Synthesis Harness Output

When developing a new module, it's very convenient to run it through your CAD tool by itself, on a smaller target FPGA, with any random automatic pin assignments, to iterate quickly and find synth and timing issues. However, you can run out of physical pins, and your logic will get scattered all over the FPGA as it tries to stay close to the pins, wrecking your timing estimates. Also, any input or output logic which isn't registered won't be part of the STA (Static Timing Analysis), so that also makes the timing estimate less accurate when synthesized in isolation.

A solution to both problems is to place your design in a harness of registers. For the outputs, we connect them all to a final bank of registers whose outputs are then XOR-reduced to a single bit, which virtually eliminates the problem of running out of pins, while still avoiding optimizing away any logic by accident since that final output is completely data-dependent. The output is meaningless for simulation, but that's not what we need it for right now.

It would seem natural to do the reverse of the Synthesis Harness Input and do a parallel-to-serial conversion to the output bits to a single bit, but that might pollute our timing analysis since synth might create multiplexers between the outputs and the registers. So we bit-reduce instead.

You must also constrain the harness registers to not be placed in the FPGA I/O registers so they will cluster around your logic, which will now tend to place all together in the center of the FPGA, giving you a reasonnably accurate timing estimate.

You can make the timing estimate more conservative by logically partioning the netlists of the design and the harness so they do not retime into eachother. You can make the timing estimate even more conservative by additionally physically partitioning (a.k.a. floorplanning) the netlists: place your design into a floorplan rectangle (or let the CAD tool do it automatically) and exclude the harness, which will then cluster around the design floorplan and approximate either connection from adjacent floorplans, or logic forced apart by congestion.

A good way to use this module is to add up the widths of all your design outputs and use that sum as the WORD_WIDTH parameter, then connect a concatenation of all your output wires to the word_in port. The remaining harness ports can be connected to any suitable device pins.

`default_nettype none

module Synthesis_Harness_Output 
    parameter   WORD_WIDTH = 0
    input       wire                        clock,
    input       wire                        clear,
    input       wire    [WORD_WIDTH-1:0]    word_in,
    input       wire                        word_in_valid,
    output      reg                         bit_out

    localparam WORD_ZERO = {WORD_WIDTH{1'b0}};

    initial begin
        bit_out = 1'b0;

    wire [WORD_WIDTH-1:0] word_out;

    // Vivado: don't put in I/O buffers, and keep netlists separate in
    // synth and implementation.
    (* IOB = "false" *)
    (* DONT_TOUCH = "true" *)

    // Quartus: don't use I/O buffers, and don't merge registers with others.
    (* useioff = 0 *)
    (* preserve *)

        .WORD_WIDTH     (WORD_WIDTH),
        .PIPE_DEPTH     (1),
        .clock          (clock),
        .clock_enable   (word_in_valid),
        .clear          (clear),
        .parallel_load  (1'b0),
        .parallel_in    (WORD_ZERO),
        // verilator lint_off PINCONNECTEMPTY
        .parallel_out   (),
        // verilator lint_on  PINCONNECTEMPTY
        .pipe_in        (word_in),
        .pipe_out       (word_out)

    always @(*) begin
        bit_out = ^word_out;


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