Confusion about ADC sampling speed and sampled value

[rakib2023]

Hello.

I am very new in using STM32. I am using STM32H747I-DISCO board. For my project, I only need one ADC in differential mode. Previously, I tried to use DMA with ADC but it never worked, even the "ADC dual mode interleaved" example provided by st didn't work. So, I started to fiddle with "PollForConversion" and interrupt methods. Now, from CubeIDE, i have the following clock setup,

CPU1 - 480 MHz, CPU2 - 240 MHz, APB1 and APB2 timer clocks - 240 MHz, ADC - 21 MHz

Only have channel 1 set to differential mode for ADC1 and Timer 2 in free running (1 MHz) to measure timings.

Here's the timer initialize,

static void MX_TIM2_Init(void)
{
  TIM_ClockConfigTypeDef sClockSourceConfig = {0};
  TIM_MasterConfigTypeDef sMasterConfig = {0};

  htim2.Instance = TIM2;
  htim2.Init.Prescaler = 240;
  htim2.Init.CounterMode = TIM_COUNTERMODE_UP;
  htim2.Init.Period = 4294967295;
  htim2.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;
  htim2.Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_DISABLE;
  if (HAL_TIM_Base_Init(&htim2) != HAL_OK)
  {
    Error_Handler();
  }
  sClockSourceConfig.ClockSource = TIM_CLOCKSOURCE_INTERNAL;
  if (HAL_TIM_ConfigClockSource(&htim2, &sClockSourceConfig) != HAL_OK)
  {
    Error_Handler();
  }
  sMasterConfig.MasterOutputTrigger = TIM_TRGO_RESET;
  sMasterConfig.MasterSlaveMode = TIM_MASTERSLAVEMODE_DISABLE;
  if (HAL_TIMEx_MasterConfigSynchronization(&htim2, &sMasterConfig) != HAL_OK)
  {
    Error_Handler();
  }
}

Here's the ADC initialize,

static void MX_ADC1_Init(void)
{
  ADC_MultiModeTypeDef multimode = {0};
  ADC_ChannelConfTypeDef sConfig = {0};

  /** Common config
  */
  hadc1.Instance = ADC1;
  hadc1.Init.ClockPrescaler = ADC_CLOCK_ASYNC_DIV1;
  hadc1.Init.Resolution = ADC_RESOLUTION_12B;
  hadc1.Init.ScanConvMode = ADC_SCAN_DISABLE;
  hadc1.Init.EOCSelection = ADC_EOC_SINGLE_CONV;
  hadc1.Init.LowPowerAutoWait = DISABLE;
  hadc1.Init.ContinuousConvMode = ENABLE;
  hadc1.Init.NbrOfConversion = 1;
  hadc1.Init.DiscontinuousConvMode = DISABLE;
  hadc1.Init.ExternalTrigConv = ADC_SOFTWARE_START;
  hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_NONE;
  hadc1.Init.ConversionDataManagement = ADC_CONVERSIONDATA_DR;
  hadc1.Init.Overrun = ADC_OVR_DATA_PRESERVED;
  hadc1.Init.LeftBitShift = ADC_LEFTBITSHIFT_NONE;
  hadc1.Init.OversamplingMode = DISABLE;
  if (HAL_ADC_Init(&hadc1) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure the ADC multi-mode
  */
  multimode.Mode = ADC_MODE_INDEPENDENT;
  if (HAL_ADCEx_MultiModeConfigChannel(&hadc1, &multimode) != HAL_OK)
  {
    Error_Handler();
  }

  /** Configure Regular Channel
  */
  sConfig.Channel = ADC_CHANNEL_1;
  sConfig.Rank = ADC_REGULAR_RANK_1;
  sConfig.SamplingTime = ADC_SAMPLETIME_8CYCLES_5;
  sConfig.SingleDiff = ADC_DIFFERENTIAL_ENDED;
  sConfig.OffsetNumber = ADC_OFFSET_NONE;
  sConfig.Offset = 0;
  sConfig.OffsetSignedSaturation = DISABLE;
  if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)
  {
    Error_Handler();
  }
}

I tried both poll and interrupt method and checked the time it takes for reading one sample. From the ADC configuration, it should have 1 Msps sampling rate, that is 1us for each sample. But when checking in action, poll method seems to take about 1 second to read 1 sample (two calls, "PollForConversion" and "GetValue"). Interrupt method seems to take about 400ms. Besides, without any input to ADC pins (0V), the values I get is still very big, roughly 32000 in 16 bit mode and 2000 in 12 bit mode.

