STM32F4 Disc - ADC sample time

[pano]

Posted on November 14, 2014 at 15:10

Hi,

I have this code, copied from anothers 3d in this forum, for a multi channel ADC:

#include <
stdio.h
>
#include <
stdlib.h
>
#include <
string.h
>
#include ''stm32f4xx.h''
//******************************************************************************
volatile uint16_t ADCConvertedValues[2];
//******************************************************************************
void GPIO_Configuration(void)
{
GPIO_InitTypeDef GPIO_InitStructure;
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);
/* Configure ADC1 Channel3 & 5 pins as analog input ******************************/
GPIO_InitStructure.GPIO_Pin = GPIO_Pin_3 | GPIO_Pin_5;
GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AN;
GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
GPIO_Init(GPIOA, &GPIO_InitStructure);
}
//******************************************************************************
void ADC1_Configuration(void)
{
ADC_InitTypeDef ADC_InitStructure;
ADC_CommonInitTypeDef ADC_CommonInitStructure;
DMA_InitTypeDef DMA_InitStructure;
/* Enable peripheral clocks *************************************************/
RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2, ENABLE);
RCC_APB2PeriphClockCmd(RCC_APB2Periph_ADC1, ENABLE);
/* DMA2_Stream0 channel0 configuration **************************************/
DMA_DeInit(DMA2_Stream0);
DMA_InitStructure.DMA_Channel = DMA_Channel_0;
DMA_InitStructure.DMA_PeripheralBaseAddr = (uint32_t)&ADC1->DR;
DMA_InitStructure.DMA_Memory0BaseAddr = (uint32_t)&ADCConvertedValues[0];
DMA_InitStructure.DMA_DIR = DMA_DIR_PeripheralToMemory;
DMA_InitStructure.DMA_BufferSize = 2;
DMA_InitStructure.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
DMA_InitStructure.DMA_MemoryInc = DMA_MemoryInc_Enable;
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);
/* DMA2_Stream0 enable */
DMA_Cmd(DMA2_Stream0, ENABLE);
/* 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);
/* ADC1 Init ****************************************************************/
ADC_InitStructure.ADC_Resolution = ADC_Resolution_12b;
ADC_InitStructure.ADC_ScanConvMode = ENABLE;
ADC_InitStructure.ADC_ContinuousConvMode = ENABLE;
ADC_InitStructure.ADC_ExternalTrigConvEdge = ADC_ExternalTrigConvEdge_None;
ADC_InitStructure.ADC_ExternalTrigConv = ADC_ExternalTrigConv_T1_CC1;
ADC_InitStructure.ADC_DataAlign = ADC_DataAlign_Right;
ADC_InitStructure.ADC_NbrOfConversion = 2;
ADC_Init(ADC1, &ADC_InitStructure);
/* ADC1 regular channel configuration ******************************/
ADC_RegularChannelConfig(ADC1, ADC_Channel_3, 1, ADC_SampleTime_480Cycles); // PA3
ADC_RegularChannelConfig(ADC1, ADC_Channel_5, 2, ADC_SampleTime_480Cycles); // PA5
/* Enable DMA request after last transfer (Single-ADC mode) */
ADC_DMARequestAfterLastTransferCmd(ADC1, ENABLE);
/* Enable ADC1 DMA */
ADC_DMACmd(ADC1, ENABLE);
/* Enable ADC1 **************************************************************/
ADC_Cmd(ADC1, ENABLE);
/* Start ADC1 Software Conversion */
ADC_SoftwareStartConv(ADC1);
}

what is the value of the sample time? how to fix it? thanks

[Tesla DeLorean]

Posted on November 14, 2014 at 15:45

It's going to be a function of the sample time you specify, and the conversion time, which is something like 12 cycles for a 12-bit value, as I recall, read the manual.

It's going to keep cycling in a hard loop, doing one sample, and then the second, 492 cycles (of the ADC clock) apart. If you want a specific time base, then configure a timer and use that to trigger the conversion. If you want the samples to occur at the same instant in time, then you'd need to use ADC1 + ADC2 in dual mode. I've posted examples of both on the forum. Perhaps look for ADC + TIM + DMA, or ADC IQ

[pano]

Posted on November 15, 2014 at 13:11

ok, but 492 cycles, what is? 492 cycles/s?

so, with the code

/* ADC1 regular channel configuration ******************************/
ADC_RegularChannelConfig(ADC1, ADC_Channel_3, 1, ADC_SampleTime_480Cycles); // PA3
ADC_RegularChannelConfig(ADC1, ADC_Channel_5, 2, ADC_SampleTime_480Cycles); // PA5

the ADC save a sample 480 cycles after the previous? and you have written ''If you want the samples to occur at the same instant in time, then you'd need to use ADC1 + ADC2 in dual mode.'' with this code, the sample for the two input analog signals is not taken at the same instant? thanks

[Tesla DeLorean]

Posted on November 15, 2014 at 14:37

ADC_Prescaler_Div2;
 So 492 cycles of the APB2 clock divided by two. This is the ADCCLK
 So 492 / (84000000 / 2) seconds between samples, what's that, about 7us or 85366 Hz
 No, they are not sampled at the same time, you are ''scanning'' through a list.

[pano]

Posted on November 15, 2014 at 21:05

ok... so,

theoretically

, if I change the prescaler I change the sample frequency, right?

[Tesla DeLorean]

Posted on November 15, 2014 at 21:37

Yes, I guess you could but your options are pretty limited in terms of division and sample settings.

The practical method is to set the periodicity with a timer, and use that to trigger the conversion(s).