Identify analogRead() input type and get right pin

This first commit is only for the micro:bit board, but it is to be
extended to the rest of the variants.

The analogRead() argument can be the "ADC Channel" or the pin directly.
Boards without the same type of pin organisation than the Uno (Analog pins
in order after the digital pins) need a look up table to convert a channel
value into the right Pin.

Author: carlosperate

cores/nRF5/wiring_analog_nRF51.c

@@ -119,6 +119,16 @@ uint32_t analogRead( uint32_t ulPin )
   uint32_t resolution;
   int16_t value;
 
+#ifdef ARDUINO_ADC_NON_STANDARD_PINS
+  ulPin = _analogChannelPin(ulPin);
+#elif NUM_ANALOG_INPUTS > 0
+  if (ulPin < A0) {
+    ulPin += A0;
+  }
+#else
+  return 0;
+#endif
+
   if (ulPin >= PINS_COUNT) {
     return 0;
   }

variants/BBCmicrobit/variant.h

@@ -40,7 +40,6 @@ extern "C"
 // LEDs
 #define PIN_LED                 (13)
 #define LED_BUILTIN             PIN_LED
-// #define
 
 // Buttons
 #define PIN_BUTTON_A            (5)
@@ -65,6 +64,23 @@ static const uint8_t A4  = PIN_A4 ;
 static const uint8_t A5  = PIN_A5 ;
 #define ADC_RESOLUTION    10
 
+// Non standard distribution of Analog Pins requires a lookup table
+#define ARDUINO_ADC_NON_STANDARD_PINS
+__inline__ uint32_t _analogChannelPin(uint32_t pin) __attribute__((always_inline));
+__inline__ uint32_t _analogChannelPin(uint32_t pin) {
+  switch (pin) {
+    case 0:  return PIN_A0;
+    case 1:  return PIN_A1;
+    case 2:  return PIN_A2;
+    case 3:  return PIN_A3;
+    case 4:  return PIN_A4;
+    case 5:  return PIN_A5;
+    case 10: return PIN_A5;
+    // Otherwise non-analog pin so return out of range pin to exit early
+    default: return PINS_COUNT;
+  }
+}
+
 /*
  * Serial interfaces
  */