combinational and sequential logic

MODULE 3 MCA-102 DIGITAL SYSTEMS & LOGIC DESIGN ADMN 2014-‘17

Dept. of Computer Science And Applications, SJCET, Palai Page 46

Combinational Logic

Logic circuits for digital systems may be combinational or sequential.

A combinational circuit consists of input variables, logic gates, and output variables.

Fig 3.1 combinational circuit

Hence, a combinational circuit can be described by:

1. A truth table that lists the output values for each combination of the input variables, or

2. m Boolean functions, one for each output variable.

Combinational vs. Sequential Circuits

Combinational circuits are memory-less. Thus, the output value depends ONLY on the current

input values.

Sequential circuits consist of combinational logic as well as memory elements (used to store certain

circuit states). Outputs depend on BOTH current input values and previous input values (kept in the

storage elements).

Fig 3.2 combinational circuits vs sequential circuits

Design Procedure

Given a problem statement:

● Determine the number of inputs and outputs



● Derive the truth table

● Simplify the Boolean expression for each output

● Produce the required circuit

Half Adder

A combinational circuit that performs the addition of two bits is called a half adder.

Fig 3.3 logical symbol of half adder

The truth table for the half adder is listed below:

x y C S

0 0 0 0

0 1 0 1

1 0 0 1

1 1 1 0

Fig 3.3 truth table for half adder

S = x’y + xy’

C = xy

Fig 3.4 half adder logic diagram



Full Adder

One that performs the addition of three bits (two significant bits and a previous carry) is a full

adder.

x y z C S

0 0 0 0 0

0 0 1 0 1

0 1 0 0 1

0 1 1 1 0

1 0 0 0 1

1 0 1 1 0

1 1 0 1 0

1 1 1 1 1

Fig 3.5 truth table for full adder

Fig 3.6 maps for full adder

S = x’y’z + x’yz’ + xy’z’ + xyz

C = xy + xz + yz

Alternative formulae using algebraic manipulation:

C = X.Y + X.Z + Y.Z

= X.Y + (X + Y).Z

= X.Y + ((X XOR Y) + X.Y).Z

= X.Y + (X XOR Y).Z + X.Y.Z

= X.Y + (X XOR Y).Z


Dept. of Computer Science And Applications, SJCET, Palai 9

S = X'.Y'.Z + X'.Y.Z' + X.Y'.Z' + X.Y.Z

= X‘. (Y'.Z + Y.Z') + X. (Y'.Z' + Y.Z)

= X'. (Y XOR Z) + X. (Y XOR Z)'

= X XOR (Y XOR Z) or (X XOR Y) XOR Z

Fig 3.7 full adder logic diagram

Fig 3.8 Full adder using two half adder

Parallel Binary Adders

• Parallel Adder is a digital circuit that produces the arithmetic sum of 2 binary numbers.

• Constructed with full adders connected in cascade, with output carry from each full adder

connected to the input carry of next full adder in the chain.

• The carries are connected in a chain through the full adders.

• Two methods to handle carries in parallel adder

1. Ripple carry: carry out of each FA is connected to the carry input of next FA

2. Carry Look ahead: it anticipates the output carry of each stage, and based on the input bits

of each stage, produces the output carry by either carry generation or carry propagation.


Dept. of Computer Science And Applications, SJCET, Palai 0

Carry generation: output carry is produced internally by the FA.carry is generated only when

both input bits are 1s.the generated carry C is expressed as the AND function

of two input bits A and B so C=AB.

Carry propagation: occurs when the i/p carry is rippled to become the o/p carry.an i/p carry

may be propagated by the full adder when either or both of the i/p bits are

1s.the propagated carry Cp is expressed as the OR function of the i/p bits ie

Cp=A+B

2- Bit Parallel Adder

• LSB of two binary numbers are represented by A1 and B1.

• The next higher bit are A2 and B2. The resulting t 1 2 and CO, in which the CO

becomes MSB.

• The carry output CO of each adder is connected as the carry input of the next higher order.

Fig 3.9 2bit adder using two full adder

Four Bit Parallel Adders

• An n-bit adder requires n full adders with each output connected to the input carry of the next

higher-order full adder.

