Почему DMA обеспечивает 16-битное значение, когда АЦП 12-битный?

Question

Используя библиотеку StdPeriph для моей платы обнаружения STM32F4, я столкнулся со странной проблемой с отладчиком / кодом.(с использованием IAR Workbench)

Переменная ADC3ConvertedValue показывает 16-битные значения, тогда как ADC только 12-битный.На самом деле они коррелируют с входным напряжением АЦП, но я ожидал, что значение 2 ^ 12 - 1 макс.

Проблема должна быть где-то в моем коротком коде.Я использую DMA в циклическом режиме из Library-Example (я только что изменил функцию main ())

Попытка только цикла while из примера Code работает нормально, но как только я добавляю свои несколько строк,значение повреждено.

__IO uint16_t ADC3ConvertedValue = 0;
__IO uint16_t ADC3ConvertedVoltage = 0;
uint32_t sum = 0;
uint16_t count = 0;
uint16_t average = 0;
float voltageSignal = 0;

void ADC3_CH12_DMA_Config(void);


int main(void)
{ 
  ADC3_CH12_DMA_Config();

  ADC_SoftwareStartConv(ADC3);

  // ADC3ConvertedVoltage = ADC3ConvertedValue *3300/0xFFF;

  for(count = 0; count < 5000; count++)
  {
    sum += ADC3ConvertedValue;
  }

  average = sum / count;

  while(1)
  {
    voltageSignal = ((ADC3ConvertedValue - average) * VOLT_PER_BIT);
  }
}

Для пояснения, это пример кода, который работает (я только что изменил основную функцию):

/* Includes ------------------------------------------------------------------*/
#include "stm32f4_discovery.h"
#include <stdio.h>

/** @addtogroup STM32F4_Discovery_Peripheral_Examples
  * @{
  */

/** @addtogroup ADC_ADC3_DMA
  * @{
  */ 

/* Private typedef -----------------------------------------------------------*/
/* Private define ------------------------------------------------------------*/
#define ADC3_DR_ADDRESS     ((uint32_t)0x4001224C)

/* Private macro -------------------------------------------------------------*/
/* Private variables ---------------------------------------------------------*/
/* You can monitor the converted value by adding the variable "ADC3ConvertedValue" 
   to the debugger watch window */
__IO uint16_t ADC3ConvertedValue = 0;
__IO uint32_t ADC3ConvertedVoltage = 0;

/* Private function prototypes -----------------------------------------------*/
/* Private functions ---------------------------------------------------------*/
void ADC3_CH12_DMA_Config(void);

/**
  * @brief  Main program
  * @param  None
  * @retval None
  */
int main(void)
{
  /*!< At this stage the microcontroller clock setting is already configured, 
       this is done through SystemInit() function which is called from startup
       file (startup_stm32f4xx.s) before to branch to application main.
       To reconfigure the default setting of SystemInit() function, refer to
       system_stm32f4xx.c file
     */
  /* ADC3 configuration *******************************************************/
  /*  - Enable peripheral clocks                                              */
  /*  - DMA2_Stream0 channel2 configuration                                   */
  /*  - Configure ADC Channel12 pin as analog input                           */
  /*  - Configure ADC3 Channel12                                              */
  ADC3_CH12_DMA_Config();

  /* Start ADC3 Software Conversion */ 
  ADC_SoftwareStartConv(ADC3);

  while (1)
  {
  /* convert the ADC value (from 0 to 0xFFF) to a voltage value (from 0V to 3.3V)*/
    ADC3ConvertedVoltage = ADC3ConvertedValue *3300/0xFFF;
  }
}


/**
  * @brief  ADC3 channel12 with DMA configuration
  * @param  None
  * @retval None
  */
void ADC3_CH12_DMA_Config(void)
{
  ADC_InitTypeDef       ADC_InitStructure;
  ADC_CommonInitTypeDef ADC_CommonInitStructure;
  DMA_InitTypeDef       DMA_InitStructure;
  GPIO_InitTypeDef      GPIO_InitStructure;

