One-Hot Demultiplexer

Connects the word_in input port to one of the words in the words_out output port, as selected by the selectors one-hot vector, and raises the corresponding valid bit in the valids_out output port. If more than one bit is set in the selectors input, then all the corresponding output words will get the input word value.

Implementation Options

Set the BROADCAST parameter to 1 to simply replicate and connect word_in to each word in words_out, without any logic. The valid bit will indicate which downstream logic should accept the data, and other logic can snoop the data if that's part of your larger design.

Set the BROADCAST parameter to 0 to send word_in to only the selected word in words_out, whose valid bit is set. All other output words are annulled to zero. If no valid bit is set, all output words stay at zero.

Not broadcasting the input to the output costs some logic, but less than it appears from a standalone synthesis of the demultiplexer: the inferred Annullers will very likely disappear into downstream LUT logic, it also makes tracing a simulation easier, and adds some design security and robustness since unselected downstream logic cannot snoop or accidentally receive other data.

If necessary, select the Annuller IMPLEMENTATION which yields the best logic synthesis. Usually, this does not matter, and can be set to "AND".

Setting BROADCAST to any value other than 1 or 0 will disconnect word_in from words_out, raise some critical warnings in your CAD tool, and generally cause a lot of downstream logic to optimize away, so you should notice...

`default_nettype none

module Demultiplexer_One_Hot
    parameter       BROADCAST           = 0,
    parameter       WORD_WIDTH          = 0,
    parameter       OUTPUT_COUNT        = 0,
    parameter       IMPLEMENTATION      = "AND",

    // Do not set at instantiation
    input   wire    [OUTPUT_COUNT-1:0]  selectors,
    input   wire    [WORD_WIDTH-1:0]    word_in,
    output  reg     [TOTAL_WIDTH-1:0]   words_out,
    output  reg     [OUTPUT_COUNT-1:0]  valids_out

    localparam OUTPUT_ZERO = {OUTPUT_COUNT{1'b0}};
    localparam TOTAL_ZERO  = {TOTAL_WIDTH{1'b0}};

    initial begin
        words_out  = TOTAL_ZERO;
        valids_out = OUTPUT_ZERO;

Pass along the selector to the downstream logic, so we know which output word is the selected one.

    always @(*) begin
        valids_out = selectors;

If we are not broadcasting, then for each output word, annul the output if its selector bit is not set. Thus, only the selected output word will have the word_in data. All others will stay at zero. Otherwise, simply replicate and connect the input to all outputs.

        if (BROADCAST == 0) begin
            wire [TOTAL_WIDTH-1:0] words_out_internal;

            genvar i;
            for (i=0; i < OUTPUT_COUNT; i=i+1) begin: per_output
                    .WORD_WIDTH     (WORD_WIDTH),
                    .annul          (selectors[i] == 1'b0),
                    .data_in        (word_in),
                    .data_out       (words_out_internal[WORD_WIDTH*i +: WORD_WIDTH])

            always @(*) begin
                words_out = words_out_internal;
        if (BROADCAST == 1) begin
            always @(*) begin
                words_out = {OUTPUT_COUNT{word_in}};


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