• The carry output of each adder is connected to the carry input of next adder called as internal

carries.

A2

A1

+ B2

B1

C0 ∑

2 ∑

1



Fig 3.10 4 bit adder

The logic symbol for a 4-bit parallel adder is shown. This 4-bit adder includes a Carry In (labeled C0) and a

Carry Out (labeled C4).

Fig 3.11 truth table for 4 bit parallel adder

74283 Four-bit binary adder

7483 is an older chip that is functionally identical to the 74283, but the pins are laid out differently

Cascading Parallel Adders

When we connect the outputs from one circuit to the inputs of another identical circuit to expand the

number of bits being operated on, we say that the circuits are cascaded together.

For example, you can cascade two 4-bit parallel adders to add two 8-bit numbers. To do this, connect the

lower-order adder’s Carry Out to the higher-order adder’s Carry In.

http://focus.ti.com/lit/ds/symlink/sn74ls283.pdf

http://people.sinclair.edu/nickreeder/eet1131/datasheets/7483.pdf



Fig 3.12 cascading of parallel adders

Binary Subtraction

• The subtraction A-B can be performed by taking the 2's complement of B and adding to A.

• The 2's complement of B can be obtained by complementing B and adding one to the result.

A-B = A + 2C(B)

= A + 1C(B) + 1

= A + B’ + 1

Fig 3.13 binary substractor circuit

Adder/Substractor

Design requires:

(i) XOR gates:



(ii) S connected to carry-in.

Fig 3.14 adder/substractor circuit

• When S=0, the circuit performs A + B. The carry in is 0, and the XOR gates simply pass B

untouched.

• When S=1, the carry into the least significant bit (LSB) is 1, and B is complemented (1’s

complement) prior to the addition; hence, the circuit adds to A the 1’s complement of B plus 1

(from the carry into the LSB).

Magnitude Comparator

A magnitude comparator is a combinational circuit that compares two numbers, A and B, and then

determines their relative magnitudes.

A > B

A = B

A < B

Fig 3.15 logical symbol of magnitude comparator

Problem: Design a magnitude comparator that compares 2 4-bit numbers A and B and determines

whether:

A > B, or

A = B, or

A < B

Inputs

First n-bit number A



Second n-bit number B

Outputs

3 output signals (GT, EQ, LT), where:

GT = 1 IFF A > B

EQ = 1 IFF A = B

LT = 1 IFF A < B

Exactly One of these 3 outputs equals 1, while the other 2 outputs are 0`s.

Solution:

Inputs: 8-bits (A ⇒ 4-bits, B ⇒ 4-bits).A and B are two 4-bit numbers. Let A = A3A2A1A0, and

Let B = B3B2B1B0 .

Design of the EQ

Define Xi = Ai xnor Bi = Ai Bi + Ai’ Bi’

i = 1 IFF Ai = Bi ∀ i =0, 1, 2 and 3

i = 0 IFF Ai ≠ Bi

Therefore the condition for A = B or EQ=1 IFF

A3= B3 → (X3 = 1), and

A2= B2 → (X2 = 1), and

A1= B1 → (X1 = 1), and

A0= B0 → (X0 = 1).

Thus, EQ=1 IFF X3 X2 X1 X0 = 1. In other words, EQ = X3 X2 X1 X0

Designing GT and LT:

GT = 1 if A > B:

If A3 > B3 3 = 1 and B3 = 0

If A3 = B3 and A2 > B2

If A3 = B3 and A2 = B2 and A1 > A1

If A3 = B3 and A2 = B2 and A1 = B1 and A0 > B0

Therefore,

GT = A3B3‘+ X3 A2 B2‘+ X3 X2 A1 B1‘+ X3 X2 X1A0 B0‘

Similarly, LT = A3’B3 + X3 A2‘B2 + X3 X2 A1’B1 + X3 X2 X1A0’ B0



Fig 3.16 4 bit magnitude comparator

Decoder

• A decoder is a logic circuit that accepts a set of inputs that represents a binary number and activates

only the output that corresponds to the input number.

• A decoder has

• N inputs

• 2N outputs

• Exactly one output will be active for each combination of the inputs.

