Designing Decoders in Verilog and SystemVerilog

In digital systems design, decoders are essential circuits that convert a binary code into a set of output signals. They are used in various applications such as address decoding, memory interfacing, and control signal generation. In this tutorial, we'll learn how to design decoders in Verilog and SystemVerilog.

Designing a 3-Bit Decoder in Verilog and SystemVerilog

Let's start by designing a simple 3-bit decoder that takes a 3-bit binary input and generates 8 output signals, one for each possible input combination. We can implement this using a combinational logic block in Verilog and SystemVerilog.

module DecoderSimple(
  input  logic [2:0] in,
  output logic [7:0] out
);

  always_comb begin
    case (in)
      3'b000: out = 8'b00000001;
      3'b001: out = 8'b00000010;
      3'b010: out = 8'b00000100;
      3'b011: out = 8'b00001000;
      3'b100: out = 8'b00010000;
      3'b101: out = 8'b00100000;
      3'b110: out = 8'b01000000;
      3'b111: out = 8'b10000000;
    endcase
  end

endmodule

In this example, we have defined a module named DecoderSimple that takes a 3-bit input signal in and generates an 8-bit output signal out. Inside the always_comb block, we use a case statement to match each possible input value with the corresponding output value.

Designing a Parameterized Decoder in Verilog and SystemVerilog

To design a parameterized decoder in Verilog and SystemVerilog, we can use a logic block to handle any arbitrary number of input bits. This implementation is more concise and efficient compared to using a case statement, especially when the number of input bits is large.

module Decoder #(
  parameter EncodeWidth = 4;
  parameter DecodeWidth = 2 ** EncodeWidth;
) (
  input  logic [EncodeWidth-1:0] in,
  output logic [DecodeWidth-1:0] out
);

  always_comb begin
    // Initialize all output bits to 0
    out = '0;

    // Set the corresponding decoded bit to 1
    out[in] = 1'b1;
  end

endmodule

In this example, we use a logic block to implement the decoder. Inside the always_comb block, we first initialize all output bits to 0 by assigning the out signal to '0. Then, we set the corresponding decoded bit to 1 by accessing the output bit at the index specified by the input signal in and assigning it the value of 1'b1.

This implementation is more elegant and efficient compared to using a case statement. We don't need to match each possible input value with the corresponding output value, which can result in a significant reduction in code complexity and simulation time.

Conclusion

In this tutorial, we've learned how to design decoders in Verilog and SystemVerilog, starting with a simple 3-bit decoder and then generalizing it into a parameterized decoder that can handle any arbitrary number of input bits. Decoders are fundamental building blocks in digital systems design, and with the knowledge gained from this tutorial, you'll be able to design your own custom decoders for your digital circuits.