  /* Enable ADC3, DMA2 and GPIO clocks ****************************************/
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2 | RCC_AHB1Periph_GPIOC, ENABLE);
  RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC3, ENABLE);

  /* DMA2 Stream0 channel0 configuration **************************************/
  DMA_InitStructure.DMA_Channel = DMA_Channel_2;  
  DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)ADC3_DR_ADDRESS;
  DMA_InitStructure.DMA_Memory0BaseAddr = (uint32_t)&ADC3ConvertedValue;
  DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralToMemory;
  DMA_InitStructure.DMA_BufferSize = 1;
  DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
  DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Disable;
  DMA_InitStructure.DMA_PeripheralDataSize = DMA_PeripheralDataSize_HalfWord;
  DMA_InitStructure.DMA_MemoryDataSize = DMA_MemoryDataSize_HalfWord;
  DMA_InitStructure.DMA_Mode = DMA_Mode_Circular;
  DMA_InitStructure.DMA_Priority = DMA_Priority_High;
  DMA_InitStructure.DMA_FIFOMode = DMA_FIFOMode_Disable;         
  DMA_InitStructure.DMA_FIFOThreshold = DMA_FIFOThreshold_HalfFull;
  DMA_InitStructure.DMA_MemoryBurst = DMA_MemoryBurst_Single;
  DMA_InitStructure.DMA_PeripheralBurst = DMA_PeripheralBurst_Single;
  DMA_Init(DMA2_Stream0, &DMA_InitStructure);
  DMA_Cmd(DMA2_Stream0, ENABLE);

  /* Configure ADC3 Channel12 pin as analog input ******************************/
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_2;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL ;
  GPIO_Init(GPIOC, &GPIO_InitStructure);

  /* ADC Common Init **********************************************************/
  ADC_CommonInitStructure.ADC_Mode = ADC_Mode_Independent;
  ADC_CommonInitStructure.ADC_Prescaler = ADC_Prescaler_Div2;
  ADC_CommonInitStructure.ADC_DMAAccessMode = ADC_DMAAccessMode_Disabled;
  ADC_CommonInitStructure.ADC_TwoSamplingDelay = ADC_TwoSamplingDelay_5Cycles;
  ADC_CommonInit(&ADC_CommonInitStructure);

  /* ADC3 Init ****************************************************************/
  ADC_InitStructure.ADC_Resolution = ADC_Resolution_12b;
  ADC_InitStructure.ADC_ScanConvMode = DISABLE;
  ADC_InitStructure.ADC_ContinuousConvMode = ENABLE;
  ADC_InitStructure.ADC_ExternalTrigConvEdge = ADC_ExternalTrigConvEdge_None;
  ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
  ADC_InitStructure.ADC_NbrOfConversion = 1;
  ADC_Init(ADC3, &ADC_InitStructure);

  /* ADC3 regular channel12 configuration *************************************/
  ADC_RegularChannelConfig(ADC3, ADC_Channel_12, 1, ADC_SampleTime_3Cycles);

 /* Enable DMA request after last transfer (Single-ADC mode) */
  ADC_DMARequestAfterLastTransferCmd(ADC3, ENABLE);

  /* Enable ADC3 DMA */
  ADC_DMACmd(ADC3, ENABLE);

  /* Enable ADC3 */
  ADC_Cmd(ADC3, ENABLE);
}

#ifdef  USE_FULL_ASSERT

/**
  * @brief  Reports the name of the source file and the source line number
  *         where the assert_param error has occurred.
  * @param  file: pointer to the source file name
  * @param  line: assert_param error line source number
  * @retval None
  */
void assert_failed(uint8_t* file, uint32_t line)
{ 
  /* User can add his own implementation to report the file name and line number,
     ex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */

  /* Infinite loop */
  while (1)
  {
  }
}
#endif

/**
  * @}
  */ 

/**
  * @}
  */ 

/******************* (C) COPYRIGHT 2011 STMicroelectronics *****END OF FILE****/