Fig 3.17 logical symbol of N-M decoder

• For each of these input combinations only one of the M outputs will be active HIGH (1), all the

other outputs are LOW (0). An AND gate can be used as the basic decoding element because it

produces a HIGH output only when all inputs are HIGH.



• If an active-LOW output (74138, one of the output will low and the rest will be high) is required for

each decoded number, the entire decoder can be implemented with NAND gates Inverters

• If an active-HIGH output (74139, one of the output will high and the rest will be low) is required

for each decoded number, the entire decoder can be implemented with AND gates Inverters

Decoder example

2-to-4-Line Decoder

Fig 3.18 2-4 decoder with active high output

Fig 3.19 logical symbol and truth table of 2-4 decoder in fig 3.19

3-8 line decoder (active-HIGH)

• It can be called a 3-line-to- 8-line decoder, because it has three input lines and eight output lines.

• It could also be called a binary-octal decoder or converters because it takes a three bit binary input

code and activates the one of the eight outputs corresponding to that code. It is also referred to as a

1-of-8 decoder, because only 1 of the 8 outputs is activated at one time.



Fig 3.20 3-8 decoder circuit

Fig 3.21 truth table for circuit in fig 3.21

Application example

A simplified computer I/O port system with a port address decoder with only four address lines

shown.



Fig 3.22 peripheral decoding in computer

• Computer must communicate with a variety of external devices called peripherals by sending and/or

receiving data through what is known as input/output (I/O) ports

• Each I/O port has a number, called an address, which uniquely identifies it. When the computer wants

to communicate with a particular device, it issues the appropriate address code for the I/O port to

which that particular device is connected. The binary port address is decoded and appropriate decoder

output is activated to enable the I/O port

• Binary data are transferred within the computer on a data bus, which is a set of parallel lines

BCD -to- Decimal decoders

• The BCD- to-decimal decoder converts each BCD code into one of Ten Positional decimal digit

indications. It is frequently referred as a 4-line -to- 10 line decoder

• The method of implementation is that only ten decoding gates are required because the BCD code

represents only the ten decimal digits 0 through 9.

• Each of these decoding functions is implemented with NAND gates to provide active -LOW

outputs. If an active HIGH output is required, AND gates are used for decoding



Fig 3. 23 a. logic circuit for BCD to Decimal decoder b. logical symbol of BCD to Decimal decoder IC

c. truth table

BCD to 7-Segment Display Decoder

Used to convert a BCD or a binary code

into a 7 segment code used to operate a 7 segment LED display. It generally has 4 input lines and 7

output lines. Here we design a simple display decoder circuit using logic gates.

Fig 3.24 logical symbol of BCD-7 segment decoder



Encoder

Encoders typically have 2N inputs and N outputs.

These are called 2N–to–N encoders.

Encoders can also be devised to encode various symbols and alphabetic characters.

The process of converting from familiar symbols or numbers to a coded format is called encoding.

Fig 3.25 Logical diagram of Encoder

8-to-3 encoder Implementation

Octal-to-Binary

An octal to binary encoder has 23

= 8 input lines D0 to D7 and 3 output lines Y0 to Y2. Below is the

truth table for an octal to binary encoder.

Fig 3. 26 truth table for 8-3 encoder

From the truth table, the outputs can be expressed by following Boolean Function.

Y0 = D1 + D3 + D5 + D7



Y1 = D2 + D3 + D6 + D7

Y2 = D4 + D5 + D6 + D7

Note: Above boolean functions are formed by ORing all the input lines for which output is 1. For instance

Y0 is 1 for D1, D3, D5, D7 input lines. The encoder can therefore be implemented with OR gates whose

inputs are determined directly from truth table as shown in the image below:

Fig 3.27 logic circuit for 8-3 encoder

Decimal – BCD encoder

• Encoder will produce a BCD output corresponding to the highest-order decimal digit input that is

active and will ignore any other lower order active inputs.

Fig 3.28 truth table for 10-4 encoder

From the truth table, the outputs can be expressed by following Boolean Function.

Note: Below Boolean functions are formed by OR ing all the input lines for which output is 1. For instance

A0 is 1 for 1,3,5,7 or 9 input lines.

