# Digital Logic Design

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CS302 - Digital Logic & Design
Lesson No. 06
LOGIC GATES & OPERATIONAL CHARACTERISTICS
NOR Gate as a Universal Gate
The NOR gate is also used as a Universal Gate as the NOR Gate can be used in a
combination to perform the function of a AND, OR and NOT gates.
4. NOT Gate Implementation
A NOT gate can be implemented using a NOR gate by connecting both the inputs of
the NOR gate together. By connecting the two inputs together, the combinations with
dissimilar inputs become redundant. The Function Table of the 2-input NOR Gate reduces to
that of the NOT gate. Figure 6.1
Logical NOR
Operation
Inputs
Output
A
B
F
0
0
1
0
1
0
1
0
0
1
1
0
Figure 6.1
Implementing a NOT Gate using a NOR gate
5. OR Gate Implementation
A NOR Gate performs the OR-NOT function. Removing the NOT gate at the output of
the NOR gate results in an OR gate. The effect of the NOT gate at the output of the NOR gate
can be cancelled by connecting a NOT gate at the output of the NOR Gate. The two NOT
gates cancel each other out. A NOT Gate implemented using a NOR gate (2) is connected to
the output of a NOR gate (1). Figure 6.2.
Figure 6.2
Implementing an OR Gate using two NOR gates
6. AND Gate Implementation
An AND Gate can be implemented using a combination of three NOR gates. The
implementation is based on the alternate symbolic representation of the AND gate. The AND
gate is represented as an OR gate with bubbles at the inputs and outputs. Figure 5.13. The
two bubbles at the input can be replaced by two NOT gates (1) & (2) implemented using two
NOR gates. If the two bubbles are removed from the two inputs, the OR gate with the bubble
at the output represents a NOR gate (3). Figure 6.3
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CS302 - Digital Logic & Design
Figure 6.3
Implementing an OR Gate using three NOR gates
NAND-NOR Universal Gates
NAND and NOR gates are known as Universal Gates as they can be used to
implement any of the three fundamental gates, AND, OR and NOT. The NAND Universal Gate
can also be used to implement a NOR gate. Similarly, a NOR gate can be used to implement a
NAND gate.
1. NAND gate Implementation using NOR gates
The AND gate implementation using three NOR gates is shown in figure 6.3. A NAND
gate implementation requires addition of an inverter (NOT) gate at the output. The NOT gate is
implemented using a NOR gate. Figure 6.4. NOR gates 1, 2 and 3 implement the AND gate.
NOR gate 4 implements the NOT gate connected at the output of the NAND gate.
Figure 6.4
Implementing a NAND Gate using four NOR gates
2. NOR gate Implementation using NAND gates
The OR gate implementation using three AND gates is shown in figure 5.20. A NOR
gate implementation requires addition of an inverter (NOT) gate at the output. The NOT gate is
implemented using a NAND gate. Figure 6.5. NAND gates 1, 2 and 3 implement the OR gate.
NAND gate 4 implements the NOT gate connected at the output of the NOR gate.
Figure 6.5
Implementing a NOR Gate using four NAND gates
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CS302 - Digital Logic & Design
NAND and NOR Gate Applications
The output of a NAND is 0 when all inputs to the NAND gate are 1s. This property of
the NAND gate can be used to activate an operation when any of the inputs to the NAND gate
are deactivated. A NOR gate on the other hand generates an output of 1 when all inputs to
NOR gate are deactivated. The output is deactivated when any input is activated.
A warehouse is used to store industrial chemicals. Toxic fumes produced by the
chemicals are removed from the ware house and dispersed in the atmosphere through three
exhaust fans. The three exhaust fans should be continuously working to remove the
dangerous toxic fumes. If any one or more fans fail an alarm should be activated to signal the
failure of one or more exhaust fans.
An electronic circuit connected to each fan generates a 1 to indicate a working fan. If
the fan fails the circuit generates a 0 output. The outputs of the three fans are connected to the
three inputs of a NAND gate. When all fans are working the input to the 3-input NAND gate is
111 and the corresponding output is a 0. When any one fan fails the output of NAND gate
becomes 1 activating an alarm connected to the output of the NAND gate. Figure 6.6
Figure 6.6
A NAND gate based exhaust fan failure detection system
A Washing Machine has three sensors to check for washing machine lid open, washing
tub filled to minimum level and weight of cloths and water in the tub. If the lid of the Washing
machine is open or the water is below the minimum level or the washing machine has been
overloaded the appropriate sensor generates an output of 1. The outputs of the three sensors
are connected to the inputs of a 3-input NOR gate. During the normal operation of the
Washing Machine all the sensors output a 0. The corresponding output of the NOR gate is a 1.
