ADC - TIMER - Sample rate confusion?

[Bogdan_ST]

Hi, I need a little explanation about what is wrong behind my reasoning..

I am using STM32F411. reading, two inputs of ADC in scan mode over circular DMA triggered via TIM2 update event.

TIM2 is triggering ADC at 100KHz, so I have 50Khz sample rate per adc input. At input of both ADC channels I connected signal generator feeding 1Vrms sine @20KHz.

Everything works ok, I am using printf of both channels over (USB-CDC) as  and I can see two sinusoide signals on serial plotter. I am using this serial plotter:

https://web-serial-plotter.atomic14.com/ 

Now the problem and confusion starts here:

If I change only TIM2 frequency to 10KHz, I am getting 10X lower sample rate also (now 5KHz pre channel, all checked with Logic Analyzer), but I still can see clear 20KHz sine at serial plotter, without degradation and this contradicts Nyquist theorem...

How is this possible and what I am missing?

[Tesla DeLorean]

In scan the trigger should cause both samples to be taken. The TIM can pace the sampling, but the sample rate of the ADC will determine how long the process takes and the saturation point.

For the sample to occur at the same instant you need DUAL mode where ADC1 and ADC2 are bound together. 

[Bogdan_ST]

Yes, both samples are taken, that parts works ok and I never mentioned "sample to occur at the same instant ", have read what was my question?

ADC - DMA -TIM2 is working in background, I only print the values in superloop. (if that is important)

[TDK]

> How is this possible and what I am missing?

It's obviously not possible to see a 20 kHz sine wave with a 5 kHz sample rate. So one or more of your assumptions are incorrect.

Probably the ADC is being converted continuously, or you are not using TIM2 to trigger. Impossible to say without seeing code.

[Bogdan_ST]

Hi, what do you think now, as I see TIM2 is working correctly. 

[TDK]

I don't think all samples are making it into your serial plotter, thus the output is showing up as a sine wave, but not with the frequency you expect. That would be the expected behavior. I would look into how you're getting data out. Send it in chunks, not one at a time.

Printf hooked up to CDC directly is not a robust output mechanism, especially not with a 5ms delay between outputs.

[MasterT]

You are misunderstanding Nyquist, there is no obstacle to to see 20.1 kHz sinewave as 100 Hz if sampling adc at 10 kHz. It's called aliasing. Same happened to 30.1, 40.1 etc. This is why anti aliasing  filters were invented

[Bogdan_ST]

Well I can see I am missing something here...

I am fooled by serial plotter, it prints slowly, it needs about 12 sec to print one sine cycle.

This is what I see with 20KHz input and only 10KHz sample rate.

/* USER CODE BEGIN Header */
/**
 ******************************************************************************
 * @file : main.c
 * @brief : Main program body
 ******************************************************************************
 * @attention
 *
 * Copyright (c) 2025 STMicroelectronics.
 * All rights reserved.
 *
 * This software is licensed under terms that can be found in the LICENSE file
 * in the root directory of this software component.
 * If no LICENSE file comes with this software, it is provided AS-IS.
 *
 ******************************************************************************
 */
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "adc.h"
#include "dma.h"
#include "tim.h"
#include "usb_device.h"
#include "gpio.h"

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include <stdio.h>
#include "usbd_cdc_if.h"
#include <stdint.h>
#include <stdbool.h>
/* USER CODE END Includes */

/* Private typedef -----------------------------------------------------------*/
/* USER CODE BEGIN PTD */

/* USER CODE END PTD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */
#define ADC1_BUFFER_SIZE 2
#define ADC_MAX 4095 // 12-bit ADC resolution (0-4095)
#define VREF_MV 3300
/* USER CODE END PD */

/* Private macro -------------------------------------------------------------*/
/* USER CODE BEGIN PM */

/* USER CODE END PM */

/* Private variables ---------------------------------------------------------*/

/* USER CODE BEGIN PV */
volatile uint16_t adc1_buf[ADC1_BUFFER_SIZE]; // Raw ADC data
volatile uint32_t adc1_mv[ADC1_BUFFER_SIZE]; // Converted to millivolts
/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */
int _write(int file, char *ptr, int len)
{
	CDC_Transmit_FS((uint8_t*) ptr, len);
	return len;
}

void HAL_ADC_ConvHalfCpltCallback(ADC_HandleTypeDef *hadc)
{
	if (hadc->Instance == ADC1)
	{

