Fix ADC prescaler reg config, add prescaler options

cores/nRF5/wiring_analog.h

@@ -26,7 +26,7 @@ extern "C" {
 #endif
 
 /*
- * \brief SAMD products have only one reference for ADC
+ * \brief nRF51 and nRF52 have different reference options.
  */
 #ifdef NRF52
 typedef enum _eAnalogReference

cores/nRF5/wiring_analog_nRF51.c

@@ -48,6 +48,7 @@ static struct PWMContext pwmContext[PWM_COUNT] = {
 static int timerEnabled = 0;
 
 static uint32_t adcReference = ADC_CONFIG_REFSEL_VBG;
+static uint32_t adcPrescaling = ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling;
 
 static uint32_t readResolution = 10;
 static uint32_t writeResolution = 8;
@@ -80,10 +81,11 @@ static inline uint32_t mapResolution( uint32_t value, uint32_t from, uint32_t to
 }
 
 /*
- * Internal Reference is at 1.0v
- * External Reference should be between 1v and VDDANA-0.6v=2.7v
+ * Internal VBG Reference is 1.2 V.
+ * External References AREF0 and AREF1 should be between 0.83 V - 1.3 V.
  *
- * Warning : On Arduino Zero board the input/output voltage for SAMD21G18 is 3.3 volts maximum
+ * Warning : ADC should not be exposed to > 2.4 V, calculated after prescaling.
+ *           GPIO pins must not be exposed to higher voltage than VDD + 0.3 V.
  */
 void analogReference( eAnalogReference ulMode )
 {
@@ -91,23 +93,34 @@ void analogReference( eAnalogReference ulMode )
     case AR_DEFAULT:
     case AR_VBG:
     default:
+      // 1.2 Reference, 1/3 prescaler = 0 V - 3.6 V range
+      // Minimum VDD for full range in safe operation = 3.3V
       adcReference = ADC_CONFIG_REFSEL_VBG;
+      adcPrescaling = ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling;
       break;
 
     case AR_SUPPLY_ONE_HALF:
+      // 1/2 VDD Reference, 2/3 prescaler = 0 V - 0.75VDD range
       adcReference = ADC_CONFIG_REFSEL_SupplyOneHalfPrescaling;
+      adcPrescaling = ADC_CONFIG_INPSEL_AnalogInputTwoThirdsPrescaling;
       break;
 
     case AR_SUPPLY_ONE_THIRD:
+      // 1/3 VDD Reference, 1/3 prescaler = 0 V - VDD range
       adcReference = ADC_CONFIG_REFSEL_SupplyOneThirdPrescaling;
+      adcPrescaling = ADC_CONFIG_INPSEL_AnalogInputOneThirdPrescaling;
       break;
 
     case AR_EXT0:
+      // ARF0 reference, 2/3 prescaler = 0 V - 1.5 ARF0
       adcReference = ADC_CONFIG_REFSEL_External | (ADC_CONFIG_EXTREFSEL_AnalogReference0 << ADC_CONFIG_EXTREFSEL_Pos);
+      adcPrescaling = ADC_CONFIG_INPSEL_AnalogInputTwoThirdsPrescaling;
       break;
 
     case AR_EXT1:
+      // ARF1 reference, 2/3 prescaler = 0 V - 1.5 ARF1
       adcReference = (ADC_CONFIG_REFSEL_External | ADC_CONFIG_EXTREFSEL_AnalogReference1 << ADC_CONFIG_EXTREFSEL_Pos);
+      adcPrescaling = ADC_CONFIG_INPSEL_AnalogInputTwoThirdsPrescaling;
       break;
   }
 }
@@ -178,7 +191,7 @@ uint32_t analogRead( uint32_t ulPin )
   uint32_t config_reg = 0;
 
   config_reg |= ((uint32_t)adcResolution << ADC_CONFIG_RES_Pos) & ADC_CONFIG_RES_Msk;
-  config_reg |= ((uint32_t)ADC_CONFIG_RES_10bit << ADC_CONFIG_INPSEL_Pos) & ADC_CONFIG_INPSEL_Msk;
+  config_reg |= ((uint32_t)adcPrescaling << ADC_CONFIG_INPSEL_Pos) & ADC_CONFIG_INPSEL_Msk;
   config_reg |= ((uint32_t)adcReference << ADC_CONFIG_REFSEL_Pos) & ADC_CONFIG_REFSEL_Msk;
 
   if (adcReference & ADC_CONFIG_EXTREFSEL_Msk)