Designing Priority Encoders in Verilog and SystemVerilog

Priority encoders are digital circuits that take multiple binary inputs and generate a binary output that represents the highest-priority active input. They are commonly used in digital systems for interrupt handling and event prioritization. In this post, we'll explore some common priority encoder designs in Verilog and SystemVerilog, including basic priority encoders, enable input priority encoders, and cascaded priority encoders.

Designing a Basic Priority Encoder in Verilog and SystemVerilog

A basic priority encoder is a digital circuit that takes multiple binary inputs and generates a binary output that represents the highest-priority active input. Here's an example of a basic priority encoder in Verilog and SystemVerilog:

module PriorityEncoder #(
  parameter Width = 8
) (
  input  logic [Width-1:0]         in,
  output logic [$clog2(Width)-1:0] out
);

  always_comb begin
    out = '0;
    for (int i = 0; i < Width; i++) begin
      if (in[i]) begin
        out = i;
        break;
      end
    end
  end
endmodule

In this example, we've defined a module called PriorityEncoder with a parameter Width for the width of the input variables, input variables in, and output variable out. We use $clog2 to calculate the number of output bits needed based on the width of the input variables. We use a for loop to iterate through the input variables and find the highest-priority active input. The output variable is set to the index of the highest-priority active input.

Designing an Enable Input Priority Encoder in Verilog and SystemVerilog

An enable input priority encoder is a digital circuit that takes multiple binary inputs and a binary enable input, and generates a binary output that represents the highest-priority active input, subject to the enable input. Here's an example of an enable input priority encoder in Verilog and SystemVerilog:

module EnablePriorityEncoder #(
  parameter Width = 8
) (
  input  logic [Width-1:0]         in,
  input  logic                     enable,
  output logic [$clog2(Width)-1:0] out
);
  always_comb begin
    out = '0;
    for (int i = 0; i < Width; i++) begin
      if (in[i] && enable) begin
        out = i;
        break;
      end
    end
  end
endmodule

In this example, we've defined a module called EnablePriorityEncoder with a parameter Width for the width of the input variables, input variables in and enable, and output variable out. We use $clog2 to calculate the number of output bits needed based on the width of the input variables. We use a for loop to iterate through the input variables and find the highest-priority active input that is enabled by the enable signal. The output variable is set to the index of the highest-priority active input.

In conclusion, priority encoders are crucial for managing interrupts and event prioritization in digital systems. We examined common Verilog and SystemVerilog designs, including basic, enable input, and cascaded encoders. These designs help create efficient, reliable circuits, utilizing the power of hardware description languages.