DIGITAL LOGIC CIRCUITS

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1 DIGITAL LOGIC CIRCUITS Introduction Logic Gates Boolean Algebra Map Specification Combinational Circuits Flip-Flops Sequential Circuits Memory Components Integrated Circuits

2 Digital Computers 2 LOGIC GATES Logic Gates - 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 binary octal signal range * Consider the calculation cost - Add

3 3 BASIC LOGIC BLOCK - GATE - Logic Gates 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

4 A B 4 COMBINATIONAL GATES Name Symbol Function Truth Table AND OR A B A B X X X X = A B or X = AB X = A + B I A X X = A Buffer A X X = A NAND A X X = (AB) B NOR XOR Eclusive OR XNOR Eclusive NOR or Equivalence A B A B X X X = (A + B) X = A B or X = A B + AB X = (A B) or X = A B + AB A B X A B X A X 0 0 A X 0 0 A B X A B X A B X A B X Logic Gates

5 Boolean Algebra 5 BOOLEAN ALGEBRA Boolean Algebra * Algebra with Binary(Boolean) Variable and Logic Operations * Boolean Algebra is useful in Analysis and Synthesis of Digital Logic Circuits Truth Table - 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 * 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 2 n minterms

6 Truth Table 6 LOGIC CIRCUIT DESIGN y z F Boolean Algebra Boolean Function F = + y z Logic Diagram y z F

7 7 BASIC IDENTITIES OF BOOLEAN ALGEBRA [] + 0 = [3] + = [5] + = [7] + = [9] + y = y + [] + (y + z) = ( + y) + z [3] (y + z) = y +z [5] ( + y) = y [7] ( ) = [2] 0 = 0 [4] = [6] = [8] X = 0 [0] y = y [2] (yz) = (y)z [4] + yz = ( + y)( + z) [6] (y) = + y [5] and [6] : De Morgan s 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 y = ( + y) + y = (y) I, AND NOR I, OR NAND Boolean Algebra

8 8 EUIVALENT CIRCUITS Boolean Algebra Many different logic diagrams are possible for a given Function F = ABC + ABC + A C... () = AB(C + C ) + A C [3]... (2) = AB + A C [7] = AB + A C [4].... (3) () A B C F (2) A B C F (3) A B C F

9 9 COMPLEMENT OF FUNCTIONS Boolean Algebra 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 subepressions in the parentheses appearing in the function epression 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, etensive applications of the De Morgan s theorem ( n ) 2... n ( 2... n )' ' + 2 ' n '

10 0 SIMPLIFICATION Map Simplification Truth Table Unique Boolean Function Many different epressions eist Simplification from Boolean function - Finding an equivalent epression that is least epensive to implement - For a simple function, it is possible to obtain a simple epression for low cost implementation - But, with comple functions, it is a very difficult task Karnaugh Map (K-map) is a simple procedure for simplifying Boolean epressions. Truth Table Boolean function Karnaugh Map Simplified Boolean Function

11 KARNAUGH MAP Map Simplification 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 2 n 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 -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. F Identification of the cell Karnaugh Map value 0 0 of F F() = (0) y F y cell y F(,y) = (,2)

12 y z F u v w F KARNAUGH MAP yz y z yz uv w w u uv w F(,y,z) = (,2,4) F(u,v,w,) = (,3,6,8,9,,4) v Map Simplification

13 3 Map Simplification MAP SIMPLIFICATION - 2 ADJACENT CELLS - Adjacent cells Rule: y +y = (y+y ) = - binary identifications are different in one bit minterms associated with the adjacent cells have one variable complemented each other Cells (,0) and (,) are adjacent Minterms for (,0) and (,) are y --> =, y=0 y --> =, y= F = y + y can be reduced to F = From the map y adjacent cells y and y merge them to a larger cell F(,y) = (2,3) = y + y =

14 4 Map Simplification MAP SIMPLIFICATION - MORE THAN 2 CELLS - u v w + u v w + u v w + u v w = u v w ( +) + u v w(+ ) = u v w + u v w = u v (w +w) = u v w uv w vw u u v v uw w uv w u v v u u v w +u v w +u vw +u vw +uvw +uvw +uv w +uv w = u v w ( +) + u vw ( +) + uvw ( +) + uv w ( +) = u (v +v)w + u(v +v)w = (u +u)w = w uv w w u w v uv u w v V u

15 5 MAP SIMPLIFICATION Map Simplification uv w (0,), (0,2), (0,4), (0,8) Adjacent Cells of Adjacent Cells of 0 (,0), (,3), (,5), (,9) Adjacent Cells of 5 (5,7), (5,), (5,3), (5,4) w v 0 0 u F(u,v,w,) = (0,,2,9,3,5) Merge (0,) and (0,2) --> u v w + u v Merge (,9) --> v w Merge (9,3) --> uw Merge (3,5) --> uv F = u v w + u v + v w + uw + uv But (9,3) is covered by (,9) and (3,5) F = u v + v w + uv

16 6 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 -cells can be realized by an AND gate, with some inversion of the input variables. y z z y F(,y,z) = (0,2,6) y z y z z Map Simplification y z y z y z y z F z y z F I AND OR

17 7 Map Simplification 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 DeMorgan s theorem F can be obtained F(,y,z) = (0,2,6) y y z z F = y + z F = (y )z = ( + y)z y z F I OR AND

