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Integer Cube Root in Verilog

Introduction


We have created a number for the binary algorithm. This code had been tested on the Cyclone IV FPGA board. How you works.

Github link: Cube root

What is a cube root?


Cube root of a number

$$ display $$ x ^ 3 = y $$ display $$



Examples:

$$ display $$ \ sqrt [3] {8} = 2 \\ \ sqrt [3] {27} = 3 \\ \ sqrt [3] {64} = 4 $$ display $$


So, we need an integer cube root .
This means that:

$$ display $$ a ^ 3 \ leqslant x, \\ (a + 1) ^ 3 \ geqslant x $$ display $$


Examples:

$$ display $$ \ sqrt [3] {26} = 2 \\ \ sqrt [3] {28} = 3 \\ sqrt [3] {63} = 3 \\ \ sqrt [3] {65} = 4 $$ display $$


Main logic


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The main module is the number during input.
It has 4 possible actions:


Main module
module cube_root( input inc, input sub, input next, input prev, input enter, input clk, output wire [7:0] leds, output wire [7:0] control ); reg signed [31:0] exit; wire ready; wire [31:0] res; reg zero = 0; // input // reg inc1 = 0; reg next1 = 0; reg prev1 = 0; reg sub1 = 0; reg enter1 = 0; reg [31:0] decimal = 1; ////////// reg [31:0] to_display; display_bcd display( .clk(clk), .value(ready == 0 ? exit : res), .control(control), .leds(leds) ); calculate calc( .clk(clk), .ready_to_calc(~enter), .num(exit), .ready(ready), .res(res) ); always @(posedge clk) begin if (enter == 1) begin if ((inc1 == 1'b0) && (~inc == 1'b1)) begin exit = exit + decimal; end inc1 = ~inc; if ((sub1 == 1'b0) && (~sub == 1'b1)) begin if (exit > 0) begin exit = exit - decimal; end end sub1 = ~sub; if ((next1 == 1'b0) && (~next == 1'b1)) begin decimal = decimal * 10; end next1 = ~next; if ((prev1 == 1'b0) && (~prev == 1'b1)) begin if (decimal >= 1 && decimal <= 9) begin decimal = 1; end else begin decimal = decimal / 10; end end prev1 = ~prev; end else begin if (ready == 1'b1) begin exit = 0; decimal = 1; end end end endmodule 


There are also two other modules: calculate and display_bcd . During the program execution.

Now, let's understand how they work.

Calculate module


Calculation module uses a binary search algorithm. It is a signal for the display module to output the answer.

Calculate module
 module calculate( input clk, input ready_to_calc, input [31:0] num, output reg ready, output [31:0] res ); integer mid; integer start; integer final; integer counter; assign res = mid; always @(posedge clk) begin if (ready_to_calc == 1) begin if (ready == 0) begin mid = (start + final )/2; if ((mid*mid*mid) > num) begin final = mid; end else begin start = mid; end if (counter == 27) begin ready = 1; counter = 0; end else begin counter = counter + 1; end end end else begin final = 465; start = 0; ready = 0; counter = 0; end end endmodule 


Why this module does not exactly 27 iterations?
- Maximum input number is 99999999. So, the maximum possible number of iterations is $ inline $ \ log_2 99999999 = 26.575424745 \ approx 27 $ inline $
Why is upper bound of binary search is initialized by 465?
- Because it can be a result. $ inline $ \ sqrt [3] {99999999} \ approx $ 464 inline $

Display module


This module is responsible for the performance. It uses eight eight-segment displays and has been manipulated by 16 pins. Where 8 pins are in charge and they are distinct digits.

So, we want to display this module. Then, it passes the binary number to the Binary Coded Decimal using the Double Dabble algorithm . After that, converted value becomes easy to display.

