DIGITAL BASIC - 1.4 : Combinational Circuits

Introduction

Index	Chapter1	Chapter2	Chapter3	Chapter4
	Digital Background	Semiconductor Background	CMOS Processing

1.1	1.2	1.3a	1.3b	1.4	1.5	1.6
Number System	Digital Arithmetic	Logic Gates	Logic Gates	Combinational Circuits	Multiplex (MUX)

Digital circuits are of two types:

• Combinational Circuits (also known as Time-Independent Logic)
• Combinational digital circuits are those circuits where output is function of present input only.
• Sequential Circuits
• Output depends not only on the present input but also on the history of the input

Let’s discuss these circuits in a comparison table format.

Combinational Circuits	Sequential Circuits
No Memory (No memory unit required)	Has Memory (Memory unit require)
Output is function of Present input. Output = ƒ(In)	Output is function of present + previous inputs. Output = ƒ(In, previous In)
For designing Basic gates (AND,OR,NOT) or Universal gates (NAND,NOR) are used	Used to construct finite state machines, a basic building block in all digital circuitry, as well as memory circuits and other devices.
Examples of combinational digital circuits are:  Half Adder, Full Adder, Half subtractor, Full subtractor, Code converter, Decoder, Multiplexer, De-multiplexer, Encoder, ROM etc	Examples for sequential digital circuits are: Registers, Shift register, Counters etc.
The construction of combinational logic is generally done using one of two methods: A sum of products (SOP)  A product of sums (POS)	The construction of Sequential logic is generally done using State tables.
Faster because the delay between the input and output  is due to the propagation delay of the gates only	Slower then the combinational  circuits
Easy to design	Compartively harder to design

Classification of Combinational logic Circuits:

• Arithmetic and logical functions
•  Adders
• Subtractors
• Comparitors
• PLDs
•  Data transmission
• Multiplexers
• Demultiplexers
• Decoders
• Encoders
• Code Converters
• Binary Converter
• BCD converter
• 7-segment display

Classification of Sequential Circuits

• Synchronous Circuits
•  Asynchronous Circuits

Synchronous Sequential Circuits	Asynchronous Sequential circuits
Memory Elements are clocked flipflop	Memory elements are either unclocked Flipflops or time delay elements
The change in the input signal can effects memory elements upon activation of clock signal	The change in the input signal can effects the memory elements at any instant of time.
Maximum operating speed of the clock depends on the time delay involved	Because of the absence of clock, Asynchronous circuits can operate faster than the Synchronous Circuits.
Easy to design	More difficult to design
the state of the device changes only at discrete times in response to a clock signal	circuits the state of the device can change at any time in response to changing inputs

Combinational Circuits:

Let’s discuss few of the combinational circuit’s details (like Block diagram, Circuit Diagram and Truth table). I am not discussing in detail about the circuit diagram here because these are very straight forward.  In few cases, if you have any confusion, you can refer any basic electronics books. Or you can use K-map to figure out the equations which is mentioned either in block-diagram or circuit diagram.

Circuit name	Block diagram	Circuit Diagram	Truth table
Half Adder			A B S C 0 0 0 0 0 1 1 0 1 0 1 0 1 1 0 1
Full Adder			A,B Cin S, Cout 0,0 0 0, 0 0,0 1 1, 0 0,1 0 1, 0 0,1 1 0, 1 1,0 0 1, 0 1,0 1 0, 1 1,1 0 0, 1 1,1 1 1, 1
Half Subtractor			A B B0 D 0 0 0 0 0 1 1 1 1 0 0 1 1 1 0 0
Decoder (Active high)			AB D­0D1D2D3 00 1000 01 0100 10 0010 11 0001
Decoder (Active low)			XY D­0D1D2D3 00 0111 01 1011 10 1101 11 1110
De-Multiplexer		E A B D0D1D2D3 1 X X 0000 0 0 0 1000 0 0 1 0100 0 1 0 0010 0 1 1 0001
Multiplexer		S1 S0 Y 0 0 I0 0 1 I1 1 0 I2 1 1 I3

Important points:

• Half adder can be converted into Half Subtractor with an additional inverter.
• Full adder can be implemented by using two half adders and an OR gate.
• Full subtractor can be implemented by using two half- subtractors and an OR gate.
• Full adder can be converted into full subtractor with an additional inverter.
• Full adder are of 2 type
• Ripple carry adder.
• Carry Look- Ahead adder
• Four bit binary parallel adder can be constructed by using
• 3 full adders and 1 half adder or
• 4 full adders with input carry for least significant bit full adder is zero.

4 Bit FA: Using 4 full Adder (LSB FA's carry bit =0)

4 Bit FA: 3 Full Adder + 1 Half Adder

Important points:

• Decoder
• Converts binary information from ‘n’ input lines to a maximum of 2ⁿ unique output lines.
• E.g. 2x4 line Decoder (it is also called one four line decoder)
• Active high output type of decoders are constructed with AND gates.
• Active low output type of decoders are constructed with NAND gates.
• 3 to 8 line decoder is also called Binary-to-Octal decoder or converter. It is also called 1of 8 decoder, because only one of the 8 outputs is active at a time.
• Decoders are widely used in the memory system of computer, where they respond to the address code input from the CPU to activate the memory storage location specified by the address code.

In the next part of the combinational circuits we will discuss about the MUX with few examples in detail. The reason, I am discussing MUX separately because it has several important things which usually asked in the interview. Also it's very important from VLSI designing point of view.