A0 = 1+3+5+7+9



A1 = 2+3+6+7

A2 = 4+5+6+7

A3 = 8+9

The decimal to bcd encoder can therefore be implemented with OR gates whose inputs are determined

directly from truth table as shown in the image below.

Fig 3.29 Logic circuit and symbol for 10-4 encoder

Multiplexer

A multiplexer or mux is a device that selects one of several analog or digital input signals and

forwards the selected input into a single line. A multiplexer of 2n inputs has n select lines, which are

used to select which input line to send to the output.”

The select lines determine which input is connected to the output.

MUX Types

-to-1 (1 select line)

-to-1 (2 select lines)





Fig 3.30 multiplexer block diagram

4-to-1 Multiplexer (MUX)

Fig 3.31 logic circuit, symbol and table for 4-1 multiplexer

The Boolean expression for this 4-to-1 Multiplexer above with inputs D0 to D3 and data select lines A, B is

given as:

Y = A’B’D0 + A’B D1 + AB’D2 + ABD3

Demultiplexer

• A DEMUX is a digital switch with a single input (source) and a multiple outputs (destinations).

• The select lines determine which output the input is connected to.

• DEMUX Types



-to-2 (1 select line)




Fig 3.32 Demultiplexer circuit symbol

1-to-4 De-Multiplexer (DEMUX)

Fig 3.33 logic circuit for 1-4 demultiplexer, symbol and table

Parity Bit Generator

The most common error detection code used is the parity bit.

A parity bit is an extra bit included with a binary message to make the total number of 1's either odd

or even.

A parity bit added to n-bit code produces (n+1)-bit code with an odd (or even) count of 1s

Odd Parity bit: count of 1s in (n+1)-bit code is odd

o So use an even function to generate the odd parity bit

Even Parity bit: count of 1s in (n+1)-bit code is even

o So use an odd function to generate the even parity bit



To check for odd parity

o Use an even function to check the (n+1)-bit code

To check for even parity

o Use an odd function to check the (n+1)-bit code

Even Parity Generators and Checkers for 3-bit codes

An even parity bit could be added to n-bit code to produce an n + 1 bit code:

• Use an odd function to produce codes with even parity

• Use odd function circuit to check code words with even parity

Example: n = 3. Generate even parity code words of length 4 with an odd function circuit (parity

generator):

The design procedure is made simple by writing the truth table for the circuit.

Fig 3.34 truth table fon parity bit generator

Fig 3.35 K-map for the truth table in fig 3.35

From this the minimal output equation is

This function can be implemented using exclusive-or gates, shown in fig 3.37.Similarly the checker circuit

can be designed using XOR gates, where



Fig 3.36 even parity generator and checker

Operation: (X,Y,Z) = (0,0,1) gives (X,Y,Z,P) = (0,0,1,1) and E = 0.If Y changes from 0 to 1

between generator and checker, then E = 1 indicates an error.

Odd Parity Generators and Checkers

Similarly, an odd parity bit could be added to n-bit code to produce an n + 1 bit code

• Use an even function to produce codes with odd parity

• Use even function circuit to check code words with odd parity

Message

X Y Z

Odd Parity

Generator

P

Checker Bit

C

0 0 0 1 0

0 0 1 0 0

0 1 0 0 0

0 1 1 1 0

1 0 0 0 0

1 0 1 1 0

1 1 0 1 0

1 1 1 0 0

Fig 3.37 Truth table for Odd parity generator

C



1

1

1

1

Fig 3.38 k-map for fig 3.39

From this the minimal output equation is

P=X’Y’Z’+X’YZ’+XYZ’+XY’Z

This function can be implemented using XOR and XNOR gates, shown in fig 3.40.Similarly the checker

circuit can be designed using XOR and XNOR gates, where

Fig 3.39 odd parity generator and checker circuit

Fig 3.40 Odd parity checker circuit

PLA [Programmable Logic Array] Combination of a programmable AND array followed by a programmable OR array.

This layout allows for a large number of logic functions to be synthesized in the sum of products

(and sometimes product of sums) canonical forms.

