Added decimator

HW/sampling.v

@@ -5,13 +5,14 @@ module sampling(
     input wire adc_B,
     output wire adc_O,
 
-    input wire clk,
+    input wire clk_15,
+    input wire clk_120,
     input wire reset_n
 );
     wire sigmadelta_sample;
 
     sigmadelta_sampler m_sdsampler(
-      .clk(clk),
+      .clk(clk_15),
       .A(adc_A),
       .B(adc_B),
       .out(sigmadelta_sample)
@@ -34,10 +35,99 @@ module sampling(
         else clamp01_q15 = v;
     endfunction
 
-    always @(posedge clk or negedge reset_n) begin
+    always @(posedge clk_15 or negedge reset_n) begin
         if (!reset_n) y_q15 <= 16'sd0000;
         else y_q15 <= clamp01_q15(y_next_q15);
     end
     // ------------------------------
 
-endmodule
+    wire signed [15:0] lpfed_q15;
+    lpf_iir_q15 #(.K(7)) m_lpf (
+        .clk(clk_15),
+        .rst_n(reset_n),
+        .x_q15(y_next_q15),
+        .y_q15(lpfed_q15)
+    );
+
+    wire signed [15:0] decimated_q15;
+    decimate_by_r #(.R(375)) m_decimator (
+        .clk(clk_15),
+        .rst_n(reset_n),
+        .in_valid(1'b1),
+        .in_q15(lpfed_q15),
+        .out_valid(),
+        .out_q15(decimated_q15)
+    );
+
+    // decimated_q15 + out_valid @ 40KHz
+
+endmodule
+
+// --------------------------------------------------------------------
+// Simple 1-pole IIR LPF, Q1.15 in/out, multiplier-less (alpha = 1/2^K)
+// --------------------------------------------------------------------
+module lpf_iir_q15 #(
+    parameter integer K = 10  // try 8..12; bigger = more smoothing
+)(
+    input  wire        clk,
+    input  wire        rst_n,
+    input  wire signed [15:0] x_q15,   // Q1.15 input (e.g., 0..0x7FFF)
+    output reg  signed [15:0] y_q15    // Q1.15 output
+);
+    wire signed [15:0] e_q15     = x_q15 - y_q15;
+    wire signed [15:0] delta_q15 = e_q15 >>> K;   // arithmetic shift
+    wire signed [15:0] y_next    = y_q15 + delta_q15;
+
+    // clamp to [0, 0x7FFF] (handy if your signal is non-negative)
+    function signed [15:0] clamp01;
+        input signed [15:0] v;
+        begin
+            if (v < 16'sd0)         clamp01 = 16'sd0;
+            else if (v > 16'sh7FFF) clamp01 = 16'sh7FFF;
+            else                    clamp01 = v;
+        end
+    endfunction
+
+    always @(posedge clk or negedge rst_n) begin
+        if (!rst_n) y_q15 <= 16'sd0;
+        else        y_q15 <= clamp01(y_next);
+    end
+endmodule
+
+// -------------------------------------------------------------
+// Decimate-by-R: passes through one sample every R input clocks
+// No $clog2 used; set CNT_W wide enough for your R.
+// -------------------------------------------------------------
+module decimate_by_r #(
+    parameter integer R      = 400, // decimation factor
+    parameter integer CNT_W  = 10   // width so that 2^CNT_W > R (e.g., 10 for 750)
+)(
+    input  wire        clk,
+    input  wire        rst_n,
+    input  wire        in_valid,          // assert 1'b1 if always valid
+    input  wire signed [15:0] in_q15,     // Q1.15 sample at full rate
+    output reg         out_valid,         // 1-cycle pulse every R samples
+    output reg  signed [15:0] out_q15     // Q1.15 sample at decimated rate
+);
+    reg [CNT_W-1:0] cnt;
+
+    always @(posedge clk or negedge rst_n) begin
+        if (!rst_n) begin
+            cnt       <= {CNT_W{1'b0}};
+            out_valid <= 1'b0;
+            out_q15   <= 16'sd0;
+        end else begin
+            out_valid <= 1'b0;
+
+            if (in_valid) begin
+                if (cnt == R-1) begin
+                    cnt       <= {CNT_W{1'b0}};
+                    out_q15   <= in_q15;
+                    out_valid <= 1'b1;
+                end else begin
+                    cnt <= cnt + 1'b1;
+                end
+            end
+        end
+    end
+endmodule

SIM/sampling_tb.v

@@ -1,11 +1,13 @@
 `timescale 1ns/1ps
 
 module sampling_tb;
-    reg clk;
+    reg clk_15;
+    reg clk_120;
     reg reset_n;
 
     sampling m_sampling(
-        .clk(clk),
+        .clk_15(clk_15),
+        .clk_120(clk_120),
         .reset_n(reset_n)
     );
 
@@ -13,15 +15,16 @@ module sampling_tb;
         $dumpfile("sampling_tb.vcd");
         $dumpvars (0, sampling_tb);
 
-        clk <= 1'b0;
+        clk_15 <= 1'b0;
+        clk_120 <= 1'b0;
         reset_n <= 1'b0;
         #50 reset_n <= 1'b1;
 
-        #1000000
+        #2000000
         $finish;
     end
 
-    //15 MHz clock
-    always #33.33 clk = ~clk;
+    always #(500/15) clk_15 = ~clk_15;
+    always #(500/120) clk_120 = ~clk_120;
 
 endmodule

SIM/sigmadelta_sampler.v

@@ -8,7 +8,7 @@ module sigmadelta_sampler(
 );
     // ===== Tunable parameters =====
     // Sine source (A input / P)
-    parameter real    F_HZ       = 2.0e3;   // input sine frequency (1 kHz)
+    parameter real    F_HZ       = 1.5e3;   // input sine frequency (1 kHz)
     parameter real    AMP        = 1.5;     // sine amplitude (V)
     parameter real    VCM        = 1.65;    // common-mode (V), centered in 0..3.3V