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// Short halfband decimator (intended to be followed by another stage)
// Implements impulse responses of the form [A 0 B 0.5 B 0 A]
//
// These taps designed by halfgen4 from ldoolittle:
// 2 * 131072 * halfgen4(.75/8,2)
module small_hb_int
#(parameter WIDTH=18)
(input clk,
input rst,
input bypass,
input stb_in,
input [WIDTH-1:0] data_in,
input [7:0] output_rate,
input stb_out,
output reg [WIDTH-1:0] data_out);
reg phase;
reg [WIDTH-1:0] d1, d2, d3, d4, d5, d6;
localparam MWIDTH = 36;
wire [MWIDTH-1:0] prod;
reg [6:0] stbin_d;
always @(posedge clk)
stbin_d <= {stbin_d[5:0],stb_in};
always @(posedge clk)
if(stb_in)
begin
d1 <= data_in;
d2 <= d1;
d3 <= d2;
d4 <= d3;
d5 <= d4;
d6 <= d5;
end
wire [WIDTH-1:0] sum_outer, sum_inner;
add2_and_round_reg #(.WIDTH(WIDTH)) add_outer (.clk(clk),.in1(d1),.in2(d4),.sum(sum_outer));
add2_and_round_reg #(.WIDTH(WIDTH)) add_inner (.clk(clk),.in1(d2),.in2(d3),.sum(sum_inner));
wire [17:0] coeff_outer = -10690;
wire [17:0] coeff_inner = 75809;
MULT18X18S mult(.C(clk), .CE(1), .R(rst), .P(prod), .A(stbin_d[1] ? coeff_outer : coeff_inner),
.B(stbin_d[1] ? sum_outer : sum_inner) );
wire [MWIDTH:0] accum;
acc #(.IWIDTH(MWIDTH),.OWIDTH(MWIDTH+1))
acc (.clk(clk),.clear(stbin_d[2]),.acc(|stbin_d[3:2]),.in(prod),.out(accum));
wire [WIDTH+2:0] accum_rnd;
round_reg #(.bits_in(MWIDTH+1),.bits_out(WIDTH+3))
final_round (.clk(clk),.in(accum),.out(accum_rnd));
wire [WIDTH-1:0] clipped;
clip_reg #(.bits_in(WIDTH+3),.bits_out(WIDTH))
final_clip (.clk(clk),.in(accum_rnd),.out(clipped));
reg [WIDTH-1:0] saved, saved_d3;
always @(posedge clk)
if(stbin_d[6])
saved <= clipped;
always @(posedge clk)
if(stbin_d[3])
saved_d3 <= d3;
always @(posedge clk)
if(bypass)
data_out <= data_in;
else if(stb_in & stb_out)
case(output_rate)
1 : data_out <= d6;
2 : data_out <= d4;
3, 4, 5, 6, 7 : data_out <= d3;
default : data_out <= d2;
endcase // case(output_rate)
else if(stb_out)
data_out <= saved;
endmodule // small_hb_int
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