Overview
This lecture explains the function, structure, truth table, and circuit design of a 3-to-8 (3x8) decoder, with an example and practical applications.
Introduction to 3x8 Decoder
- A decoder converts n input lines into 2ⁿ output lines.
- A 3x8 decoder has 3 inputs (A, B, C) and 8 outputs (Y0 to Y7).
- The enable (E) terminal controls whether the decoder is active.
Working Principle
- When E = 0, the decoder is disabled and all outputs are 0.
- When E = 1, the outputs depend on the values of A, B, and C.
- Inputs A, B, C are also called selection lines.
Truth Table and Output Selection
- With E = 1, each combination of ABC selects exactly one output to be 1, all others stay 0:
- ABC = 000 → Y0 = 1
- ABC = 001 → Y1 = 1
- ABC = 010 → Y2 = 1
- ABC = 011 → Y3 = 1
- ABC = 100 → Y4 = 1
- ABC = 101 → Y5 = 1
- ABC = 110 → Y6 = 1
- ABC = 111 → Y7 = 1
Circuit Implementation
- The decoder uses AND gates to generate each output from the combinations of inputs and their inverses.
- The E (enable) terminal is connected to all AND gates to ensure outputs are only active when E = 1.
- Each output Yi is generated by a unique combination of A, B, C (and their inverses) ANDed with E.
Example
- If E = 1 and ABC = 110, then Y6 = 1 and all other outputs are 0.
Boolean Functions for Outputs
- Example: Y7 = A·B·C·E; Y6 = A·B·C̅·E.
- Each output Yi can be written as an AND of the relevant form of A, B, C (either normal or inverted) and E.
Applications
- Used to select one of eight devices, such as ICs in memory interfacing.
- Popular in microprocessor memory interfacing (e.g., 8085, 8086).
Key Terms & Definitions
- Decoder — A circuit that converts n input lines to 2ⁿ unique output lines.
- Enable (E) — Control signal that activates or deactivates the decoder.
- Selection Lines — Input lines that determine which output is active.
Action Items / Next Steps
- Review the Boolean functions for each output.
- Study applications of decoders in memory interfacing systems.