vsrc: add ClockDivider3 used to simulate unaligned clocks

vsrc/ClockDivider3.v

@@ -0,0 +1,37 @@
+// See LICENSE.SiFive for license details.
+
+/** This black-boxes a Clock Divider by 3.
+  * The output clock is phase-aligned to the input clock.
+  * Do NOT use this in synthesis; the duty cycle is 2:1.
+  *
+  * Because Chisel does not support
+  * blocking assignments, it is impossible
+  * to create a deterministic divided clock.
+  *
+  *  @param clk_out Divided Clock
+  *  @param clk_in  Clock Input
+  *
+  */
+
+module ClockDivider3 (output reg clk_out, input clk_in);
+
+   reg delay;
+
+   initial begin
+     clk_out = 1'b0;
+     delay = 1'b0;
+   end
+
+   always @(posedge clk_in) begin
+      if (clk_out == 1'b0) begin
+        clk_out = 1'b1;
+        delay <= 1'b0;
+      end else if (delay == 1'b1) begin
+        clk_out = 1'b0;
+        delay <= 1'b0;
+      end else begin
+        delay <= 1'b1;
+      end
+   end
+
+endmodule // ClockDivider3