital system Microprocessor, memory module Classification by Technology TTL - Transistor-Transistor Logic Bipolar transistors NAND ECL Emitter-coupled Logic Bipolar transistor NOR MOS - Metal-Oxide Semiconductor Unipolar transistor High density CMOS - Complementary MOS Low power consumption Computer Organization Prof. H. Yoon

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