ADC轉換是指將輸入的模擬量轉化爲數字量，並且將數字量讀取出來。完成這個動作的時間叫做ADC的採樣週期。
採樣週期 = 轉換時間+讀取時間。轉換時間 = 採樣時間+12.5個週期。採樣時間可以通過軟件設置ADC採樣時間寄存器來設置ADC的採樣時間。對應的庫函數如下：
/**
* @brief Configures for the selected ADC regular channel its corresponding
* rank in the sequencer and its sample time.
* @param ADCx: where x can be 1, 2 or 3 to select the ADC peripheral.
* @param ADC_Channel: the ADC channel to configure.
* This parameter can be one of the following values:
* @arg ADC_Channel_0: ADC Channel0 selected
* @arg ADC_Channel_1: ADC Channel1 selected
* @arg ADC_Channel_2: ADC Channel2 selected
* @arg ADC_Channel_3: ADC Channel3 selected
* @arg ADC_Channel_4: ADC Channel4 selected
* @arg ADC_Channel_5: ADC Channel5 selected
* @arg ADC_Channel_6: ADC Channel6 selected
* @arg ADC_Channel_7: ADC Channel7 selected
* @arg ADC_Channel_8: ADC Channel8 selected
* @arg ADC_Channel_9: ADC Channel9 selected
* @arg ADC_Channel_10: ADC Channel10 selected
* @arg ADC_Channel_11: ADC Channel11 selected
* @arg ADC_Channel_12: ADC Channel12 selected
* @arg ADC_Channel_13: ADC Channel13 selected
* @arg ADC_Channel_14: ADC Channel14 selected
* @arg ADC_Channel_15: ADC Channel15 selected
* @arg ADC_Channel_16: ADC Channel16 selected
* @arg ADC_Channel_17: ADC Channel17 selected
* @param Rank: The rank in the regular group sequencer. This parameter must be between 1 to 16.
* @param ADC_SampleTime: The sample time value to be set for the selected channel.
* This parameter can be one of the following values:
* @arg ADC_SampleTime_1Cycles5: Sample time equal to 1.5 cycles
* @arg ADC_SampleTime_7Cycles5: Sample time equal to 7.5 cycles
* @arg ADC_SampleTime_13Cycles5: Sample time equal to 13.5 cycles
* @arg ADC_SampleTime_28Cycles5: Sample time equal to 28.5 cycles
* @arg ADC_SampleTime_41Cycles5: Sample time equal to 41.5 cycles
* @arg ADC_SampleTime_55Cycles5: Sample time equal to 55.5 cycles
* @arg ADC_SampleTime_71Cycles5: Sample time equal to 71.5 cycles
* @arg ADC_SampleTime_239Cycles5: Sample time equal to 239.5 cycles
* @retval None
*/
void ADC_RegularChannelConfig(ADC_TypeDef* ADCx, uint8_t ADC_Channel, uint8_t Rank, uint8_t ADC_SampleTime)
{
uint32_t tmpreg1 = 0, tmpreg2 = 0;
/* Check the parameters */
assert_param(IS_ADC_ALL_PERIPH(ADCx));
assert_param(IS_ADC_CHANNEL(ADC_Channel));
assert_param(IS_ADC_REGULAR_RANK(Rank));
assert_param(IS_ADC_SAMPLE_TIME(ADC_SampleTime));
/* if ADC_Channel_10 ... ADC_Channel_17 is selected */
if (ADC_Channel > ADC_Channel_9)
{
/* Get the old register value */
tmpreg1 = ADCx->SMPR1;
/* Calculate the mask to clear */
tmpreg2 = SMPR1_SMP_Set << (3 * (ADC_Channel - 10));
/* Clear the old channel sample time */
tmpreg1 &= ~tmpreg2;
/* Calculate the mask to set */
