lib/Strobe: add a Strobe module to generate synchronization pulses

lib/Strobe.bsv

@@ -0,0 +1,57 @@
+package Strobe;
+
+import Real::*;
+import Printf::*;
+
+// A Strobe provides a synchronization signal to other modules, when
+// an event happens at a cadence other than the module clock.
+(* always_ready *)
+interface Strobe;
+   method Bool _read();
+   // reset resets the strobe cycle, starting with a strobe on the
+   // cycle following reset.
+   method Action reset();
+endinterface
+
+// mkStrobe returns a Strobe that triggers at the given
+// target_frequency, assuming mkStrobe is being clocked at the given
+// higher clock_frequency.
+module mkStrobe(Integer clock_frequency, Integer target_frequency, Strobe ifc);
+   if (target_frequency > clock_frequency)
+      error("mkStrobe target_frequency must be less than clock_frequency");
+   let strobe_every = round(fromInteger(clock_frequency)/fromInteger(target_frequency));
+
+   // Because we're using integer counters to divide frequencies,
+   // unless the clock and target frequencies divide cleanly we'll end
+   // up with a small amount of error.
+   //
+   // Strobes like this tend to be used for relatively short
+   // operations before some other synchronization event happens
+   // (e.g. sending one byte on UART), so we can allow a small amount
+   // of frequency error. For now, the target frequency error is fixed
+   // at <=0.1%.
+   Real actual_frequency = fromInteger(clock_frequency)/fromInteger(strobe_every);
+   Real frequency_error_pct = abs(fromInteger(target_frequency)-actual_frequency) / fromInteger(target_frequency) * 100;
+   if (frequency_error_pct > 0.1)
+      error(sprintf("mkStrobe actual frequency is %0f, %0f%% error vs. requested %0d. Your clock_frequency and target_frequency are probably too near each other.", actual_frequency, frequency_error_pct, target_frequency));
+
+   Reg#(UInt#(32)) cnt[2] <- mkCReg(2, 0);
+
+   (* no_implicit_conditions, fire_when_enabled *)
+   rule increment;
+      if (cnt[0] == fromInteger(strobe_every-1))
+         cnt[0] <= 0;
+      else
+         cnt[0] <= cnt[0]+1;
+   endrule
+
+   method Bool _read();
+      return cnt[0] == 0;
+   endmethod
+
+   method Action reset();
+      cnt[1] <= 0;
+   endmethod
+endmodule
+
+endpackage

lib/Strobe_Test.bsv

@@ -0,0 +1,53 @@
+package Strobe_Test;
+
+import Assert::*;
+import StmtFSM::*;
+
+import Strobe::*;
+import Testing::*;
+
+module mkTB();
+   let testflags <- mkTestFlags();
+   let cycles <- mkCycleCounter();
+
+   // For this test, we assume we're clocked at 25MHz, and want a
+   // 115_200bps strobe for a serial port. That translates to a strobe
+   // every 217 cycles.
+   let dut <- mkStrobe(25_000_000, 115_200);
+   let want_pulse_every = 217;
+
+   function Action check_dut(Bool want);
+      return action
+                if (testflags.verbose)
+                   $display("%0d (%0d): strobe = %0d, want %0d", cycles.all, cycles, dut, want);
+                dynamicAssert(dut == want, "incorrect strobe state");
+             endaction;
+   endfunction
+
+   function Stmt check_one_cycle();
+      return seq
+                action
+                   check_dut(True);
+                   cycles.reset();
+                endaction
+                while (cycles < want_pulse_every)
+                   check_dut(False);
+             endseq;
+   endfunction
+
+   runTest(2000,
+      mkTest("Strobe", seq
+         dut.reset();
+         repeat(3) check_one_cycle();
+
+         // Reset should actually reset
+         repeat(10) noAction;
+         par
+            check_dut(False);
+            dut.reset();
+         endpar
+         repeat(3) check_one_cycle();
+      endseq));
+endmodule
+
+endpackage