Added decimation

project.cfg

@@ -24,6 +24,7 @@ files_verilog       = rtl/toplevel/top_generic.v
                       rtl/core/sigmadelta_rcmodel_q15.v
                       rtl/core/mul_const.v
                       rtl/core/lpf_iir_q15_k.v
+                      rtl/core/decimate_by_r_q15.v
                       rtl/arch/spartan-6/lvds_comparator.v
                       rtl/arch/spartan-6/clk_gen.v
 files_con           = boards/mimas_v1/constraints.ucf
@@ -52,6 +53,7 @@ files_verilog       = sim/tb/tb_nco_q15.v
                       rtl/core/sigmadelta_rcmodel_q15.v
                       rtl/core/mul_const.v
                       rtl/core/lpf_iir_q15_k.v
+                      rtl/core/decimate_by_r_q15.v
                       sim/overrides/sigmadelta_sampler.v
                       sim/overrides/clk_gen.v
 files_other         = rtl/util/conv.vh

rtl/core/decimate_by_r_q15.v

@@ -0,0 +1,74 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// Decimator by R
+// Reduces the effective sample rate by an integer factor R by selecting every
+// R-th input sample. Generates a one-cycle 'out_valid' pulse each time a new
+// decimated sample is produced.
+//
+// Implements:
+//      For each valid input sample:
+//          if (count == R-1):
+//              count     <= 0
+//              out_q15   <= in_q15
+//              out_valid <= 1
+//          else:
+//              count     <= count + 1
+//
+// parameters:
+//      -- R      : integer decimation factor (e.g., 400)
+//                   output sample rate = input rate / R
+//      -- CNT_W  : counter bit width, must satisfy 2^CNT_W > R
+//
+// inout:
+//      -- clk       : input clock (same rate as 'in_valid')
+//      -- rst_n     : active-low synchronous reset
+//      -- in_valid  : input data strobe; assert 1'b1 if input is always valid
+//      -- in_q15    : signed 16-bit Q1.15 input sample (full-rate)
+//      -- out_valid : single-cycle pulse every R samples (decimated rate strobe)
+//      -- out_q15   : signed 16-bit Q1.15 output sample (decimated stream)
+//
+// Notes:
+//  - This module performs *pure downsampling* (sample selection only).
+//    It does not include any anti-alias filtering; high-frequency content
+//    above the new Nyquist limit (Fs_out / 2) will alias into the baseband.
+//  - For most applications, an anti-alias low-pass filter such as
+//    lpf_iir_q15 or a FIR stage should precede this decimator.
+//  - The output sample rate is given by:
+//          Fs_out = Fs_in / R
+//  - Typical usage: interface between high-rate sigma-delta or oversampled
+//    data streams and lower-rate processing stages.
+// =============================================================================
+module decimate_by_r_q15 #(
+    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/tb/tb_sigmadelta.v

@@ -35,10 +35,17 @@ module tb_sigmadelta();
     );
 
     wire signed [15:0] y_q15;
-    lpf_iir_q15_k #(8) lpf(
+    lpf_iir_q15_k #(9) lpf(
         .clk(clk), .rst_n(resetn),
         .x_q15(sample_q15),
         .y_q15(y_q15)
     );
 
+    wire signed [15:0] decimated_q15;
+    decimate_by_r_q15 #(400, 10) decimate(
+        .clk(clk), .rst_n(resetn),
+        .in_valid(1'b1), .in_q15(y_q15),
+        .out_valid(), .out_q15(decimated_q15)
+    );
+
 endmodule