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/**
* Collatz Conjecture module.
*
* Takes a start integer and calculates the number of steps
* required for that number to get down to 1 using the collatz
* conjecture.
*/
module collatz (
/* Input clock and reset pin. */
input clk,
input rst,
/* The number to calculate. */
input wire [15:0] in_start,
/* Pulsing the start_int pin will start the calculation. */
input wire start_int,
/* Output count. */
output reg [15:0] o_count,
/* Set to high when the module is idle and ready to start.
* While computing, this output is set to low.
*
* A positive edge of this will indicate the output is ready
* to be consumed. */
output idle
);
// Define some state parameters.
localparam integer IDLE = 0;
localparam integer SPINNING = 1;
reg state = 0; // IDLE or SPINNING
reg [15:0] count = 0; // Current count
reg [15:0] n = 0; // Current value
wire is_even = ~n[0]; // Is the current value even.
assign idle = state == IDLE;
// When the start_int input is set to high, copy the in_start to the current
// number and set the state to SPINNING.
always @(posedge start_int) begin
if (state == IDLE) begin
state <= SPINNING;
n <= in_start;
end
end
// Each clock cycle, determine what needs to happen.
always @(posedge clk or posedge rst) begin
// On reset, reset the state.
if (rst == 1'b1) begin
state <= IDLE;
count <= 0;
n <= 0;
// If the state is idle, reset the count to 0.
end else if (state == IDLE) begin
n <= 0;
// If the state is calculating, do the next step.
end else if (state == SPINNING) begin
// If n is 1, then set the output and go back to the IDLE state.
if (n <= 1) begin
o_count = count;
state = IDLE;
end else begin
// If is_even, divide by two, otherwise multiply by 3 and add 1.
if (is_even) begin
n <= n >> 1;
end else begin
n <= n * 3 + 1;
end
// Increment the count.
count <= count + 1;
end
end
end
endmodule
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