00

01

11

10

0 1

Z

Y

X

P

Y

X

C

P

Z



Fig 3.41 block diagram of PLA

Example :Design a PLA to realize the following logic functions

f0 (A'.B'+ A. C')

f1 (A. C'+ B)

f2 (A'.B'+ B. C’)

f3 (B+ AC)

Fig 3.42 PLA table

Fig 3.43 realisation of PLA



Example: Design a PLA to realise the following three logic functions

f1 (A, B, C, D, E) = A'.B'.D' + B'.C.D' + A'.B.C.D.E'

f2 (A, B, C, D, E) = A'.B.E + B'.C.D'.E

f3 (A, B, C, D, E) = A'.B'.D' + B'.C'.D'.E + A'.B.C.D

Fig 3.44 realisation of PLA

SEQUENTIAL LOGIC CIRCUITS

Sequential circuits are constructed using combinational logic and a number of memory elements

with some or all of the memory outputs fed back into the combinational logic forming a feedback path or

loop.

Sequential circuit = Combinational logic + Memory Elements



Fig 3.45 sequential circuit

There are two types of sequential circuits:

synchronous: outputs change only at specific time

asynchronous: outputs change at any time

A state variable in a sequential circuit represents the single-bit variable Q stored in a memory element in

circuit.

– Each memory element may be in state 0 or state 1 depending on the current value stored in the memory

element.

• The State of A sequential Circuit:

– The collection of all state variables (memory element stored values) that at any time contain all the

information about the past necessary to account for the circuit’s future behavior.

– A sequential circuit that contains n memory elements could be in one of a maximum of 2n states at any

given time depending on the stored values in the memory elements.

– Sequential Circuit State transition: A change in the stored values in memory elements thus changing the

sequential circuit from one state to another.

Clock Signals & Synchronous Sequential Circuits

• A clock signal is a periodic square wave that indefinitely switches values from 0 to 1 and 1 to 0 at fixed

intervals.

Clock cycle time or clock period: The time interval between two consecutive rising or falling edges of the

clock.



Fig 3.46 clock signal

Synchronous Sequential Circuits: Sequential circuits that have a clock signal as one of its inputs:

– All state transitions in such circuits occur only when the clock value is either 0 or 1 or happen at the

rising or falling edges of the clock depending on the type of memory elements used in the circuit.

Sequential Circuit Memory Elements: Latches, Flip-Flops

• Latches and flip-flops are the basic single-bit memory elements used to build sequential circuit with one

or two inputs/outputs, designed using individual logic gates and feedback loops.

• Latches:

– The output of a latch depends on its current inputs and on its previous inputs and its change of state can

happen at any time when its inputs change.

• Flip-Flop:

– The output of a flip-flop also depends on current and previous input but the change in output (change of

state or state transition) occurs at specific times determined by a clock input.

S-R Latch

• An S-R (set-reset) latch can be built using two NOR gates forming a feedback loop.

• The output of the S-R latch depends on current as well as previous inputs or state, and its state (value

stored) can change as soon as its inputs change.

When Q is HIGH, the latch is in SET state.

When Q is LOW, the latch is in RESET state.

Fig 3.47 SR latch circuit diagram, truth table and symbol

S

R

Q

Q'



S-R Flip flop

Since the S-R latch is responsive to its inputs at all times an enable line C is used to disable or

enable state transitions.

Behaves similar to a regular S-R latch when enable C=1

Fig 3.48 gated SR flip flop

S = 0, R = 0; this is the normal resting state of the circuit and it has no effect of the output states. Q and Q’

will remain in whatever state they were in prior to the occurrence of this input condition. It works in

HOLD (no change) mode operation. • S = 0, R = 1; this will reset Q to 0, it works in RESET mode

operation.

S = 1, R = 0; this will set Q to 1, it works in SET mode operation.

S = 1, R = 1; this condition tries to set and reset the NOR gate latch at the same time, and it

produces Q = ¯ = 0. This is an unexpected condition and are not used.

Since the two outputs should be inverse of each other. If the inputs are returned to 1 simultaneously, the

output states are unpredictable. This input condition should not be used and when circuits are constructed,

the design should make this condition SET=RESET = 1 never arises.

Clocked SR Flip Flop

Additional clock input is added to change the SR flip-flop from an element used in asynchronous

sequential circuits to one, which can be used in synchronous circuits.

