stm32/adc: Add workaround for ADC errata with G4 MCUs.

For STM32G4, there is a errata on ADC that may get wrong ADC result.
According to the errata sheet, this can be avoid by performing two
consecutive ADC conversions and keep second result.
Signed-off-by: NYuuki NAGAO <wf.yn386@gmail.com>

ports/stm32/adc.c

@@ -458,9 +458,18 @@ STATIC void adc_config_channel(ADC_HandleTypeDef *adc_handle, uint32_t channel)
 }
 
 STATIC uint32_t adc_read_channel(ADC_HandleTypeDef *adcHandle) {
+    uint32_t value;
+    #if defined(STM32G4)
+    // For STM32G4 there is errata 2.7.7, "Wrong ADC result if conversion done late after
+    // calibration or previous conversion".  According to the errata, this can be avoided
+    // by performing two consecutive ADC conversions and keeping the second result.
+    for (uint8_t i = 0; i < 2; i++)
+    #endif
+    {
         HAL_ADC_Start(adcHandle);
         adc_wait_for_eoc_or_timeout(adcHandle, EOC_TIMEOUT);
-    uint32_t value = adcHandle->Instance->DR;
+        value = adcHandle->Instance->DR;
+    }
     HAL_ADC_Stop(adcHandle);
     return value;
 }

ports/stm32/machine_adc.c

@@ -371,13 +371,22 @@ STATIC void adc_config_channel(ADC_TypeDef *adc, uint32_t channel, uint32_t samp
 }
 
 STATIC uint32_t adc_read_channel(ADC_TypeDef *adc) {
+    uint32_t value;
+    #if defined(STM32G4)
+    // For STM32G4 there is errata 2.7.7, "Wrong ADC result if conversion done late after
+    // calibration or previous conversion".  According to the errata, this can be avoided
+    // by performing two consecutive ADC conversions and keeping the second result.
+    for (uint8_t i = 0; i < 2; i++)
+    #endif
+    {
         #if ADC_V2
         adc->CR |= ADC_CR_ADSTART;
         #else
         adc->CR2 |= ADC_CR2_SWSTART;
         #endif
         adc_wait_eoc(adc, ADC_EOC_TIMEOUT_MS);
-    uint32_t value = adc->DR;
+        value = adc->DR;
+    }
     return value;
 }