This chapter is about the basic building blocks of all digital circuits. Understanding the rules for each gate is essential.

1. Short Notes: Core Concepts

a) Logic Gates

• A logic gate is a tiny electronic circuit that takes one or more binary inputs (0 or 1) and produces a single binary output (0 or 1).

b) Basic Logic Gates

• AND Gate: The output is 1 ONLY IF ALL inputs are 1.

  • Real-world example: A bank vault that needs two keys (Key A AND Key B) to open.

• OR Gate: The output is 1 IF AT LEAST ONE input is 1.

  • Real-world example: A passenger can exit a bus from the front door OR the back door.

• NOT Gate (Inverter): The output is the opposite of the single input.

c) Combinational Logic Gates

• NAND Gate (NOT-AND): The output is the opposite of an AND gate.

• NOR Gate (NOT-OR): The output is the opposite of an OR gate.

d) Truth Tables

• A truth table is a chart that shows every possible input combination for a logic gate and the corresponding output.

• For a gate with 2 inputs, there will be 2² = 4 rows in the truth table.

Example: 2-Input AND Gate Truth Table | Input A | Input B | Output Q | |:---:|:---:|:---:| | 0 | 0 | 0 | | 0 | 1 | 0 | | 1 | 0 | 0 | | 1 | 1 | 1 |

2. Tips and Tricks for the Exam

• Memorization Tip: Focus on the "unique" case for each gate.

  • AND: The only way to get a 1 is 1 AND 1.

  • OR: The only way to get a 0 is 0 OR 0.

  • NAND: The opposite of AND. The only way to get 0 is 1 NAND 1.

  • NOR: The opposite of OR. The only way to get 1 is 0 NOR 0.

• Symbol Tip: The AND gate has a flat back, like the 'D' in AND. The OR gate has a curved back. The small circle on NAND and NOR gates means "NOT".

3. Important Points to Remember

• Logic gates are the fundamental building blocks of the CPU.

• Inputs and outputs are always binary (0 or 1).

• A truth table is the best way to represent the function of a logic gate.