tmpreg2 = (uint32_t)ADC_SampleTime << (3 * (ADC_Channel - 10));
/* Set the new channel sample time */
tmpreg1 |= tmpreg2;
/* Store the new register value */
ADCx->SMPR1 = tmpreg1;
}
else /* ADC_Channel include in ADC_Channel_[0..9] */
{
/* Get the old register value */
tmpreg1 = ADCx->SMPR2;
/* Calculate the mask to clear */
tmpreg2 = SMPR2_SMP_Set << (3 * ADC_Channel);
/* Clear the old channel sample time */
tmpreg1 &= ~tmpreg2;
/* Calculate the mask to set */
tmpreg2 = (uint32_t)ADC_SampleTime << (3 * ADC_Channel);
/* Set the new channel sample time */
tmpreg1 |= tmpreg2;
/* Store the new register value */
ADCx->SMPR2 = tmpreg1;
}
/* For Rank 1 to 6 */
if (Rank < 7)
{
/* Get the old register value */
tmpreg1 = ADCx->SQR3;
/* Calculate the mask to clear */
tmpreg2 = SQR3_SQ_Set << (5 * (Rank - 1));
/* Clear the old SQx bits for the selected rank */
tmpreg1 &= ~tmpreg2;
/* Calculate the mask to set */
tmpreg2 = (uint32_t)ADC_Channel << (5 * (Rank - 1));
/* Set the SQx bits for the selected rank */
tmpreg1 |= tmpreg2;
/* Store the new register value */
ADCx->SQR3 = tmpreg1;
}
/* For Rank 7 to 12 */
else if (Rank < 13)
{
/* Get the old register value */
tmpreg1 = ADCx->SQR2;
/* Calculate the mask to clear */
tmpreg2 = SQR2_SQ_Set << (5 * (Rank - 7));
/* Clear the old SQx bits for the selected rank */
tmpreg1 &= ~tmpreg2;
/* Calculate the mask to set */
tmpreg2 = (uint32_t)ADC_Channel << (5 * (Rank - 7));
/* Set the SQx bits for the selected rank */
tmpreg1 |= tmpreg2;
/* Store the new register value */
ADCx->SQR2 = tmpreg1;
}
/* For Rank 13 to 16 */
else
{
/* Get the old register value */
tmpreg1 = ADCx->SQR1;
/* Calculate the mask to clear */
tmpreg2 = SQR1_SQ_Set << (5 * (Rank - 13));
/* Clear the old SQx bits for the selected rank */
tmpreg1 &= ~tmpreg2;
/* Calculate the mask to set */
tmpreg2 = (uint32_t)ADC_Channel << (5 * (Rank - 13));
/* Set the SQx bits for the selected rank */
tmpreg1 |= tmpreg2;
/* Store the new register value */
ADCx->SQR1 = tmpreg1;
}
}
通道的採樣時間會影響採樣的精度。下面舉例來進行說明：
例如ADCLCK = 14MHZ，設置 1.5週期的採樣時間。那麼 轉換時間 = 1.5+12.5 =14週期 = 1us。
一般情況下，如果是軟件啓動，那麼轉換時間就是採樣週期。
如果通過定時器進行觸發啓動ADC，則還需要加上定時器的相關時間。
如果已知待採集的週期性信號的頻率，怎樣確定ADC的採樣週期？
已知信號的頻率爲50hz（週期20ms），初步定一個週期採集2500個採樣點，（一個週期最少要採集20個點，即採樣率最少爲1K），因此每兩個點的採樣間隔爲20ms/2500 = 8us；如果ADC可編程通道的採樣週期時間選擇71.5週期，便可以知道ADC的時鐘頻率爲9MHZ.、
ADC+DMA傳輸數據
採用ADC+DMA的形式傳輸數據，遇到的問題：ADC正常採集數據，DMA不進入傳輸完成中斷。經過以下調整解決問題。順便總結ADC+DMA的配置方式
1、配置DMA,
DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)&ADC1->DR;
DMA_InitStructure.DMA_MemoryBaseAddr = (uint32_t)ADC1_Value_buff;
DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralSRC; // 內存作爲數據傳輸的目的地址
2、配置用於ADC採集數據的引腳；
3、配置ADC。如果要採用DMA的方式一定要記得ADC_DMACmd(ADC1, ENABLE);這個地方坑了自己，開始配置時沒有開啓該DMA功能。
4、在DMA傳輸完成中斷完成時，要重新設置傳輸量的大小。