18 8 IMPLEMENTATION OF K-MAPS - Don t-care Conditions - Map Simplification In some logic circuits, the output responses for some input conditions are don t care whether they are or 0. In K-maps, don t-care conditions are represented by d s in the corresponding cells. Don t-care conditions are useful in minimizing the logic functions using K-map. - Can be considered either or 0 - Thus increases the chances of merging cells into the larger cells --> Reduce the number of variables in the product terms y d d d z yz y z F

19 9 COMBINATIONAL LOGIC CIRCUITS Half Adder y c s Full Adder y c n- c n s y c n- y c = y c n y 0 c n- y 0 0 s = y + y = y c n = y + c n- + yc n- = y + ( y)c n- s = y c n- + yc n- +y c n- +yc n- = y c n- = ( y) c n- S c n 0 0 s y 0 0 Combinational Logic Circuits c n- y c s

20 20 Combinational Logic Circuits COMBINATIONAL LOGIC CIRCUITS Other Combinational Circuits Multipleer Encoder Decoder Parity Checker Parity Generator etc

21 2 MULTIPLEXER Combinational Logic Circuits 4-to- Multipleer Select Output S S 0 Y 0 0 I 0 0 I 0 I 2 I 3 I 0 I I 2 Y I 3 S 0 S

22 22 ENCODER/DECODER Combinational Logic Circuits Octal-to-Binary Encoder D D 2 D 3 D 4 D 5 D 6 D 7 A 0 A A 2 2-to-4 Decoder D 0 E A A 0 D 0 D D 2 D d d A 0 A E D D 2 D 3

23 23 FLIP FLOPS Flip Flops Characteristics - 2 stable states - Memory capability - Operation is specified by a Characteristic Table state -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 eternally. R S S R (t+) 0 0 (t) indeterminate (forbidden)

24 24 CLOCKED FLIP FLOPS 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 c (clock) 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 operates when clock is high S c R operates when clock is low

25 25 Flip Flops RS-LATCH WITH PRESET AND CLEAR INPUTS P(preset) R c (clock) S clr(clear) S P c R clr S P c R clr S P c R clr S P c R clr

26 26 D-LATCH Flip Flops D-Latch Forbidden input values are forced not to occur by using an inverter between the inputs D E (enable) E D(data) D D (t+) 0 0 E

27 27 EDGE-TRIGGERED FLIP FLOPS 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

28 28 POSITIVE EDGE-TRIGGERED Flip Flops D-Flip Flop D C S SR C R ' S2 2 SR2 C2 R2 2' ' D D-FF C ' JK-Flip Flop SR inactive SR2 active SR2 inactive SR2 inactive SR active SR active J K C S SR C R ' S2 2 SR2 C2 R2 2' ' J C K ' 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.

29 29 CLOCK PERIOD Flip Flops 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... Combinational Logic Circuit... FF FF Delay Combinational Logic Circuit Combinational logic Delay t d FF FF Setup Time FF Hold Time t s,t h clock period T = t d + t s + t h

30 30 DESIGN EXAMPLE Design Procedure: Specification State Diagram State Table Ecitation Table Karnaugh Map Circuit Diagram Eample: 2-bit Counter -> 2 FF's =0 =0 = 0 = = = = =0 current net state input state FF inputs A B A B Ja Ka Jb Kb d 0 d d d d d d d d 0 0 d 0 d 0 d 0 d 0 d d d Sequential Circuits A B d d d d Ja A B d d d d Ka A B d d d d Jb A B d d d d Kb Ja = B Ka = B Jb = Kb = clock J C K ' A J C K ' B

31 3 SEUENTIAL CIRCUITS - Registers Sequential Circuits A 0 A A 2 A 3 D C D C D C D C Clock Shift Registers I 0 I I 2 I 3 Serial Input Clock D C Bidirectional Shift Register with Parallel Load D C D C D C A 0 A A 2 A 3 Serial Output D C D C D C D C 4 MUX 4 MUX 4 MUX 4 MUX Clock S 0 S SeriaI Input I 0 I I 2 I 3 Serial Input

32 32 SEUENTIUAL CIRCUITS - Sequential Circuits Counters A 0 A A 2 A 3 Clock J K J K J K J K Counter Enable Output Carry

33 33 MEMORY COMPONENTS 0 Logical Organization Memory Components words (byte, or n bytes) Random Access Memory N - - 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 2 k Words (n bits/word) Write n data output lines

34 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 Memory Components m n ROM (m=2 k ) Information on the data output line depends only on the information on the address input lines. --> Combinational Logic Circuit address Canonical minterms n data output lines X 0 =A B + B C X =A B C + A BC X 2 =BC + AB C X 3 =A BC + AB X 4 =AB X 0 =A B C + A B C + AB C X =A B C + A BC X 2 =A BC + AB C + ABC X 3 =A BC + AB C + AB C X 4 =ABC + ABC Output ABC X 0 X X 2 X 3 X

35 35 TYPES OF ROM Memory Components 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 user s 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 user s 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

36 36 INTEGRATED CIRCUITS Memory Components Classification by the Circuit Density SSI - MSI - LSI - VLSI - several (less than 0) independent gates 0 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 - ECL - MOS - Transistor-Transistor Logic Bipolar transistors NAND Emitter-coupled Logic Bipolar transistor NOR Metal-Oide Semiconductor Unipolar transistor High density CMOS - Complementary MOS Low power consumption

DIGITAL LOGIC CIRCUITS

DIGITAL LOGIC CIRCUITS Introduction Logic Gates Boolean Algebra Map Specification Combinational Circuits Flip-Flops Sequential Circuits Memor Components Integrated Circuits BASIC LOGIC BLOCK - GATE - Logic