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module cic3_pdm (
input wire clk, // PDM clock
input wire rst, // active-high synchronous reset
input wire pdm_in, // 1-bit PDM data input
input wire [7:0] hpf_alpha, // HPF coefficient (0-255, 255=bypass)
input wire [2:0] scale_shift, // Right shift amount (0-7 bits)
output reg signed [15:0] pcm_out, // Decimated PCM output
output reg pcm_valid // Output valid pulse
);
// CIC filter parameters
localparam DECIMATION = 64;
localparam CIC_WIDTH = 17; // 17 bits for safe margin against overflow
// CIC integrator and comb stages
reg signed [CIC_WIDTH-1:0] integrator1, integrator2;
reg signed [CIC_WIDTH-1:0] comb1, comb2;
reg signed [CIC_WIDTH-1:0] comb1_d1, comb2_d1;
// Decimation counter
reg [6:0] decim_count; // 7 bits to match DECIMATION width
// CIC output and scaling
reg signed [CIC_WIDTH-1:0] cic_out;
reg signed [15:0] scaled_out;
reg cic_valid;
// Leaky integrator HPF registers
reg signed [23:0] hpf_integrator; // 24-bit for precision
reg signed [15:0] hpf_output;
// HPF calculation wires
wire signed [8:0] hpf_complement;
wire signed [23:0] hpf_scaled_input;
wire signed [23:0] hpf_dc_estimate;
/* verilator lint_off UNUSEDSIGNAL */
wire signed [23:0] hpf_temp_result;
/* verilator lint_on UNUSEDSIGNAL */
// HPF wire assignments
assign hpf_complement = 9'd256 - {1'b0, hpf_alpha};
assign hpf_scaled_input = {{8{scaled_out[15]}}, scaled_out};
assign hpf_dc_estimate = hpf_integrator >>> 8;
assign hpf_temp_result = hpf_scaled_input - hpf_dc_estimate;
wire signed [CIC_WIDTH-1:0] pdm_signed = pdm_in ? 17'sd1 : -17'sd1;
// CIC Integrator stages (run at PDM rate)
always @(posedge clk) begin
if (rst) begin
integrator1 <= 0;
integrator2 <= 0;
end else begin
integrator1 <= integrator1 + pdm_signed;
integrator2 <= integrator2 + integrator1;
end
end
// Decimation counter and CIC comb stages
always @(posedge clk) begin
if (rst) begin
decim_count <= 0;
comb1 <= 0;
comb2 <= 0;
comb1_d1 <= 0;
comb2_d1 <= 0;
cic_out <= 0;
cic_valid <= 0;
end else begin
cic_valid <= 0;
if (decim_count == DECIMATION - 1) begin
decim_count <= 0;
// Comb stage 1
comb1 <= integrator2 - comb1_d1;
comb1_d1 <= integrator2;
// Comb stage 2
comb2 <= comb1 - comb2_d1;
comb2_d1 <= comb1;
cic_out <= comb2;
cic_valid <= 1;
end else begin
decim_count <= decim_count + 1;
end
end
end
// Output scaling
always @(*) begin
case (scale_shift)
3'd0: scaled_out = cic_out[15:0];
3'd1: scaled_out = cic_out[16:1];
3'd2: scaled_out = {{1{cic_out[16]}}, cic_out[16:2]}; // Sign extend from bit 16
3'd3: scaled_out = {{2{cic_out[16]}}, cic_out[16:3]}; // Sign extend from bit 16
3'd4: scaled_out = {{3{cic_out[16]}}, cic_out[16:4]}; // Sign extend from bit 16
3'd5: scaled_out = {{4{cic_out[16]}}, cic_out[16:5]}; // Sign extend from bit 16
3'd6: scaled_out = {{5{cic_out[16]}}, cic_out[16:6]}; // Sign extend from bit 16
3'd7: scaled_out = {{6{cic_out[16]}}, cic_out[16:7]}; // Sign extend from bit 16
endcase
end
// Leaky integrator-based DC blocker
// y[n] = x[n] - dc_estimate
// dc_estimate[n] = alpha/256 * dc_estimate[n-1] + (1-alpha/256) * x[n]
always @(posedge clk) begin
if (rst) begin
hpf_integrator <= 0;
hpf_output <= 0;
pcm_out <= 0;
pcm_valid <= 0;
end else begin
pcm_valid <= 0;
if (cic_valid) begin
if (hpf_alpha == 8'd255) begin
// Bypass HPF completely
pcm_out <= scaled_out;
end else begin
// Update leaky integrator (DC estimate)
// integrator = alpha * integrator + (256-alpha) * input
hpf_integrator <= (hpf_alpha * hpf_dc_estimate) +
(hpf_complement * hpf_scaled_input);
// Output = input - DC estimate
hpf_output <= hpf_temp_result[15:0];
pcm_out <= hpf_output;
end
pcm_valid <= 1;
end
end
end
endmodule
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