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// Final halfband decimator
// Implements impulse responses of the form [A 0 B 0 C .. 0 H 0.5 H 0 .. C 0 B 0 A]
// Strobe in cannot come faster than every 2nd clock cycle
// These taps designed by halfgen4 from ldoolittle
// myfilt = round(2^18 * halfgen4(.7/4,8))
module hb_dec
#(parameter IWIDTH=18, OWIDTH=18, CWIDTH=18, ACCWIDTH=24)
(input clk,
input rst,
input bypass,
input [8:0] cpi, // Clocks per input -- equal to the decimation ratio ahead of this block
input stb_in,
input [IWIDTH-1:0] data_in,
output reg stb_out,
output reg [OWIDTH-1:0] data_out);
// Control
reg [3:0] addr_odd_a, addr_odd_b, addr_odd_c, addr_odd_d;
wire write_odd, write_even, do_mult;
reg odd;
reg [2:0] phase, phase_d1;
reg stb_out_int;
wire clear, do_acc;
assign do_mult = 1;
always @(posedge clk)
if(rst)
odd <= 0;
else if(stb_in)
odd <= ~odd;
assign write_odd = stb_in & odd;
assign write_even = stb_in & ~odd;
always @(posedge clk)
if(rst)
phase <= 0;
else if(stb_in & odd)
phase <= 1;
else if(phase == 4)
phase <= 0;
else if(phase != 0)
phase <= phase + 1;
always @(posedge clk)
phase_d1 <= phase;
reg [15:0] stb_out_pre;
always @(posedge clk)
if(rst)
stb_out_pre <= 0;
else
stb_out_pre <= {stb_out_pre[14:0],(stb_in & odd)};
always @*
case(phase)
1 : begin addr_odd_a = 0; addr_odd_b = 15; end
2 : begin addr_odd_a = 1; addr_odd_b = 14; end
3 : begin addr_odd_a = 2; addr_odd_b = 13; end
4 : begin addr_odd_a = 3; addr_odd_b = 12; end
default : begin addr_odd_a = 0; addr_odd_b = 15; end
endcase // case(phase)
always @*
case(phase)
1 : begin addr_odd_c = 4; addr_odd_d = 11; end
2 : begin addr_odd_c = 5; addr_odd_d = 10; end
3 : begin addr_odd_c = 6; addr_odd_d = 9; end
4 : begin addr_odd_c = 7; addr_odd_d = 8; end
default : begin addr_odd_c = 4; addr_odd_d = 11; end
endcase // case(phase)
assign do_acc = |stb_out_pre[6:3];
assign clear = stb_out_pre[3];
// Data
wire [IWIDTH-1:0] data_odd_a, data_odd_b, data_odd_c, data_odd_d;
wire [IWIDTH-1:0] sum1, sum2;
wire [OWIDTH-1:0] final_sum;
reg [CWIDTH-1:0] coeff1, coeff2;
wire [35:0] prod1, prod2;
always @* // Outer coeffs
case(phase_d1)
1 : coeff1 = -107;
2 : coeff1 = 445;
3 : coeff1 = -1271;
4 : coeff1 = 2959;
default : coeff1 = -107;
endcase // case(phase)
always @* // Inner coeffs
case(phase_d1)
1 : coeff2 = -6107;
2 : coeff2 = 11953;
3 : coeff2 = -24706;
4 : coeff2 = 82359;
default : coeff2 = -6107;
endcase // case(phase)
srl #(.WIDTH(IWIDTH)) srl_odd_a
(.clk(clk),.write(write_odd),.in(data_in),.addr(addr_odd_a),.out(data_odd_a));
srl #(.WIDTH(IWIDTH)) srl_odd_b
(.clk(clk),.write(write_odd),.in(data_in),.addr(addr_odd_b),.out(data_odd_b));
srl #(.WIDTH(IWIDTH)) srl_odd_c
(.clk(clk),.write(write_odd),.in(data_in),.addr(addr_odd_c),.out(data_odd_c));
srl #(.WIDTH(IWIDTH)) srl_odd_d
(.clk(clk),.write(write_odd),.in(data_in),.addr(addr_odd_d),.out(data_odd_d));
add2_reg /*_and_round_reg*/ #(.WIDTH(IWIDTH)) add1 (.clk(clk),.in1(data_odd_a),.in2(data_odd_b),.sum(sum1));
add2_reg /*_and_round_reg*/ #(.WIDTH(IWIDTH)) add2 (.clk(clk),.in1(data_odd_c),.in2(data_odd_d),.sum(sum2));
wire [IWIDTH-1:0] data_even;
reg [3:0] addr_even;
always @(posedge clk)
case(cpi)
// 1 is an error
2 : addr_even <= 9; // Maximum speed (overall decim by 4)
3, 4, 5, 6, 7 : addr_even <= 8;
default : addr_even <= 7;
endcase // case(cpi)
srl #(.WIDTH(IWIDTH)) srl_even
(.clk(clk),.write(write_even),.in(data_in),.addr(addr_even),.out(data_even));
localparam MWIDTH = ACCWIDTH-2;
wire [MWIDTH-1:0] sum_of_prod;
MULT18X18S mult1(.C(clk), .CE(do_mult), .R(rst), .P(prod1), .A(coeff1), .B(sum1) );
MULT18X18S mult2(.C(clk), .CE(do_mult), .R(rst), .P(prod2), .A(coeff2), .B(sum2) );
add2_and_round_reg #(.WIDTH(MWIDTH))
add3 (.clk(clk),.in1(prod1[35:36-MWIDTH]),.in2(prod2[35:36-MWIDTH]),.sum(sum_of_prod));
wire [ACCWIDTH-1:0] acc_out;
acc #(.IWIDTH(MWIDTH),.OWIDTH(ACCWIDTH))
acc (.clk(clk),.clear(clear),.acc(do_acc),.in(sum_of_prod),.out(acc_out));
localparam SHIFT_FACTOR = ACCWIDTH-IWIDTH-5;
wire [ACCWIDTH-1:0] data_even_signext;
wire [ACCWIDTH:0] final_sum_unrounded;
sign_extend #(.bits_in(IWIDTH),.bits_out(ACCWIDTH-SHIFT_FACTOR))
signext_data_even (.in(data_even),.out(data_even_signext[ACCWIDTH-1:SHIFT_FACTOR]));
assign data_even_signext[SHIFT_FACTOR-1:0] = 0;
add2_reg /* add2_and_round_reg */ #(.WIDTH(ACCWIDTH+1))
final_adder (.clk(clk), .in1({acc_out,1'b0}), .in2({data_even_signext,1'b0}), .sum(final_sum_unrounded));
round_reg #(.bits_in(ACCWIDTH-4),.bits_out(OWIDTH))
final_round (.clk(clk),.in(final_sum_unrounded[ACCWIDTH-5:0]),.out(final_sum));
// Output
always @(posedge clk)
if(bypass)
data_out <= data_in;
else if(stb_out_pre[9])
data_out <= final_sum;
always @(posedge clk)
if(rst)
stb_out <= 0;
else if(bypass)
stb_out <= stb_in;
else
stb_out <= stb_out_pre[9];
endmodule // hb_dec
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