For my project, I need very fast sampling rate, at least 0.1 Msps, preferably 1 Msps. The processor doesn't have to do anything during sampling. Even the sampled value conversion can be done later. This should be feasible in STM32 microprocessors but I had been unable to do so.

Any help or guidance on what I could be doing wrong or what I can improve would be very appreciated. Thank you.

[rakib2023]

Additionally, since I can't edit the post, if there is any available working sample code for a different board (nucleo, eval or disco) for this particular application, that would also be very appreciated. I don't necessarily have to use H7 series, I can use other boards if it works better or easier.

[TDK]

> Besides, without any input to ADC pins (0V), the values I get is still very big, roughly 32000 in 16 bit mode and 2000 in 12 bit mode.

Those values are consistent with ~0 V when in differential mode. (32768 and 2048). (Although "no input/floating" and "0V" are not the same.)

Polling for conversion is not practical at 1 Msps. Use HAL_ADC_DMA_Start, put many values into a buffer, and process them periodically on the half- and full-complete interrupt callbacks.

See the ADC_DMA_Transfer examples.

https://github.com/STMicroelectronics/STM32CubeH7/blob/96dd7e5b69d2b10406ac3f358e44b14cececa611/Projects/NUCLEO-H743ZI/Examples/ADC/ADC_DMA_Transfer/Src/main.c

As a side note, I want to make sure you're aware that in even in differential mode both inputs must be between 0 V and VREF+.

[AScha.3]

Hi,

>I don't necessarily have to use H7 series, I can use other boards if it works better or easier.

So why you choose a super-complicated-hi-speed-dual-core-cpu ? to make it more difficult ? :)

What boards you can use ?  (maybe :  nucleo ... F303 ? )

[rakib2023]

My initial attempt was to use DMA, of course. But I could not get DMA working at all, even by following official sample codes. From what I gathered, H7 series DMA seems to have some complication. I will try the code you provided. I know that 1 Msps is not realistic without DMA but I will take 100 Ksps if it can be done with poll or interrupt method. But measuring the time shows that either way it's roughly 1~3 sample/s max speed which seems unusually low. Any reason why this might be happening?

[rakib2023]

Initially, I had plans to add some computation in the MCU but now I will take anything as long as I get a high-speed ADC working. I am ordering F4 and F7 series nucleo. I will try them separately. I would still prefer to use H7 if I can get the ADC working right since I can offload a good amount of computation to MCU.

[TDK]

Probably your code has a ton of overhead and/or isn't measuring the duration correctly. Show your code.

There's no way an ADC conversion is taking an entire second or even hundreds of ms.

[AScha.3]

No big problem with a H7 cpu also - just more complex to set up everything.

STM32H747I-DISCO  has display also... What you wanna build ?

[rakib2023]

Just this after all the initialization. Nothing else, not even in the second CPU. This gives roughly 996000 in "dur" which is about 1 second.

uint32_t ADC_result = 0;
uint8_t message[32];
uint32_t start = 0, end = 0;
uint32_t dur = 0;

HAL_TIM_Base_Start(&htim2);
HAL_ADC_Start(&hadc1);

while (1)
{
    start = TIM2->CNT;
    HAL_ADC_PollForConversion(&hadc1, 1000);
    ADC_result = HAL_ADC_GetValue(&hadc1);
    end = TIM2->CNT;
    dur = end-start;
    sprintf(message, "%lu in %lu\r\n", ADC_result, dur);
    HAL_UART_Transmit(&huart1, message, strlen(message), 1000);
    HAL_Delay(1000);
}

[rakib2023]

An analyzer system, preferably with a GUI. The plan was to build it as a standalone analyzer and that's why I wanted two CPUs, one to sample and convert analog signal and one to handle computation and GUI.