Display module
 module display_bcd ( input clk, input [31:0] value, output [7:0] control, output [7:0] leds ); bcd_convert #(32, 8) bcd_convert( .i_Clock(clk), .i_Binary(value_temp), .i_Start(1'b1), .o_BCD(bcd_number), .o_DV(bcd_ready) ); integer delay = 0; integer final_bcd; reg [2:0] ctrl = 0; reg [4:0] digit; wire bcd_ready; wire [31:0] bcd_number; wire [31:0] digits; assign digits = final_bcd; wire [31:0] value_temp; assign value_temp = value; assign control = ~(1 << ctrl); assign leds = ~ (digit == 0 ? 8'b00111111 : (digit == 1 ? 8'b00000110 : (digit == 2 ? 8'b01011011 : (digit == 3 ? 8'b01001111 : (digit == 4 ? 8'b01100110 : (digit == 5 ? 8'b01101101 : (digit == 6 ? 8'b01111101 : (digit == 7 ? 8'b00000111 : (digit == 8 ? 8'b01111111 : (digit == 9 ? 8'b01101111 : 8'b00000000)))))))))); always @(posedge clk) begin if (bcd_ready) final_bcd = bcd_number; case(ctrl) 0: digit = digits[3:0]; 1: digit = digits[31:4] ? digits[7:4] : 10; 2: digit = digits[31:8] ? digits[11:8] : 10; 3: digit = digits[31:12] ? digits[15:12] : 10; 4: digit = digits[31:16] ? digits[19:16] : 10; 5: digit = digits[31:20] ? digits[23:20] : 10; 6: digit = digits[31:24] ? digits[27:24] : 10; 7: digit = digits[31:28] ? digits[31:28] : 10; endcase delay = delay + 1; if (delay == 10000) ctrl = ctrl + 1; end endmodule 


BCD convert
 module bcd_convert #(parameter INPUT_WIDTH, parameter DECIMAL_DIGITS) ( input i_Clock, input [INPUT_WIDTH-1:0] i_Binary, input i_Start, output [DECIMAL_DIGITS*4-1:0] o_BCD, output o_DV ); parameter s_IDLE = 3'b000; parameter s_SHIFT = 3'b001; parameter s_CHECK_SHIFT_INDEX = 3'b010; parameter s_ADD = 3'b011; parameter s_CHECK_DIGIT_INDEX = 3'b100; parameter s_BCD_DONE = 3'b101; reg [2:0] r_SM_Main = s_IDLE; // The vector that contains the output BCD reg [DECIMAL_DIGITS*4-1:0] r_BCD = 0; // The vector that contains the input binary value being shifted. reg [INPUT_WIDTH-1:0] r_Binary = 0; // Keeps track of which Decimal Digit we are indexing reg [DECIMAL_DIGITS-1:0] r_Digit_Index = 0; // Keeps track of which loop iteration we are on. // Number of loops performed = INPUT_WIDTH reg [7:0] r_Loop_Count = 0; wire [3:0] w_BCD_Digit; reg r_DV = 1'b0; always @(posedge i_Clock) begin case (r_SM_Main) // Stay in this state until i_Start comes along s_IDLE : begin r_DV <= 1'b0; if (i_Start == 1'b1) begin r_Binary <= i_Binary; r_SM_Main <= s_SHIFT; r_BCD <= 0; end else r_SM_Main <= s_IDLE; end // Always shift the BCD Vector until we have shifted all bits through // Shift the most significant bit of r_Binary into r_BCD lowest bit. s_SHIFT : begin r_BCD <= r_BCD << 1; r_BCD[0] <= r_Binary[INPUT_WIDTH-1]; r_Binary <= r_Binary << 1; r_SM_Main <= s_CHECK_SHIFT_INDEX; end // Check if we are done with shifting in r_Binary vector s_CHECK_SHIFT_INDEX : begin if (r_Loop_Count == INPUT_WIDTH-1) begin r_Loop_Count <= 0; r_SM_Main <= s_BCD_DONE; end else begin r_Loop_Count <= r_Loop_Count + 1; r_SM_Main <= s_ADD; end end // Break down each BCD Digit individually. Check them one-by-one to // see if they are greater than 4. If they are, increment by 3. // Put the result back into r_BCD Vector. s_ADD : begin if (w_BCD_Digit > 4) begin r_BCD[(r_Digit_Index*4)+:4] <= w_BCD_Digit + 3; end r_SM_Main <= s_CHECK_DIGIT_INDEX; end // Check if we are done incrementing all of the BCD Digits s_CHECK_DIGIT_INDEX : begin if (r_Digit_Index == DECIMAL_DIGITS-1) begin r_Digit_Index <= 0; r_SM_Main <= s_SHIFT; end else begin r_Digit_Index <= r_Digit_Index + 1; r_SM_Main <= s_ADD; end end s_BCD_DONE : begin r_DV <= 1'b1; r_SM_Main <= s_IDLE; end default : r_SM_Main <= s_IDLE; endcase end // always @ (posedge i_Clock) assign w_BCD_Digit = r_BCD[r_Digit_Index*4 +: 4]; assign o_BCD = r_BCD; assign o_DV = r_DV; endmodule // Binary_to_BCD 


Authors: Tyurin Leonid, Tikhonov Nikita.

Source: https://habr.com/ru/post/431938/


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