Added source and testbench

verilog/source/run_tb.bat

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+mkdir bin
+del bin\source.vvp
+C:\iverilog\bin\iverilog -o bin\source.vvp -m va_math -g2005 -s SOURCE_TB source.v source_tb.v
+cd bin
+C:\iverilog\bin\vvp source.vvp -lxt2
+cd ..

verilog/source/source.v

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+// 
+// Source part of speech synth
+//
+// Niels Moseley - Moseley Instruments 2017
+// http://www.moseleyinstruments.com
+//
+// 
+
+module SOURCE (
+        clk, 
+        rst_an, 
+        period,         // period in 10kHz samples        
+        amplitude,      // unsigned 15-bit desired amplitude of source output
+        strobe,         // when strobe == '1' a new source_out will be generated
+        source_out      // signed 16-bit source output
+    );
+
+	//////////// CLOCK //////////
+	input clk;
+
+    //////////// RESET, ACTIVE LOW //////////
+    input rst_an;
+
+	//////////// OUTPUTS //////////
+	output reg signed [15:0] source_out;
+
+	//////////// INPUTS //////////
+    input [14:0] amplitude;
+    input [7:0]  period;
+    input        strobe;
+
+    // internal counter and data registers
+    reg signed [7:0] periodcnt;
+    reg        [16:0] lfsr;
+
+    always @(posedge clk, negedge rst_an)
+    begin
+        if (rst_an == 0)
+        begin
+            // reset values
+            periodcnt <= 0;
+            source_out <= 0;
+            lfsr <= 17'h1;   //note: never reset the LFSR to zero!
+        end
+        else
+        begin
+            if (strobe == 1)
+            begin
+                // if period == 0, we need to generate noise
+                // else we generate a pulse wave
+                if (period == 0)
+                begin
+                    // ------------------------------------------------------------
+                    //   LFSR NOISE GENERATOR
+                    // ------------------------------------------------------------
+                    if (periodcnt == 64)
+                        periodcnt <= 0;                
+                    else
+                        periodcnt <= periodcnt + 1;
+                    
+                    // lfsr polynomial is X^17 + X^3 + 1
+                    lfsr = {lfsr[15:0], lfsr[16] ^ lfsr[2]};
+                    source_out <= lfsr[0] ? {2'b00, amplitude[14:1]} : {2'b11, (~amplitude[14:1])};
+                end
+                else
+                begin
+                    // ------------------------------------------------------------
+                    //   PULSE GENERATOR
+                    // ------------------------------------------------------------        
+                    // make periodcnt count from 0 .. period-1
+                    if (periodcnt == period)
+                        periodcnt <= 0;                
+                    else
+                        periodcnt <= periodcnt + 1;
+
+                    if (periodcnt < 8)
+                        source_out <= {1'b0, amplitude};
+                    else
+                        source_out <= 16'h0000;                            
+                end
+            end
+        end
+    end
+
+endmodule

verilog/source/source_tb.v

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+// 
+// PWMDAC testbench
+//
+// Niels Moseley - Moseley Instruments 2017
+// http://www.moseleyinstruments.com
+//
+
+module SOURCE_TB;
+    reg clk, rst_an, strobe; 
+    reg signed [14:0] amp;
+    reg [7:0] period;
+
+    reg [7:0] cnt;
+
+    wire signed [15:0] source_out;
+
+    SOURCE u_source (
+        .clk     (clk),
+        .rst_an  (rst_an),
+        .period  (period),
+        .amplitude (amp),
+        .strobe  (strobe),
+        .source_out  (source_out)
+    );
+
+    integer fd; // file descriptor
+
+    initial
+    begin
+        fd = $fopen("audio.sw","wb");
+        $dumpfile ("source.vcd");
+        $dumpvars;
+        clk = 0;
+        rst_an = 0;
+        strobe = 0;
+        period = 50;
+        amp    = 15000;
+        cnt    = 0;
+        #3
+        rst_an = 1;
+        #300000
+        period = 0; // switch to noise mode
+        #300000
+        $fclose(fd);
+        $finish;
+    end
+
+    always @(posedge clk)
+    begin        
+        if (cnt == 4)
+        begin
+            cnt <= 0;
+            strobe <= 1;            
+            $fwrite(fd,"%u",{ {16{source_out[15]}} ,source_out});
+        end
+        else
+        begin
+            strobe <= 0;
+            cnt <= cnt + 1;
+        end        
+    end
+
+    always
+        #5 clk = !clk;
+
+endmodule