8-to-3 Priority Encoder
Accepts up to 8 inputs (I0–I7) and outputs the 3-bit binary code of the highest-priority active input. I7 has the highest priority. Implements IC 74HC148. GS = 1 when any input is active.
8-to-3 Priority Encoder — IC 74HC148
Priority: None
Code: — (0b—)
Priority Table
| Highest Active | A2 | A1 | A0 | GS |
|---|---|---|---|---|
| None (all 0) | 0 | 0 | 0 | 0 |
| I0 (lowest) | 0 | 0 | 0 | 1 |
| I1 | 0 | 0 | 1 | 1 |
| I2 | 0 | 1 | 0 | 1 |
| I3 | 0 | 1 | 1 | 1 |
| I4 | 1 | 0 | 0 | 1 |
| I5 | 1 | 0 | 1 | 1 |
| I6 | 1 | 1 | 0 | 1 |
| I7 ★ (highest) | 1 | 1 | 1 | 1 |
Priority Resolution
When multiple inputs are active simultaneously, the output always reflects the highest-indexed active input (I7 has maximum priority).
GS — Group Select
GS goes HIGH when any input is active. Used for cascading multiple 74HC148 chips to create 16-to-4 or larger encoders.
Encoder vs Decoder
An encoder collapses N inputs → log₂N outputs. A decoder expands log₂N inputs → N outputs. They are inverse operations.