Bitmask: Thermometer from Count

Credit: Hacker's Delight, Section 2-1: Manipulating Rightmost Bits

Sets the first N least-significant bits given integer N. If N is greater than WORD_WIDTH, then the mask is all-ones.

This thermometer bitmask is also the lexicographically first member of the set of bitmasks with a given number of set bits (that is, with a constant population count). The last member is the same sequence, but with the bit order reversed (see: Word Reverser). To generate all members, use Bitmask: Next with Constant Popcount.

`default_nettype none

module Bitmask_Thermometer_from_Count
    parameter WORD_WIDTH    = 0
    input   wire    [WORD_WIDTH-1:0]    count_in,
    output  reg     [WORD_WIDTH-1:0]    word_out

    localparam ONE = {{WORD_WIDTH-1{1'b0}},1'b1};

    initial begin
        word_out = {WORD_WIDTH{1'b0}};

    always @(*) begin
        word_out = (ONE << count_in) - ONE;


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