ADC just giving raw value of 4095 on all channels

[harshpanchal_6]

Hello Community,

I’m working on a custom board using the STM32G474VET3 MCU and encountering an issue with the ADC readings. I’m consistently getting a raw value of 4095 on all the ADC channels I’m using. It seems like something might be misconfigured, but I haven’t been able to pinpoint the cause.

Any insights or suggestions would be greatly appreciated.

Thank you!

[Ozone]

On a related note, I remember a similiar issue with the PCF8591 IC, an (older) I2C extender IC with ADC channels.
When the input voltage at one single ADC pin on this IC exceeded Vref, all the ADC channels returned max. value.

Not sure if this applies to this case, but analog MCU inputs are not 5V tolerant. And discharge currents through protective diodes on such inputs definitely affect ADC operation.

[harshpanchal_6]

 

 

  

static void MX_ADC3_Init(void)
{

  /* USER CODE BEGIN ADC3_Init 0 */

  /* USER CODE END ADC3_Init 0 */

  ADC_MultiModeTypeDef multimode = {0};
  ADC_ChannelConfTypeDef sConfig = {0};

  /* USER CODE BEGIN ADC3_Init 1 */

  /* USER CODE END ADC3_Init 1 */

  /** Common config
  */
  hadc3.Instance = ADC3;
  hadc3.Init.ClockPrescaler = ADC_CLOCK_SYNC_PCLK_DIV2;
  hadc3.Init.Resolution = ADC_RESOLUTION_12B;
  hadc3.Init.DataAlign = ADC_DATAALIGN_RIGHT;
  hadc3.Init.GainCompensation = 0;
  hadc3.Init.ScanConvMode = ADC_SCAN_ENABLE;
  hadc3.Init.EOCSelection = ADC_EOC_SINGLE_CONV;
  hadc3.Init.LowPowerAutoWait = DISABLE;
  hadc3.Init.ContinuousConvMode = DISABLE;
  hadc3.Init.NbrOfConversion = 1;
  hadc3.Init.DiscontinuousConvMode = DISABLE;
  hadc3.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc3.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc3.Init.DMAContinuousRequests = DISABLE;
  hadc3.Init.Overrun = ADC_OVR_DATA_PRESERVED;
  hadc3.Init.OversamplingMode = DISABLE;
  if (HAL_ADC_Init(&hadc3) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure the ADC multi-mode
  */
  multimode.Mode = ADC_MODE_INDEPENDENT;
  if (HAL_ADCEx_MultiModeConfigChannel(&hadc3, &multimode) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_10;
  sConfig.Rank = ADC_REGULAR_RANK_1;
  sConfig.SamplingTime = ADC_SAMPLETIME_2CYCLES_5;
  sConfig.SingleDiff = ADC_SINGLE_ENDED;
  sConfig.OffsetNumber = ADC_OFFSET_NONE;
  sConfig.Offset = 0;
  if (HAL_ADC_ConfigChannel(&hadc3, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
  /* USER CODE BEGIN ADC3_Init 2 */

  /* USER CODE END ADC3_Init 2 */

}

[harshpanchal_6]

It may cause because of ADC line length on board ?

[Andrew Neil]

You still haven't shown a schematic with the stm32 !

[harshpanchal_6]

  

[harshpanchal_6]

uint16_t Read_ADC_Raw_Debug()
{
    uint16_t val = 0;

    HAL_ADC_Start(&hadc3);

    if (HAL_ADC_PollForConversion(&hadc3, HAL_MAX_DELAY) == HAL_OK)
    {
        val = HAL_ADC_GetValue(&hadc3);
    }

    HAL_ADC_Stop(&hadc3);

    // Optional: print value over UART/RS485
    char debug_msg[50];
    sprintf(debug_msg, "Raw ADC Value: %u\r\n", val);
    RS485_Transmit((uint8_t*)debug_msg, strlen(debug_msg));

    return val;
}

[Ozone]

I still think this is an input voltage problem.

The maximal ADC value corresponds to the ADC reference voltage Vref, and you need to check the datasheet / reference manual to see how your specific MCU handles this. Some variants have separate Vref pins, but usually it is tied to Vdda by default.

[Andrew Neil]

---

@Ozone wrote:

I still think this is an input voltage problem.

---

+1

@harshpanchal_6 and you still haven't said what voltages you measure at the STM32 ADC input pins.

As @Ozone said earlier, you need to get rid of all the other stuff and just concentrate on getting the ADC working.

The easiest way to do that would be on a Nucleo board.

[harshpanchal_6]

I have check the input voltage also which is perfect 

[Hl_st]

So is  on PD13 pin 2.8 V? And is on VDD, VDDA and VREF+ pin 3.3V?