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Digital Priority Encoder

Priority Encoder - Truth Table, Boolean Expression & Logic Circuit | Logicroo

Learn about Digital Priority Encoders with interactive circuit diagrams, truth tables, Boolean expressions, priority logic, logic gate implementation, and digital logic examples.

4-to-2 Priority Encoder

Digital Priority Encoder

A priority encoder takes several data lines and outputs the binary index of the highest-priority active line, ignoring lower-priority lines even if they're also active. With D3 as highest priority down to D0 as lowest, the circuit encodes which line won into Y1Y0, and raises V to flag that at least one input is active — Y1 = D3+D2, Y0 = D3+D2'·D1, V = D3+D2+D1+D0. The D2'·D1 term needs an inverter, so the critical path is a three gate delay circuit (NOT, AND, OR).

Data inputs (D3 = highest priority)
Winning line
D0
Y1 Y0
0 0
Valid
1
Delay
0Δ
Y1 = D3+D2  Y0 = D3+D2'·D1  V = D3+D2+D1+D0 (3 gate delay — NOT, AND, OR)
Higher-index inputs always override lower ones; V is low only when no input is active.
Priority table (don't-cares collapse lower-priority lines)
D3D2D1D0WinnerY1Y0V