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/**********************************************************************

 * File  : signed_mult_const_asic.v

 * Author: Ivan Rezki

 * email : irezki@gmail.com

 * Topic : RTL Core

 *                2-Dimensional Fast Hartley Transform

 *

 *

 * Function: Signed Multiplier - constant sqrt(2) = 1.41421

 * 1.41421*a = (256*1.41421)*a/256 = 362.03776*a/256 = 362*a/256

 * product = 362*a/2^8

 *

 * RIGHT TO USE: This code example, or any portion thereof, may be

 * used and distributed without restriction, provided that this entire

 * comment block is included with the example.

 *

 * DISCLAIMER: THIS CODE EXAMPLE IS PROVIDED "AS IS" WITHOUT WARRANTY

 * OF ANY KIND, EITHER EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED

 * TO WARRANTIES OF MERCHANTABILITY, FITNESS OR CORRECTNESS. IN NO

 * EVENT SHALL THE AUTHOR OR AUTHORS BE LIABLE FOR ANY DAMAGES,

 * INCLUDING INCIDENTAL OR CONSEQUENTIAL DAMAGES, ARISING OUT OF THE

 * USE OF THIS CODE.

 **********************************************************************/

module signed_mult_const_asic (

        rstn,

        clk,

        valid,

        a,

        p

);

parameter               N = 8;

input                   rstn;

input                   clk;

input                   valid;

input  [N-1:0] a; // variable - positive/negative

output [N  :0] p; // product output

// FHT constant

//wire [8:0] mult_constant; // always positive

//assign mult_constant = 9'd362;

parameter mult_constant = 9'd362;

reg [N-1:0] a_FF;

always @(posedge clk)

if              (!rstn) a_FF <= #1 0;

else if (valid) a_FF <= #1 a;

// Convert into 2's complement if (a_FF) is negative

wire [N-1:0] b;

assign b = a_FF[N-1] ? {~a_FF[N-1:0] + {{N-1{1'b0}},1'b1} } : a_FF[N-1:0];

// Multiply 2 positive numbers 

// b[N-2:0] * mult_constant[8:0]

// output result mult_wo_sign

// N-2+1 - number of (b) bits

// 8+1   - number of mult_constant bits

// N-2+1+8+1 - number of bits on the output

// = N+8 = [N+7:0]

wire [N+7:0] mult_wo_sign; // mult without sign

assign mult_wo_sign = b[N-2:0]*mult_constant;

// Divided by 256 - [N+7-8:0] = [N-1:0]

wire [N-1:0] div256; // divided 256

assign div256 = mult_wo_sign >> 8;

assign p = a_FF[N-1] ?

                                        {1'b1,{~div256[N-1:0] + {{N-1{1'b0}},1'b1}} } :

                                        {1'b0,  div256[N-1:0]}

                                        ;

endmodule

// Update Log:

// 27 Jul. 2011

// wire [8:0] mult_constant replaced by parameter