Fig 3.49 Clocked SR flip flop

Its means that the flip flop can change the output states only when clock signal makes a transition from

LOW to HIGH.

S

EN

R

Q

Q'



Fig 3.50 truth table for clocked SR flip flop

Clocked SR Flip Flop Circuit with PRESET and CLEAR

Some flip-flops have asynchronous preset Pr and clear Cl signals.

Output changes once these signals change, however the input signals must wait for a change in

clock to change the output

Fig 3.51 SR flip flop with preset and clear

JK Flip Flop

Another types of Flip flop is JK flip flop.

It differs from the RS flip flops when J=K=1 condition is not indeterminate but it is defined to give

a very useful changeover (toggle) action.

Toggle means that Q and Q ¯ will switch to their opposite states.

The JK Flip flop has clock input Cp and two control inputs J and K.

Operation of Jk Flip Flop is completely described by truth table

Fig 3.52 JK flip flop



JK Flip Flop with preset and clear

This flip flop can also have other inputs called Preset (or SET) and clear that can be used for setting the flip

Flop to 1 or resetting it to 0 by applying the appropriate signal to the Preset and Clear inputs. These inputs

can change the state of the flip flop regardless of synchronous inputs or the clock.

Fig 3.53 JK flip flop with PRESET and CLEAR

T Flip Flop

The T flip flop has only the Toggle and Hold Operation. If Toggle mode operation. The output will toggle

from 1 to 0 or vice versa.

Fig 3.54 T flip flop



D Flip Flop

Also Known as Data Flip flop

Can be constructed from RS Flip Flop or JK Flip flop by addition of an inverter.

Inverter is connected so that the R input is always the inverse of S (or J input is always

complementary of K).

The D flip flop will act as a storage element for a single binary digit (Bit).

Fig 3.55 D flip flop

EDGE TRIGGERED FLIP FLOP

Edge triggered flip-flop changes only when the clock C changes

The three basic types are introduced here: S-R, J-K and D.

Edge-triggered S-R flip-flop

The basic operation is illustrated below, along with the truth table for this type of flip-flop. The operation

and truth table for a negative edge-triggered flip-flop are the same as those for a positive except that the

falling edge of the clock pulse is the triggering edge.

As S = 1, R = 0. Flip-flop SETS on the rising clock

edge.

Fig 3.56 edge triggered SR flip flop

Edge-triggered J-K flip-flop

The J-K flip-flop works very similar to S-R flip-flop. The only difference is that this flip-flop has NO

invalid state. The outputs toggle (change to the opposite state) when both J and K inputs are HIGH.



Edge-triggered D flip-flop

The operations of a D flip-flop is much simpler. It has only one input addition to the clock. It is very

useful when a single data bit (0 or 1) is to be stored. If there is a HIGH on the D input when a clock pulse

is applied, the flip-flop SETs and stores a 1. If there is a LOW on the D input when a clock pulse is

applied, the flip-flop RESETs and stores a 0. The truth table below summarize the operations of the

positive edge-triggered D flip-flop. As before, the negative edge-triggered flip-flop works the same except

that the falling edge of the clock pulse is the triggering edge.

MASTER-SLAVE FLIP FLOP

Is designed using two separate flip flops. Out of these, one acts as the master and the other as a

slave. The figure of a master-slave J-K flip flop is shown below.

Fig 3.57 master slave flip flop

From the above figure you can see that both the J-K flip flops are presented in a series connection. The

output of the master J-K flip flop is fed to the input of the slave J-K flip flop. The output of the slave J-K

flip flop is given as a feedback to the input of the master J-K flip flop. The clock pulse [Clk] is given to the

master J-K flip flop and it is sent through a NOT Gate and thus inverted before passing it to the slave J-K

flip flop.

When Clk=1, the master J-K flip flop gets disabled. The Clk input of the master input will be the

opposite of the slave input. So the master flip flop output will be recognized by the slave flip flop only

when the Clk value becomes 0. Thus, when the clock pulse males a transition from 1 to 0, the locked

outputs of the master flip flop are fed through to the inputs of the slave flip-flop making this flip flop edge

or pulse-triggered.

combinational and sequential logic

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