DISCO_L496AG : enable ADC

See User Manual, VREF+ is not connected by default

NB: Use 2.5V as reference (instead of 3.3V)
for internal channels calculation

targets/TARGET_STM/TARGET_STM32L4/analogin_device.c

@@ -83,12 +83,23 @@ void analogin_init(analogin_t *obj, PinName pin)
  obj->handle.Init.Overrun = ADC_OVR_DATA_OVERWRITTEN; // DR register is overwritten with the last conversion result in case of overrun
  obj->handle.Init.OversamplingMode = DISABLE; // No oversampling
 
+#if defined(TARGET_DISCO_L496AG)
+ /* VREF+ is not connected to VDDA by default */
+ /* Use 2.5V as reference (instead of 3.3V) for internal channels calculation */
+ __HAL_RCC_SYSCFG_CLK_ENABLE();
+ HAL_SYSCFG_VREFBUF_VoltageScalingConfig(SYSCFG_VREFBUF_VOLTAGE_SCALE1); /* VREF_OUT2 = 2.5 V */
+ HAL_SYSCFG_VREFBUF_HighImpedanceConfig(SYSCFG_VREFBUF_HIGH_IMPEDANCE_DISABLE);
+ if (HAL_SYSCFG_EnableVREFBUF() != HAL_OK) {
+ error("HAL_SYSCFG_EnableVREFBUF issue\n");
+ }
+#endif /* TARGET_DISCO_L496AG */
+
  // Enable ADC clock
  __HAL_RCC_ADC_CLK_ENABLE();
  __HAL_RCC_ADC_CONFIG(RCC_ADCCLKSOURCE_SYSCLK);
 
  if (HAL_ADC_Init(&obj->handle) != HAL_OK) {
- error("Cannot initialize ADC");
+ error("Cannot initialize ADC\n");
  }
 
  // ADC calibration is done only once