lib/GlitchFilter: a glitch filter to debounce input pins

lib/GlitchFilter.bsv

@@ -0,0 +1,30 @@
+package GlitchFilter;
+
+import Cntrs::*;
+
+(* always_enabled="_write",always_ready="_read" *)
+module mkGlitchFilter(Integer hysteresis, bit init_value, Reg#(bit) ifc);
+   Wire#(bit) in <- mkBypassWire();
+   UCount cnt <- mkUCount(init_value == 0 ? 0 : hysteresis, hysteresis);
+   Reg#(bit) out[2] <- mkCReg(2, init_value);
+
+   (* no_implicit_conditions, fire_when_enabled *)
+   rule incr (cnt.isLessThan(fromInteger(hysteresis)) && in == 1);
+      cnt.incr(1);
+   endrule
+
+   (* no_implicit_conditions, fire_when_enabled *)
+   rule decr (cnt.isGreaterThan(0) && in == 0);
+      cnt.decr(1);
+   endrule
+
+   (* no_implicit_conditions, fire_when_enabled *)
+   rule set_out (cnt.isEqual(0) || cnt.isEqual(fromInteger(hysteresis)));
+      out[0] <= cnt.isEqual(0) ? 0 : 1;
+   endrule
+
+   method _write = in._write;
+   method _read = out[1]._read;
+endmodule
+
+endpackage

lib/GlitchFilter_Test.bsv

@@ -0,0 +1,76 @@
+package GlitchFilter_Test;
+
+import Assert::*;
+import StmtFSM::*;
+
+import GlitchFilter::*;
+import Testing::*;
+
+module mkTB();
+   let testflags <- mkTestFlags();
+   let cycles <- mkCycleCounter();
+
+   let dut <- mkGlitchFilter(3, 0);
+
+   // Slight indirection to appease the compiler: GlitchFilter
+   // requires a write on every cycle, and with just the test FSM, the
+   // compiler can't prove that this is satisfied. So, give it a
+   // default value when the test isn't driving the input, on the
+   // first few cycles of the test.
+   Wire#(bit) pin_in <- mkDWire(0);
+   (* no_implicit_conditions,fire_when_enabled *)
+   rule push_in;
+      dut <= pin_in;
+   endrule
+
+   function Action check_dut(bit in, bit want_out);
+      return action
+                if (testflags.verbose)
+                   $display("%0d: GlitchFilter(%0d) => %0d, want %0d", cycles.all, in, dut, want_out);
+                dynamicAssert(dut == want_out, "wrong GlitchFilter output");
+                pin_in <= in;
+             endaction;
+   endfunction
+
+   runTest(100,
+      mkTest("GlitchFilter", seq
+                                // Simple 0->1 transition
+                                check_dut(0, 0);
+                                check_dut(1, 0);
+                                check_dut(1, 0);
+                                check_dut(1, 0);
+                                check_dut(1, 1);
+                                check_dut(1, 1);
+
+                                // Simple 1->0 transition
+                                check_dut(0, 1);
+                                check_dut(0, 1);
+                                check_dut(0, 1);
+                                check_dut(0, 0);
+                                check_dut(0, 0);
+
+                                // Glitchy 0->1
+                                check_dut(1, 0);
+                                check_dut(1, 0);
+                                check_dut(0, 0);
+                                check_dut(1, 0);
+                                check_dut(0, 0);
+                                check_dut(1, 0);
+                                check_dut(1, 0);
+                                check_dut(1, 1);
+                                check_dut(1, 1);
+
+                                // Glitchy 1->0
+                                check_dut(0, 1);
+                                check_dut(0, 1);
+                                check_dut(1, 1);
+                                check_dut(0, 1);
+                                check_dut(1, 1);
+                                check_dut(0, 1);
+                                check_dut(0, 1);
+                                check_dut(0, 0);
+                                check_dut(0, 0);
+                             endseq));
+endmodule
+
+endpackage