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Priority encoder

What is priority encoder?

A priority encoder is a combinational circuit that, when given an input bit vector, outputs the position of the first 1 bit in the vector. For example, a 8-bit priority encoder given the input 8'b10010000 would output 3'd4, because bit[4] is first bit that is high.

Practice

Problem statement

Build a 4-bit priority encoder. For this problem, if none of the input bits are high (i.e., input is zero), output zero. Note that a 4-bit number has 16 possible combinations.

Verilog
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// synthesis verilog_input_version verilog_2001
module top_module (
    input [3:0] in,
    output reg [1:0] pos  );

endmodule

Solution

Verilog
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// synthesis verilog_input_version verilog_2001
module top_module (
    input [3:0] in,
    output reg [1:0] pos  );
    always @(*) begin
        case (in)
            4'b0000:pos=0;
            4'b0001:pos=2'd0;
            4'b0010:pos=2'd1;
            4'b0011:pos=2'd0;
            4'b0100:pos=2'd2;
            4'b0101:pos=2'd0;
            4'b0110:pos=2'd1;
            4'b0111:pos=2'd0;
            4'b1000:pos=2'd3;
            4'b1001:pos=2'd0;
            4'b1010:pos=2'd1;
            4'b1011:pos=2'd0;
            4'b1100:pos=2'd2;
            4'b1101:pos=2'd0;
            4'b1110:pos=2'd1;
            4'b1111:pos=2'd0;
        endcase
    end
endmodule
  • There is an easier way to code this. See the next problem (always_casez).