cores/vco: fix and modularize vco example (#36)

gateware/cores/util/wavetable_osc.sv

@@ -1,30 +1,29 @@
 module wavetable_osc #(
     parameter W = 16,
+    parameter FRAC_BITS = 10,
     parameter WAVETABLE_PATH = "cores/util/vco/wavetable.hex",
-    parameter WAVETABLE_SIZE = 256,
+    parameter WAVETABLE_SIZE = 256
 )(
     input rst,
     input sample_clk,
     input [31:0] wavetable_inc,
-    output signed [W-1:0] out,
+    output logic signed [W-1:0] out
 );
 
 logic [W-1:0] wavetable [0:WAVETABLE_SIZE-1];
 initial $readmemh(WAVETABLE_PATH, wavetable);
 
-logic [31:0] wavetable_pos = 32'h0;
+// Position in wavetable - N.F fixed-point where BIT_START is size of F.
+logic [31:0] wavetable_pos = 0;
 
 always_ff @(posedge sample_clk) begin
-    // TODO: linear interpolation between frequencies, silence oscillator
-    // whenever we are outside the LUT bounds.
-    wavetable_pos <= wavetable_pos + wavetable_inc;
+    if (rst) begin
+        wavetable_pos <= 0;
+    end else begin
+        wavetable_pos <= wavetable_pos + wavetable_inc;
+        // Take top N bits of wavetable_pos as output.
+        out <= wavetable[wavetable_pos[FRAC_BITS+$clog2(WAVETABLE_SIZE)-1:FRAC_BITS]];
+    end
 end
 
-// Top 8 bits of the N.F fixed-point representation are index into wavetable.
-localparam BIT_START = 10;
-wire [$clog2(WAVETABLE_SIZE)-1:0] wavetable_idx =
-    wavetable_pos[BIT_START+$clog2(WAVETABLE_SIZE)-1:BIT_START];
-
-assign out = wavetable[wavetable_idx];
-
 endmodule

gateware/cores/vco.sv

@@ -2,7 +2,7 @@
 //
 // Mapping:
 // - Input 0: V/Oct input, C3 is +3V
-// - Output 0-3: VCO output (from wavetable) phased at 0, 90, 120, 270deg.
+// - Output 0: VCO output (from wavetable)
 
 module vco #(
     parameter W = 16,
@@ -35,31 +35,30 @@ module vco #(
 logic [W-1:0] v_oct_lut [0:V_OCT_LUT_SIZE-1];
 initial $readmemh(V_OCT_LUT_PATH, v_oct_lut);
 
-logic [W-1:0] wavetable [0:WAVETABLE_SIZE-1];
-initial $readmemh(WAVETABLE_PATH, wavetable);
-
 // For < 0V input, clamp to bottom note.
-logic signed [W-1:0] lut_index;
-logic [$clog2(V_OCT_LUT_SIZE)-1:0] lut_index_clamped;
-logic [31:0] wavetable_pos = 32'h0;
-
-assign lut_index = sample_in0 >>> 6;
-assign lut_index_clamped = $clog2(V_OCT_LUT_SIZE)'(lut_index < 0 ? W'(0) : lut_index);
+logic signed [W-1:0] lut_index = 0;
+logic signed [W-1:0] lut_index_clamp_lo = 0;
 
 always_ff @(posedge sample_clk) begin
-    // TODO: linear interpolation between frequencies, silence oscillator
-    // whenever we are outside the LUT bounds.
-    wavetable_pos <= wavetable_pos + 32'(v_oct_lut[lut_index_clamped]);
+    if (rst) begin
+        lut_index <= 0;
+        lut_index_clamp_lo <= 0;
+    end else begin
+        lut_index <= sample_in0 >>> 6;
+        lut_index_clamp_lo <= lut_index < 0 ? 0 : lut_index;
+    end
 end
 
-// Top 8 bits of the N.F fixed-point representation are index into wavetable.
-localparam BIT_START = 10 + FDIV;
-wire [$clog2(WAVETABLE_SIZE)-1:0] wavetable_idx =
-    wavetable_pos[BIT_START+$clog2(WAVETABLE_SIZE)-1:BIT_START];
-
-assign sample_out0 = wavetable[wavetable_idx];
-assign sample_out1 = wavetable[wavetable_idx+WAVETABLE_SIZE/4];
-assign sample_out2 = wavetable[wavetable_idx+WAVETABLE_SIZE/2];
-assign sample_out3 = wavetable[wavetable_idx+WAVETABLE_SIZE/2+WAVETABLE_SIZE/4];
+wavetable_osc #(
+    .W(W),
+    .FRAC_BITS(10),
+    .WAVETABLE_PATH(WAVETABLE_PATH),
+    .WAVETABLE_SIZE(WAVETABLE_SIZE)
+) osc_0 (
+    .rst(rst),
+    .sample_clk(sample_clk),
+    .wavetable_inc(32'(v_oct_lut[$clog2(V_OCT_LUT_SIZE)'(lut_index_clamp_lo)])),
+    .out(sample_out0)
+);
 
 endmodule

gateware/mk/common.mk

@@ -20,4 +20,5 @@ SRC_COMMON = eurorack_pmod.sv \
 		     cores/util/transpose.sv \
 		     cores/util/echo.sv \
 		     cores/util/delayline.sv \
-			 cores/util/dc_block.sv
+			 cores/util/dc_block.sv \
+			 cores/util/wavetable_osc.sv

gateware/scripts/verilator_lint.sh

@@ -35,10 +35,11 @@ verilator --lint-only -Icores clkdiv.sv
 verilator --lint-only -Icores sampler.sv
 verilator --lint-only -Icores seqswitch.sv
 verilator --lint-only -Icores vca.sv
-verilator --lint-only -Icores vco.sv
+verilator --lint-only -Icores -Icores/util vco.sv
 verilator --lint-only cores/util/filter/karlsen_lpf.sv
 verilator --lint-only cores/util/filter/karlsen_lpf_pipelined.sv
 verilator --lint-only -Icores -Icores/util/filter filter.sv
 verilator --lint-only -Icores -Icores/util pitch_shift.sv
 verilator --lint-only -Icores -Icores/util stereo_echo.sv
 verilator --lint-only -Icores -Icores/util dc_block.sv
+verilator --lint-only -Icores -Icores/util wavetable_osc.sv