If an erroneous condition is detected by any one or more sensors, the corresponding sensor
output(s) is set to 1, setting the NOR gate output to a 0. The NOR gate output is connected to
the main switch which switches off the washing machine. Figure 6.7.
Figure 6.7
A NOR gate based Washing Machine Controller
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CS302 - Digital Logic & Design
Exclusive-OR and Exclusive-NOR Gates
The XOR and XNOR gates are frequently used in Digital Logic. These two additional
gates are used to detect dissimilar and similar inputs respectively.
1. Exclusive-OR Gate
The Exclusive-OR Gate or XOR Gate performs a function that is equivalent to the
combination of NOT, AND and OR gates. XOR gates are extensively used in digital
applications; therefore XOR gates are available as basic components. Most commonly used
XOR Gates have two inputs. The XOR gate is represented by symbol shown in figure 6.8.
Figure 6.8
Symbolic representation of XOR Gate
The function performed by the XOR gate is represented by the Function Table for a
two input XOR Gate. Figure 6.9. The function table for a 3, 4 or multiple input XOR Gate is
similar. The output of an XOR gate is 1 when the inputs are dissimilar and a 0 when all the
inputs are the same.
Logical XOR
Operation
Inputs
Output
A
B
F
0
0
0
0
1
1
1
0
1
1
1
0
Figure 6.9
Function Table of an XOR Gate
The expression describing the operation of the two inputs XOR Gate is F = A B . The
is an XOR operator and the expression for multiple input XOR Gates is
F = A B C .....N , where N is the total number of inputs.
The timing diagram of the two input XOR gate with the input varying over a period of 7
time intervals is shown in the diagram. Figure 6.10.
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CS302 - Digital Logic & Design
Figure 6.10
Timing diagram of operation of a XOR gate
2. Exclusive-NOR Gate
The Exclusive-NOR Gate or XNOR Gate performs a function that is equivalent to the
combination of NOT, AND and OR gates. XNOR gate is extensively used in digital
applications; therefore XNOR gates are available as basic components. Most commonly used
XNOR Gates have two inputs. The XNOR gate is represented by symbol shown in figure 6.11.
Figure 6.11
Symbolic representation of XNOR Gate
The function performed by the XNOR Gate is represented by the Function Table for a
two input XNOR Gate. Figure 6.12. The function table for a 3, 4 or multiple input XNOR Gate
is similar. The output of an XNOR gate is 1 when the all the inputs are same and a 0 when the
inputs are dissimilar.
Logical XNOR
Operation
Inputs
Output
A
B
F
0
0
1
0
1
0
1
0
0
1
1
1
Figure 6.12
Function Table of an XNOR Gate
The expression describing the operation of the two inputs XNOR Gate is F = A B .
The expression for multiple input XNOR Gates is F = A B C .....N , where N is the total
number of inputs.
The timing diagram of the two input XNOR gate with the input varying over a period of
7 time intervals is shown in the diagram. Figure 6.13.
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CS302 - Digital Logic & Design
A
B
t0
t1
t2
t3
t4
t5
t6
F
Figure 6.13
Timing diagram of operation of a XNOR gate
XOR and XNOR Gate Applications
XOR and XNOR gates are used to detect dissimilar and similar inputs. This property of
XOR and XNOR gates is used to detect odd and even number of 1s in a Parity Detection
Circuit.
Consider the three XOR gate logic circuit which is used to detect odd number of 1's in
a 4-bit binary input combination. Figure 6.14
A
1
B
3
C
2
D
Figure 6.14
XOR gate based Odd number of 1s detector
A 4-bit binary number 0000 applied at the inputs A, B, C and D respectively of XOR
gates 1 and 2. The output of XOR Gates 1 and 2 is 0 and 0. The output of XOR gate 3 is also
zero. Similarly, a binary number 0011 applied at the inputs A, B, C and D respectively. The
output of XOR gate 1 with inputs 00 is 0. The output of XOR gate 2 with inputs 11 is 0. The
output of gate 3 is 0. Thus the output indicates that the binary number 0011 does not have odd
number of 1's. Consider the binary number 1011 applied at the inputs A, B, C and D
respectively. The output of XOR gate 1 with inputs 10 is 1. The output of XOR gate 2 with
inputs 11 is 0. The output of gate 3 is 1. Thus the output indicates that the binary number 1011
has odd number of 1's
The logic circuit based on two XOR and a single XNOR gate which is used to detect
even number of 1's in a 4-bit binary input combination. Figure 6.15
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CS302 - Digital Logic & Design
A
1
B
3
C
2
D
Figure 6.15
XOR-XNOR gate based Even number of 1s detector
A 4-bit binary number 0000 applied at the inputs A, B, C and D respectively of XOR
gates 1 and 2. The output of XOR Gates 1 and 2 is 0 and 0. The output of XNOR gate 3 is a 1.
