ad_dds: Add selectable out data width and fair rounding

The CORDIC has a selectable width range for phase and data of 8-24.
Regarding the width of phase and data, the wider they are the smaller
the precision loss when shifting but with the cost of more FPGA
utilization. The user must decide between precision and utilization.

The DDS_WD parameter is independent of CORDIC(CORDIC_DW) or
Polynomial(16bit), letting the user chose the output width.
Here we encounter two scenarios:
 * DDS_DW < DDS data width - in this case, a fair rounding will be
implemented corresponding to the truncated bits
 * DDS_DW > DDS data width - DDS out data left shift to get the
corresponding concatenation bits.

library/common/ad_dds.v

@@ -37,51 +37,83 @@
 
 module ad_dds #(
 
-  // data path disable
-
+  // Disable DDS
   parameter   DISABLE = 0,
+  // Range = 8-24
+  parameter   DDS_DW = 16,
+  // Set 1 for CORDIC or 2 for Polynomial
   parameter   DDS_TYPE = 1,
+  // Range = 8-24
   parameter   CORDIC_DW = 16,
+  // Range = 8-24 ( make sure CORDIC_PHASE_DW < CORDIC_DW)
   parameter   CORDIC_PHASE_DW = 16) (
 
   // interface
 
-  input           clk,
-  input           dds_format,
-  input   [15:0]  dds_phase_0,
-  input   [15:0]  dds_scale_0,
-  input   [15:0]  dds_phase_1,
-  input   [15:0]  dds_scale_1,
-  output  [15:0]  dds_data);
+  input                 clk,
+  input                 dds_format,
+  input   [      15:0]  dds_phase_0,
+  input   [      15:0]  dds_scale_0,
+  input   [      15:0]  dds_phase_1,
+  input   [      15:0]  dds_scale_1,
+  output  [DDS_DW-1:0]  dds_data);
+
+ // Local parameters
+
+ localparam CORDIC = 1;
+ localparam POLYNOMIAL = 2;
+
+ // The width for Polynomial DDS is fixed (16)
+ localparam DDS_D_DW = (DDS_TYPE == CORDIC) ? CORDIC_DW : 16;
+ localparam DDS_P_DW = (DDS_TYPE == CORDIC) ? CORDIC_PHASE_DW : 16;
+ // concatenation or truncation width
+ localparam C_T_WIDTH = (DDS_D_DW > DDS_DW) ? (DDS_D_DW - DDS_DW) : (DDS_DW - DDS_D_DW);
 
   // internal registers
 
-  reg     [15:0]  dds_data_int = 'd0;
-  reg     [15:0]  dds_data_out = 'd0;
-  reg     [15:0]  dds_scale_0_d = 'd0;
-  reg     [15:0]  dds_scale_1_d = 'd0;
+  reg     [  DDS_DW-1:0]  dds_data_width = 0;
+  reg     [DDS_D_DW-1:0]  dds_data_rownd = 0;
+  reg     [DDS_D_DW-1:0]  dds_data_int = 0;
+  reg     [        15:0]  dds_scale_0_d = 0;
+  reg     [        15:0]  dds_scale_1_d = 0;
+  reg     [  DDS_DW-1:0]  dds_data_out = 0;
 
   // internal signals
 
   wire    [15:0]  dds_data_0_s;
   wire    [15:0]  dds_data_1_s;
 
-  // disable
-
+  // disable DDS
   generate
     if (DISABLE == 1) begin
-      assign dds_data = 16'd0;
+      // assign 0 for the exact buss width to avoid warnings
+      assign dds_data = {DDS_DW{1'b0}};
     end else begin
 
+      // dds channel output
       assign dds_data = dds_data_out;
 
-       // dds channel output
-
-       always @(posedge clk) begin
-         dds_data_int <= dds_data_0_s + dds_data_1_s;
-         dds_data_out[15:15] <= dds_data_int[15] ^ dds_format;
-         dds_data_out[14: 0] <= dds_data_int[14:0];
-       end
+      // output data format
+      always @(posedge clk) begin
+        dds_data_out[DDS_DW-1] <= dds_data_width[DDS_DW-1] ^ dds_format;
+        dds_data_out[DDS_DW-2: 0] <= dds_data_width[DDS_DW-2: 0];
+      end
+
+      // set desired data width
+      always @(posedge clk) begin
+        if (DDS_DW <= DDS_D_DW) begin // truncation
+          // fair rownding
+          dds_data_rownd <= dds_data_int + {(C_T_WIDTH){dds_data_int[DDS_D_DW-1]}};
+          dds_data_width <= dds_data_rownd[DDS_D_DW-1:DDS_D_DW-DDS_DW];
+        end else begin // concatenation
+          dds_data_width <= dds_data_int << C_T_WIDTH;
+        end
+      end
+
+      // dual tone
+      always @(posedge clk) begin
+        dds_data_int <= dds_data_0_s + dds_data_1_s;
+      end
 
        always @(posedge clk) begin
          dds_scale_0_d <= dds_scale_0;