stm32f103 ADC strange behaviour

Question

I am using stm32f103c8t6 discovery board and I was doing a practise by using ADC peripherals. While using keil debugger I have realized something so strange to me.If discovery board is not connected to voltage source or ground, It is giving like 1900-2200. The adc is configured as 12 bit read mode.It came so strange to me. I will leave my code ıf you want to check it.

#include "main.h"
#include "stm32f1xx_hal.h"

ADC_HandleTypeDef hadc1;

void SystemClock_Config(void);
static void MX_GPIO_Init(void);
static void MX_ADC1_Init(void);
uint16_t adc_voltage;
int main(void)
{
  HAL_Init();
  SystemClock_Config();
  MX_GPIO_Init();
  MX_ADC1_Init();
  while (1)
  {
HAL_ADC_Start(&hadc1);
while(HAL_ADC_PollForConversion(&hadc1,500));
adc_voltage = HAL_ADC_GetValue(&hadc1);
HAL_ADC_Stop(&hadc1);
HAL_Delay(500);     
  }

}

void SystemClock_Config(void)
{

  RCC_OscInitTypeDef RCC_OscInitStruct;
  RCC_ClkInitTypeDef RCC_ClkInitStruct;
  RCC_PeriphCLKInitTypeDef PeriphClkInit;

  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
  RCC_OscInitStruct.HSICalibrationValue = 16;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }

  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK|RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
  RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSI;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV8;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }

  PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_ADC;
  PeriphClkInit.AdcClockSelection = RCC_ADCPCLK2_DIV8;
  if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK)
  {
    _Error_Handler(__FILE__, __LINE__);
  }

  HAL_SYSTICK_Config(HAL_RCC_GetHCLKFreq()/1000);
  HAL_SYSTICK_CLKSourceConfig(SYSTICK_CLKSOURCE_HCLK);
  HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
}

static void MX_ADC1_Init(void)
{
  ADC_ChannelConfTypeDef sConfig;

  hadc1.Instance = ADC1;
  hadc1.Init.ScanConvMode = ADC_SCAN_ENABLE;
  hadc1.Init.ContinuousConvMode = ENABLE;
  hadc1.Init.DiscontinuousConvMode = DISABLE;
  hadc1.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc1.Init.DataAlign = ADC_DATAALIGN_RIGHT;
  hadc1.Init.NbrOfConversion = 1;
  HAL_ADC_Init(&hadc1);

  sConfig.Channel = ADC_CHANNEL_0;
  sConfig.Rank = ADC_REGULAR_RANK_1;
  sConfig.SamplingTime = ADC_SAMPLETIME_41CYCLES_5;
  HAL_ADC_ConfigChannel(&hadc1, &sConfig);

}

static void MX_GPIO_Init(void)
{

  GPIO_InitTypeDef GPIO_InitStruct;

  __HAL_RCC_GPIOC_CLK_ENABLE();
  __HAL_RCC_GPIOD_CLK_ENABLE();
  __HAL_RCC_GPIOA_CLK_ENABLE();

  HAL_GPIO_WritePin(GPIOC, GPIO_PIN_13, GPIO_PIN_RESET);

  GPIO_InitStruct.Pin = GPIO_PIN_13;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);

}

void _Error_Handler(char *file, int line)
{

  while(1)
  {
  }

}

#ifdef  USE_FULL_ASSERT

void assert_failed(uint8_t* file, uint32_t line)
{ 

}
#endif 

Answer 1

I think you are mentioning the case of a so called floating input. When there is nothing connected to an input (either digitally or analog), the value you get is indetermined, and it is instable (meaning it can change for every reading you make). Always make sure you have something connected that you want to read to an input, AND in cases the input is not driven by an output (in case of an 'uninitialized' output connected to the input, or in case the MCU starts up), use a pull up/pull down resistor (some MCUs have one or both of them already built in and you can select it when you determine the pin properties).

Answer 2

You shouldn't have any particular expectation for the reading of an unconnected ADC input or one not definitively driven to a voltage with respect to the analog ground.

If you consult the data sheet you will find there is a maximum drive impedance for meaningful results determined in part by the selected conversion time.