Similarly, a binary number 0011 applied at the inputs A, B, C and D respectively. The output of
XOR gate 1 with inputs 00 is 0. The output of XOR gate 2 with inputs 11 is 0. The output of
XNOR gate 3 is also a 1. Thus the output indicates that the binary number 0011 has even
number of 1's. Consider the binary number 1011 applied at the inputs A, B, C and D
respectively. The output of XOR gate 1 with inputs 10 is 1. The output of XOR gate 2 with
inputs 11 is 0. The output of XNOR gate 3 is 0 because of dissimilar inputs. Thus the output
indicates that the binary number 1011 does not have even number of 1's.
Digital Circuits and Operational Characteristics
The Logic Gates discussed provide the basic building blocks for implementing the large
digital systems. The logic gates discussed so far has been described in terms of the functions
they perform. Practical implementation of digital systems by using the logic gates in
combination requires some additional information. For example, theoretically the output of an
Inverter can be connected to the inputs of an unlimited number of AND Gates. However, the
practical limitation to the circuit shown is that the total current sourced by the Inverter is
distributed amongst the 10 AND Gates. The Inverter is not able to provide the total current
required by the ten AND gates. The current sunk by each AND gate is not enough to drive the
AND gate circuitry thus its behavior is unpredictable resulting in unpredictable behavior of the
system.
The binary 1 and 0 are represented by +5V and 0 V. What if the output of an AND Gate
is +3 V? Does this output voltage level represent a binary 1 or 0? If the output of the AND Gate
is connected to the input of an Inverter, what would be the response of the Inverter? Another
important aspect is the frequency of the input signal. Electronic circuits operate at certain
frequencies. If the frequency of the input signal increases beyond the operational specification
of the circuit, the circuit will not be able to respond fast enough resulting in unpredictable
behavior.
Digital circuits that depend upon battery for their power should consume low power to
allow the device to function for longer periods of time before replacing or recharging the
battery. Thus the digital system should be implemented keeping in view the power
requirements of the application.
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CS302 - Digital Logic & Design
TTL/CMOS NOT Gate Operation
Logic Gates are implemented using transistors. These transistors are connected in
various combinations to form a switching circuit. The transistor itself is configured to work like
a switch. On the application of a bias voltage the transistor is switched on and by removing the
bias voltage the transistor is turned off. Different technologies are used to manufacture the
Logic Gates based on the transistors. The performance or the Operational characteristics of a
Logic Gate are determined by the transistors and the technologies used to implement the
switching transistors. Certain technologies allow transistor and thereby the Logic Gates to
operate at high frequencies. Other technologies allow transistors to operate with low voltages,
consuming minimal power, similarly certain other implementation technologies allow very
dense logic circuits to be manufactured.
The Inversion function of the NOT gate is performed by the switching circuit shown
in figure 6.16. The Bipolar Junction Transistor (BJT) based NOT shown on the left is switched
on when a Voltage is applied at the base of the BJT. The transistor when switched on short
circuits the VCC, the output voltage is therefore 0 volts. When the BJT base pin is connected to
0 volts, the transistor is switched off. The Vo/p is at potential VCC = 5 Volts. The actual
implementation is different.
Figure 6.16
BJT & CMOS based NOT Gate Implementation
The CMOS based implementation, shown on the right, uses a P-type and a N-type
MOSFETs. When the input is connected to +V, the P-type MOSFET is switched off and the N-
type MOSFET is switched on. The Vo/p is at ground potential. When the input is connected to
ground, the P-type and N-type MOSFETs are switched on and off respectively. The Vo/p is at
potential VDD = 5 Volts.
Integrated Circuit Technologies
The practical implementation of the Logic gates is through the Integrated Circuits (IC)
technologies. The logic gates implemented through these technologies are available to be
connected and practical implementation of a digital circuit. Different types of Integrated Circuit
technologies are used to implement the digital circuits. These technologies differ in terms of
the circuit density, power consumptions, frequency response etc.
