Source

Pulse Divider

Outputs a single-cycle high pulse when divisor input pulses have been received, thus dividing their number. For example, if divisor is 3, then:

Pulses do not have to be distinct: holding the input high for a number of clock cycles will have the same effect as the same number of separate, single-cycle pulses.

The output pulse occurs in the same cycle as the input pulse, which means there is a combinational path from pulses_in to pulse_out. This is necessary to avoid a cycle of latency between an input pulse arriving and signalling that it is a multiple of divisor. (e.g. signalling that the current load fills the last empty space in a buffer).

The divisor is reloaded automatically after each output pulse. Any changes to the divisor input before then do not affect the current pulse division. However, asserting restart for one cycle will force the divisor to reload and the pulse division to restart. Holding restart high will halt the pulse divider. A restart pulse is not required after startup.

Loading a divisor of zero will disable the output pulses, raises div_by_zero, and will load divisor every cycle until it becomes non-zero.

Uses

`default_nettype none

module Pulse_Divider
#(
    parameter                   WORD_WIDTH      = 0,
    parameter [WORD_WIDTH-1:0]  INITIAL_DIVISOR = 0
)
(
    input  wire                     clock,
    input  wire                     restart,
    input  wire [WORD_WIDTH-1:0]    divisor,
    input  wire                     pulses_in,
    output reg                      pulse_out,
    output reg                      div_by_zero
);

    initial begin
        pulse_out   = 1'b0;
        div_by_zero = 1'b0;
    end

These are the two counter values which are important: WORD_ONE signifies we have received divisor or INITIAL_DIVISOR pulses, and WORD_ZERO is never reached unless a division by zero is attempted by loading or initializing with a value of zero.

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

The core of the pulse divider is a simple down counter whose value is interpreted as "input pulses remaining before an output pulse". So a loaded divisor of 3 would accept 3 input pulses to count "3, 2, 1", at which point pulse_out goes high on the third pulses_in while the counter is at 1, the counter reloads the divisor, and everything returns to start conditions at the next clock edge. The count of zero is never reached by counting.

    reg                     run     = 1'b0;
    reg                     load    = 1'b0;
    wire [WORD_WIDTH-1:0]   count;

    Counter_Binary
    #(
        .WORD_WIDTH     (WORD_WIDTH),
        .INCREMENT      (WORD_ONE),
        .INITIAL_COUNT  (INITIAL_DIVISOR)
    )
    pulse_counter
    (
        .clock          (clock),
        .clear          (1'b0),
        .up_down        (1'b1), // down
        .run            (run),
        .load           (load),
        .load_count     (divisor),
        .carry_in       (1'b0),
        // verilator lint_off PINCONNECTEMPTY
        .carry_out      (),
        // verilator lint_on PINCONNECTEMPTY
        .count          (count)
    );

Finally, we implement the control logic. We split out the calculation of div_by_zero into a parallel procedural block, otherwise the linter could see a false combinational loop between div_by_zero and pulse_out when the divider is used in an enclosing module, since it cannot see into the module hierarchy (I'm not certain of this).

    always @(*) begin
        div_by_zero = (count == WORD_ZERO);
    end

    reg division_done = 1'b0;

    always @(*) begin
        run             = (pulses_in     == 1'b1) && (count     != WORD_ZERO);
        division_done   = (pulses_in     == 1'b1) && (count     == WORD_ONE);
        load            = (division_done == 1'b1) || (restart   == 1'b1) || (div_by_zero == 1'b1);
        pulse_out       = (division_done == 1'b1) && (restart   == 1'b0);
    end

endmodule

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