Map pin number argument to internal pin number using digitalPinToPin

Adjust loop count to microsecond computations

cores/nRF5/pulse.c

@@ -27,27 +27,25 @@ extern unsigned long countPulseASM(const volatile uint32_t *port, uint32_t bit,
  * or LOW, the type of pulse to measure.  Works on pulses from 2-3 microseconds
  * to 3 minutes in length, but must be called at least a few dozen microseconds
  * before the start of the pulse. */
-uint32_t pulseIn(uint32_t pin, uint32_t state, uint32_t timeout)
+uint32_t pulseIn(uint32_t ulPin, uint32_t state, uint32_t timeout)
 {
   // cache the port and bit of the pin in order to speed up the
   // pulse width measuring loop and achieve finer resolution.  calling
   // digitalRead() instead yields much coarser resolution.
   // PinDescription p = g_APinDescription[pin];
-  uint32_t bit = 1 << pin; //p.ulPin;
+  uint32_t bit = 1 << digitalPinToPin(ulPin);
   uint32_t stateMask = state ? bit : 0;
 
   // 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;
-
-  uint32_t width = countPulseASM(&(NRF_GPIO->IN), bit, stateMask, maxloops);
-
-  // convert the reading to microseconds. The loop has been determined
-  // to be 13 clock cycles long and have about 16 clocks between the edge
-  // and the start of the loop. There will be some error introduced by
-  // the interrupt handlers.
-  if (width)
-    return clockCyclesToMicroseconds(width * 13 + 16);
-  else
-    return 0;
+  // the initial loop; it takes (roughly) 10 clock cycles per iteration.
+  uint32_t maxloops = microsecondsToClockCycles(timeout) / 10;
+
+  // count low-level loops during the pulse (or until maxLoops)
+  // a zero loopCount means that a complete pulse was not detected within the timeout
+  uint32_t loopCount = countPulseASM(&(NRF_GPIO->IN), bit, stateMask, maxloops);
+
+  // convert the reading to (approximate) microseconds. The loop time as measured with an
+  // oscilloscope is 10 cycles on a BBC micro:bit 1.3 (nRF51822). There is error because the
+  // time is quantized to an integral number of loops and because interrupt may steal cycles.
+  return clockCyclesToMicroseconds(10 * loopCount);
 }