·
CMOS: Complementary Metal-Oxide Semiconductor
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CS302 - Digital Logic & Design
The most extensively used technology, characterized by low power consumption,
switching speed which is slower but comparable to TTL. Has higher chip density than
TTL. Due to high input impedance is easily damaged due to accumulated static charge
·
TTL: Transistor-Transistor Logic
Extensively used technology, characterized by fast switching speed and high power
consumption
Offers a wide variety of gates, devices, arithmetic units etc.
·
ECL: Emitter-Coupled Logic
Used in specialized applications where switching speed is of prime importance such as
high speed transmission, high speed memories and high speed arithmetic units.
·
PMOS: p-channel and NMOS: n-channel MOS transistor
PMOS and NMOS technologies are used in LSI requiring high chip density. Large
memories and microprocessors are implemented using these technologies
These ICs have very low power consumption.
2
·
E CMOS: a combination of CMOS and NMOS technologies
Used to implement Programmable Logic Devices
Types of IC Logic Gates
The most common form of logic Gate ICs are listed. To identify and use the Integrated
Circuits or ICs in implementing logic circuits, some sort of identification code has to be used
which is printed on the IC package.
Logic Gates are identified by the codes. The prefix 74 is used to identify a commercial
version of the device from the military version device identified by the prefix 54. Military
versions are designed to withstand harsh and severe environmental conditions. The XX part of
the code identifies the switching speed of the gate.
74XX00
o
74XX02
o
74XX04
Hex Inverter
o
74XX08
o
74XX10
Triple 3-input NAND Gate
o
74XX11
Triple 3-input AND Gate
o
74XX20
Dual 4-input NAND Gate
o
74XX21
Dual 2-input AND Gate
o
74XX27
Triple 3-input NOR Gate
o
74XX30
Single 8-input NAND Gate
o
74XX32
o
74XX86
o
74XX133
Single 13-input NAND Gate
o
The Integrated Circuit packages of the seven gates that have been discussed so far
are shown. Figure 6.17. The 7408 14-pin chip has 4 or Quad, 2-input AND gates. The input
pins and the output pins of each of the four gates are shown. To use any one or all four gates
the appropriate pins are connected. Pins 7 and 14 are connected to ground and Supply
voltage respectively.
The 7432 14-pin IC package has 4 or Quad, 2-input OR Gates. Connections to the OR
gates are identical to those of the 7408 AND gate IC. The 7404 14-pin chip has 6 or hex,
inverters. The input and output connections of each of the 6 NOT gates are shown. Pins 7 and
14 are used for ground and supply voltage respectively.
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CS302 - Digital Logic & Design
The 7400, Quad, 2-input NAND Gate IC, the 7402, Quad, 2-input NOR Gate IC, the
7486, Quad, 2-input XOR Gate IC and the 74266, Quad, 2-input XNOR Gate IC are similar.
12
11
8
14
13
10
9
14
13
12
11
10
9
8
1
2
3
4
5
6
7
1
2
3
4
5
6
7
7400
7402
Four 2-Input NAND Gate
Four 2-Input NOR Gate
12
11
8
14
13
10
9
14
13
12
11
10
9
8
7
1
2
3
4
5
6
7
1
2
3
4
5
6
7408
7404
Four 2-Input AND Gate
Hex Inverters
14
13
12
11
10
9
8
14
13
12
11
10
9
8
7
7
1
2
3
4
5
6
1
2
3
4
5
6
7432
7486
Four 2-Input OR Gate
Four 2-Input XOR Gate
14
13
12
11
10
9
8
7
1
2
3
4
5
6
74266
Four 2-Input XNOR Gate
Figure 6.17
Commonly used Integrated Circuit Logic Gates
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CS302 - Digital Logic & Design
Performance Characteristics and Parameters
A number of performance characteristics and parameters determine the suitability of a
particular IC technology for a particular application. The important parameters that are
considered whilst designing Digital Logic Circuits are mentioned briefly.
·  DC Supply Voltage:
o  The supply voltage at which the Gate operates
·  Noise Margin:
o  The maximum and minimum voltages that represent binary 0 and 1 respectively. These
voltage ranges determine the suitability of a gate to work in noisy environments.
·  Power Dissipation:
o  Gates consume power during their operation. The power dissipation varies with the
frequency at which these gates operate.
·  Frequency Response and Propagation Delay:
o  Gates do not instantaneously switch to a new output state after the inputs are changed.
The delay between the input and output limits the frequency at which the inputs to a
logic gate can be changed and the logic circuit can operate.
·  Fan-Out:
o  The number of gates that can be connected to the output of a single gate.
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