Added lvds and sampler

project.cfg

@@ -29,6 +29,8 @@ xst_opts            = -vlgincdir rtl
 #files_vhdl          = 
 files_verilog       = rtl/toplevel/top_generic.v
                       rtl/core/nco_q15.v
+                      rtl/core/sigmadelta_sampler.v
+                      rtl/arch/spartan-6/lvds_comparator.v
 files_con           = boards/mimas_v1/constraints.ucf
 files_other         = rtl/util/conv.vh
 
@@ -42,4 +44,6 @@ ivl_opts            = -Irtl
 #files_sysverilog    = 
 files_verilog       = sim/tb/tb_nco_q15.v
                       rtl/core/nco_q15.v
+                      rtl/core/lvds_comparator.v
+                      sim/overrides/sigmadelta_sampler.v
 files_other         = rtl/util/conv.vh

rtl/arch/gw1/lvds_comparator.v

@@ -0,0 +1,17 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// LVDS comparator
+// Instantiating a GW1 TLVDS_IBUF
+// =============================================================================
+module lvds_comparator(
+    input wire a,
+    input wire b,
+    output wire o
+);
+    TLVDS_IBUF lvds_buf (
+        .O(o),
+        .I(a),
+        .IB(b)
+    );
+endmodule

rtl/arch/spartan-6/lvds_comparator.v

@@ -0,0 +1,20 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// LVDS comparator
+// Instantiating a spartan-6 IBUFDS
+// =============================================================================
+module lvds_comparator(
+    input wire a,
+    input wire b,
+    output wire o
+);
+    IBUFDS #(
+        .DIFF_TERM("FALSE"),
+        .IOSTANDARD("LVDS33")
+    ) lvds_buf (
+        .O(o),
+        .I(a),
+        .IB(b)
+    );
+endmodule

rtl/core/lvds_comparator.v

@@ -0,0 +1,13 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// LVDS comparator
+// Simple pass-though model
+// =============================================================================
+module lvds_comparator(
+    input wire a,
+    input wire b,
+    output wire o
+);
+    assign o = a;
+endmodule

rtl/core/sigmadelta_sampler.v

@@ -0,0 +1,23 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// Sigma-Delta sampler
+// Samples A>B at clk
+// =============================================================================
+module sigmadelta_sampler(
+    input wire clk,
+    input wire a,
+    input wire b,
+    output wire o
+);
+
+    wire comp_out;
+    lvds_comparator comp (
+        .a(a), .b(b), .o(comp_out)
+    );
+
+    reg registered_comp_out;
+    always @(posedge clk) registered_comp_out <= o;
+    assign o = registered_comp_out;
+
+endmodule

sim/overrides/sigmadelta_sampler.v

@@ -0,0 +1,111 @@
+`timescale 1ns/1ps
+
+// =============================================================================
+// Sigma-Delta sampler
+// Simulates an RC circuit between O and B and a sine at A
+// =============================================================================
+module sigmadelta_sampler(
+    input wire clk,
+    input wire a,
+    input wire b,
+    output wire o
+);
+
+    // Sine source (A input / P)
+    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
+
+    // Comparator behavior
+    parameter real    VTH        = 0.0;     // threshold on (vp - vn)
+    parameter real    VHYST      = 0.05;     // symmetric hysteresis half-width (V)
+    parameter integer ADD_HYST   = 0;       // 1 to enable hysteresis
+
+    // 1-bit DAC rails (feedback into RC)
+    parameter real    VLOW       = 0.0;     // DAC 0 (V)
+    parameter real    VHIGH      = 3.3;     // DAC 1 (V)
+
+    // RC filter (B input / N)
+    parameter real    R_OHMS     = 3300.0;  // 3.3k
+    parameter real    C_FARADS   = 220e-12; // 220 pF
+
+    // Integration step (ties to `timescale`)
+    parameter integer TSTEP_NS   = 10;      // sim step in ns (choose << tau)
+
+    // ===== Internal state (simulation only) =====
+    real vp, vn;          // comparator A/B inputs
+    real v_rc;            // RC node voltage (== vn)
+    real v_dac;           // DAC output voltage from O
+    real t_s;             // time in seconds
+    real dt_s;            // step in seconds
+    real tau_s;           // R*C time constant in seconds
+    real two_pi;
+    reg  q;               // comparator latched output (pre-delay)
+    reg  out;
+    reg  sampler;
+
+    initial sampler <= 1'b0;
+    always @(posedge clk) begin
+        sampler <= out;
+    end
+    assign o = sampler;
+
+
+    // Helper task: update comparator with optional hysteresis
+    task automatic comp_update;
+        real diff;
+    begin
+        diff = (vp - vn);
+
+        if (ADD_HYST != 0) begin
+            // simple symmetric hysteresis around VTH
+            if (q && (diff < (VTH - VHYST))) q = 1'b0;
+            else if (!q && (diff > (VTH + VHYST))) q = 1'b1;
+            // else hold
+        end else begin
+            q = (diff > VTH) ? 1'b1 : 1'b0;
+        end
+    end
+    endtask
+
+    initial begin
+        // Init constants
+        two_pi = 6.283185307179586;
+        tau_s  = R_OHMS * C_FARADS;       // ~7.26e-7 s
+        dt_s   = TSTEP_NS * 1.0e-9;
+
+        // Init states
+        t_s  = 0.0;
+        q    = 1'b0;                      // start low
+        out  = 1'b0;
+        v_dac= VLOW;
+        v_rc = (VHIGH + VLOW)/2.0;        // start mid-rail to reduce start-up transient
+        vn   = v_rc;
+        vp   = VCM;
+
+        // Main sim loop
+        forever begin
+            #(TSTEP_NS);                  // advance discrete time step
+            t_s  = t_s + dt_s;
+
+            // 1) Update DAC from previous comparator state
+            v_dac = sampler ? VHIGH : VLOW;
+
+            // 2) RC low-pass driven by DAC: Euler step
+            //    dv = (v_dac - v_rc) * dt/tau
+            v_rc  = v_rc + (v_dac - v_rc) * (dt_s / tau_s);
+            vn    = v_rc;
+
+            // 3) Input sine on A
+            vp    = VCM + AMP * $sin(two_pi * F_HZ * t_s);
+
+            // 4) Comparator decision (with optional hysteresis)
+            comp_update();
+
+            // 5) Output with propagation delay
+            out = q;
+        end
+    end
+
+
+endmodule