Overview
This lecture explains the 3-to-8 (3x8) decoder, its operation, truth table, output-circuit design, and applications in digital electronics, especially memory interfacing.
3x8 Decoder Basics
- A decoder converts n input lines into 2ⁿ unique outputs.
- The 3x8 decoder has 3 input terminals (A, B, C) and 8 outputs (Y0–Y7).
- There is an 'Enable' terminal (E) that activates or deactivates the decoder.
- If E = 0, all outputs Y0–Y7 remain 0 regardless of input values.
- If E = 1, only one output line corresponding to the input combination is set to 1.
Truth Table & Output Selection
- For each combination of ABC (from 000 to 111), a unique output (Y0–Y7) is set to 1; all others stay 0.
- Example: When E=1 and ABC=110, Y6=1 and Y0–Y5, Y7=0.
- If E=0, all outputs remain 0 for any ABC.
Circuit Implementation
- Outputs are created using AND gates, with each output connected to combinations of A, B, C, and their inverses.
- Y0 = A' B' C' E
- Y1 = A' B' C E
- Y2 = A' B C' E
- Y3 = A' B C E
- Y4 = A B' C' E
- Y5 = A B' C E
- Y6 = A B C' E
- Y7 = A B C E
- Enable (E) input is connected to all AND gates to control output activation.
Applications
- 3x8 decoders are commonly used to select among 8 memory ICs in memory interfacing.
- Popular in systems like 8085 and 8086 microprocessor memory interfacing.
Key Terms & Definitions
- Decoder — A circuit that translates n input signals to 2ⁿ output lines, selecting one at a time.
- Enable Terminal (E) — Control input that activates the decoder when set to 1.
- Selection Lines — The input lines (A, B, C) determining which output is activated.
Action Items / Next Steps
- Review the Boolean expressions for each output.
- Examine circuit diagrams or build simple 3x8 decoder circuits.
- Read about decoder application in microprocessor memory interfacing.