4-to-2 Priority Encoder
📌 Introduction
A priority encoder converts a multi-bit input into the binary index of the highest-priority 1 (or lowest).
This example converts 4 bits to 2 bits.
MSB Priority
| I3 | I2 | I1 | I0 | O0 | O1 | | :— | :— | :— | :— | :— | :— | | 0 | 0 | 0 | X | 0 | 0 | | 0 | 0 | 1 | X | 0 | 1 | | 0 | 1 | X | X | 1 | 0 | | 1 | X | X | X | 1 | 1 |
LSB Priority
| I3 | I2 | I1 | I0 | O0 | O1 | | :— | :— | :— | :— | :— | :— | | X | X | X | 1 | 0 | 0 | | X | X | 1 | 0 | 0 | 1 | | X | 1 | 0 | 0 | 1 | 0 | | 1 | 0 | 0 | 0 | 1 | 1 |
🧑💻 Code Example (LSB Priority)
Case
module top_module (
input [3:0] in,
output reg [1:0] pos );
always@(*) begin
case(in)
4'b0000: pos = 0;
4'b0001: pos = 0;
4'b0010: pos = 1;
4'b0011: pos = 0;
4'b0100: pos = 2;
4'b0101: pos = 0;
4'b0110: pos = 1;
4'b0111: pos = 0;
4'b1000: pos = 3;
4'b1001: pos = 0;
4'b1010: pos = 1;
4'b1011: pos = 0;
4'b1100: pos = 2;
4'b1101: pos = 0;
4'b1110: pos = 1;
4'b1111: pos = 0;
endcase
end
endmodule
casez
module top_module (
input [3:0] in,
output reg [1:0] pos );
always@(*) begin
pos = 2'd0; // avoid latch
casez(in)
4'b???1: pos = 0;
4'b??10: pos = 1;
4'b?100: pos = 2;
4'b1000: pos = 3;
endcase
end
endmodule
if-else (v1)
// 4-bit priority encoder
module top_module (
input [3:0] in,
output reg [1:0] pos );
always@(*) begin
if (in & 1) pos = 0;
else if (in & 1<<1) pos = 1;
else if (in & 1<<2) pos = 2;
else if (in & 1<<3) pos = 3;
else pos = 0;
end
endmodule
if-else (v2)
// 4-bit priority encoder
module top_module (
input [3:0] in,
output reg [1:0] pos );
always@(*) begin
if (in[0]) pos = 0;
else if (in[1]) pos = 1;
else if (in[2]) pos = 2;
else if (in[3]) pos = 3;
else pos = 0;
end
endmodule