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

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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.

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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.

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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.

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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.

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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.

Figure 6.9 Function Table of an XOR Gate
Logical XOR Operation
Inputs Output
A B F
0 0 0
0 1 1
1 0 1
1 1 0

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.

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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.

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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.

Figure 6.12 Function Table of an XNOR Gate
Logical XNOR Operation
Inputs Output
A B F
0 0 1
0 1 0
1 0 0
1 1 1

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.

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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.

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.

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

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

• 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.
    • E2CMOS: 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.

  1. o 74XX00 Quad 2-input NAND Gate
  2. o 74XX02 Quad 2-input NOR Gate
  3. o 74XX04 Hex Inverter
  4. o 74XX08 Quad 2-input AND Gate
  5. o 74XX10 Triple 3-input NAND Gate
  6. o 74XX11 Triple 3-input AND Gate
  7. o 74XX20 Dual 4-input NAND Gate
  8. o 74XX21 Dual 2-input AND Gate
  9. o 74XX27 Triple 3-input NOR Gate
  10. o 74XX30 Single 8-input NAND Gate
  11. o 74XX32 Quad 2-input OR Gate
  12. o 74XX86 Quad 2-input XOR Gate
  13. o 74XX133 Single 13-input NAND Gate

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.

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.

1413121110 9 8 1413121110 9 8

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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
14 13 12 11 10 9 8 14 13 12 11 10 9 8

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

1 2 3 4 5 6 7 1 2 3 4 5 6 7
7404 7408
Hex Inverters Four 2-Input AND Gate
14 13 12 11 10 9 8 14 13 12 11 10 9 8

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

1 2 3 4 5 6 7432 Four 2-Input OR Gate 7 1 2 3 4 5 6 7486 Four 2-Input XOR Gate 7
14 13 12 11 10 9 8

digital logic design  LOGIC GATES & OPERATIONAL CHARACTERISTICS

1234567

74266 Four 2-Input XNOR Gate

Figure 6.17 Commonly used Integrated Circuit Logic Gates

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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