Fix pulseIn() on SAMD51 by writing it in plain C, so that (arduino#76)

the CM0+-specific pulse_asm won't need to be linked.
The SAMD51 is fast enough that we can time pulses with micros()
Tested with a bunch of pulse lengths from <1us to >1s

Author: WestfW

cores/arduino/pulse.c

@@ -34,6 +34,36 @@ uint32_t pulseIn(uint32_t pin, uint32_t state, uint32_t timeout)
   uint32_t bit = 1 << p.ulPin;
   uint32_t stateMask = state ? bit : 0;
 
+#if defined(__SAMD51__)
+  /*
+   * The SAMD51 is fast enough to use really obvious code (similar to
+   * what was used to produce pulse_asm.S, but using micros() for timing.
+   * No assembly required, no conversion of loop counts to times (which is
+   * worrisome in the presence of cache.)
+   */
+  volatile uint32_t *port = &(PORT->Group[p.ulPort].IN.reg);
+  uint32_t usCallStart;  // microseconds at start of call, for timeout.
+  uint32_t usPulseStart; // microseconds at start of measured pulse.
+  usCallStart = micros();
+  // wait for any previous pulse to end
+  while ((*port & bit) == stateMask) {
+      if (micros() - usCallStart > timeout)
+          return -1;
+  }
+  // wait for the pulse to start
+  while ((*port & bit) != stateMask) {
+      usPulseStart = micros();
+      if (usPulseStart - usCallStart > timeout)
+          return -2;
+  }
+
+  // wait for the pulse to stop
+  while ((*port & bit) == stateMask) {
+      if (micros() - usCallStart > timeout)
+          return -3;
+  }
+  return micros() - usPulseStart;
+#else
   // convert the timeout from microseconds to a number of times through
   // the initial loop; it takes (roughly) 13 clock cycles per iteration.
   uint32_t maxloops = microsecondsToClockCycles(timeout) / 13;
@@ -48,5 +78,6 @@ uint32_t pulseIn(uint32_t pin, uint32_t state, uint32_t timeout)
     return clockCyclesToMicroseconds(width * 13 + 16);
   else
     return 0;
+#endif // SAMD51
 }