SS2 WEEK SIX LESSON NOTE

TOPIC: LOGIC CIRCUITS: Alternative Logic Circuits

1.           NOR GATE
The NOR gate out is the direct inverse of the OR gate output for all possible input conditions as can be seen from the truth table below. It is short for NOT OR.



The NOR gate works like an OR gate followed by an INVERTER so that circuits in the diagram below are equivalent.





2.           NAND GATE
This is a logic gate whose output is zero (0) if both of the inputs are 1and 1 otherwise. A NAND gate is equivalent to an AND gate followed by a NOT gate. The NAND gate is the complement of the AND function. The word NAND is got from the abbreviation NOT-AND. The NAND gate is represented graphically below.


The truth table is shown below




DIGITAL COMPARATOR

A digital comparator is a very useful application of logic circuit.

A digital comparator is an electronic device that takes two numbers as input in binary form and determines whether one number is greater than, less than or equal to the other number. A comparator compares two input voltages and indicates which is higher. Comparators are used in central processing units (CPUs) and microcontrollers (MCUs).

Note: A XNOR gate is a basic comparator, because its output is "1" only if its two input bits are equal. The XNOR gate is graphically represented below:


A
B
X
0
0
1
0
1
0
1
0
0
1
1
1
          
                                                                   
         

The analog equivalent of digital comparator is the voltage comparator. Many microcontrollers have analog comparators on some of their inputs that can be read or trigger an interrupt

Construction of a simple comparator using XOR

One of the special logic circuit that occurs quite often in digital system is the exclusive-OR (XOR) circuits.

The XOR produces a high voltage only when the two inputs are at opposite levels. This means that the XOR yields true (1) if and only if one of the inputs is true (1) and the other is false (0). The XOR gate is graphically represented below.

               


The XOR is represented in the truth table below


Uses of Logic Gates

Logic gates are widely used in various applications such as:

1.           Electronics: Logic gates are the building blocks of digital electronics, they are formed by combining transistors to realize some digital operations. Every digital product such as computers, mobile phones contain logic gates

2.           Decision making:  The term logic is usually used to refer to a decision making process. A logic gate can give a yes or no response based on the input it receives

3.           Alarms: Logic gates are used to construct alarms used in cars and homes. Pressing the alarm button gives an output of 1. When the bell is released, on output of 0 is recorded.

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