analogRead() fix for nRF52 by setting related gain. (#66)

* analogRead() fix for nRF52 by setting related gain.
Signed-off-by: Joerg Hubschneider <joerg.hubschneider@bosch-si.com>

cores/nRF5/wiring_analog_nRF52.c

@@ -28,6 +28,9 @@
 extern "C" {
 #endif
 
+static uint32_t saadcReference = SAADC_CH_CONFIG_REFSEL_Internal;
+static uint32_t saadcGain      = SAADC_CH_CONFIG_GAIN_Gain1_5;
+
 #define PWM_COUNT 3
 
 static NRF_PWM_Type* pwms[PWM_COUNT] = {
@@ -41,8 +44,6 @@ static uint32_t pwmChannelPins[PWM_COUNT] = {
   0xFFFFFFFF,
   0xFFFFFFFF
 };
-
-static uint32_t saadcReference = SAADC_CH_CONFIG_REFSEL_Internal;
 static uint16_t pwmChannelSequence[PWM_COUNT];
 
 static int readResolution = 10;
@@ -76,7 +77,7 @@ static inline uint32_t mapResolution( uint32_t value, uint32_t from, uint32_t to
 }
 
 /*
- * Internal Reference is at 1.0v
+ * Internal Reference is at 0.6v!
  * External Reference should be between 1v and VDDANA-0.6v=2.7v
  *
  * Warning : On Arduino Zero board the input/output voltage for SAMD21G18 is 3.3 volts maximum
@@ -88,10 +89,12 @@ void analogReference( eAnalogReference ulMode )
     case AR_INTERNAL:
     default:
       saadcReference = SAADC_CH_CONFIG_REFSEL_Internal;
+      saadcGain      = SAADC_CH_CONFIG_GAIN_Gain1_5;
       break;
 
     case AR_VDD4:
       saadcReference = SAADC_CH_CONFIG_REFSEL_VDD1_4;
+      saadcGain      = SAADC_CH_CONFIG_GAIN_Gain1_4;
       break;
   }
 }
@@ -169,7 +172,7 @@ uint32_t analogRead( uint32_t ulPin )
   }
   NRF_SAADC->CH[0].CONFIG =   ((SAADC_CH_CONFIG_RESP_Bypass   << SAADC_CH_CONFIG_RESP_Pos)   & SAADC_CH_CONFIG_RESP_Msk)
                             | ((SAADC_CH_CONFIG_RESP_Bypass   << SAADC_CH_CONFIG_RESN_Pos)   & SAADC_CH_CONFIG_RESN_Msk)
-                            | ((SAADC_CH_CONFIG_GAIN_Gain1    << SAADC_CH_CONFIG_GAIN_Pos)   & SAADC_CH_CONFIG_GAIN_Msk)
+                            | ((saadcGain                     << SAADC_CH_CONFIG_GAIN_Pos)   & SAADC_CH_CONFIG_GAIN_Msk)
                             | ((saadcReference                << SAADC_CH_CONFIG_REFSEL_Pos) & SAADC_CH_CONFIG_REFSEL_Msk)
                             | ((SAADC_CH_CONFIG_TACQ_3us      << SAADC_CH_CONFIG_TACQ_Pos)   & SAADC_CH_CONFIG_TACQ_Msk)
                             | ((SAADC_CH_CONFIG_MODE_SE       << SAADC_CH_CONFIG_MODE_Pos)   & SAADC_CH_CONFIG_MODE_Msk);