	}
}

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef *hadc)
{
	if (hadc->Instance == ADC1)
	{
		for (uint8_t i = 0; i < ADC1_BUFFER_SIZE; i++)
		{
			adc1_mv[i] = (uint32_t) adc1_buf[i] * VREF_MV / ADC_MAX;
		}
		HAL_GPIO_TogglePin(LED_BUILTIN_GPIO_Port, LED_BUILTIN_Pin);

	}
}
/* USER CODE END 0 */

/**
 * @brief The application entry point.
 * @retval int
 */
int main(void)
{

 /* USER CODE BEGIN 1 */

 /* USER CODE END 1 */

 /* MCU Configuration--------------------------------------------------------*/

 /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
 HAL_Init();

 /* USER CODE BEGIN Init */

 /* USER CODE END Init */

 /* Configure the system clock */
 SystemClock_Config();

 /* USER CODE BEGIN SysInit */

 /* USER CODE END SysInit */

 /* Initialize all configured peripherals */
 MX_GPIO_Init();
 MX_DMA_Init();
 MX_ADC1_Init();
 MX_USB_DEVICE_Init();
 MX_TIM2_Init();
 /* USER CODE BEGIN 2 */
	HAL_TIM_Base_Start(&htim2);
	HAL_ADC_Start_DMA(&hadc1, (uint32_t*) adc1_buf, ADC1_BUFFER_SIZE);
 /* USER CODE END 2 */

 /* Infinite loop */
 /* USER CODE BEGIN WHILE */
	while (1)
	{

		printf("adc1_mv[0] = %ld", adc1_mv[0]);
		printf(",");
		printf("adc1_mv[1] = %ld\n", adc1_mv[1]);
		HAL_Delay(5);

 /* USER CODE END WHILE */

 /* USER CODE BEGIN 3 */
	}
 /* USER CODE END 3 */
}

/**
 * @brief System Clock Configuration
 * @retval None
 */
void SystemClock_Config(void)
{
 RCC_OscInitTypeDef RCC_OscInitStruct = {0};
 RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

 /** Configure the main internal regulator output voltage
 */
 __HAL_RCC_PWR_CLK_ENABLE();
 __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE1);

 /** Initializes the RCC Oscillators according to the specified parameters
 * in the RCC_OscInitTypeDef structure.
 */
 RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
 RCC_OscInitStruct.HSEState = RCC_HSE_ON;
 RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
 RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
 RCC_OscInitStruct.PLL.PLLM = 25;
 RCC_OscInitStruct.PLL.PLLN = 192;
 RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
 RCC_OscInitStruct.PLL.PLLQ = 4;
 if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
 {
 Error_Handler();
 }

 /** Initializes the CPU, AHB and APB buses clocks
 */
 RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
 |RCC_CLOCKTYPE_PCLK1|RCC_CLOCKTYPE_PCLK2;
 RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
 RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
 RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV2;
 RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;

 if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_3) != HAL_OK)
 {
 Error_Handler();
 }
}

/* USER CODE BEGIN 4 */

/* USER CODE END 4 */

/**
 * @brief This function is executed in case of error occurrence.
 * @retval None
 */
void Error_Handler(void)
{
 /* USER CODE BEGIN Error_Handler_Debug */
	/* User can add his own implementation to report the HAL error return state */
	__disable_irq();
	while (1)
	{
	}
 /* USER CODE END Error_Handler_Debug */
}
#ifdef USE_FULL_ASSERT
/**
 * @brief Reports the name of the source file and the source line number
 * where the assert_param error has occurred.
 * file: pointer to the source file name
 * line: assert_param error line source number
 * @retval None
 */
void assert_failed(uint8_t *file, uint32_t line)
{
 /* USER CODE BEGIN 6 */
 /* 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) */
 /* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */

/* USER CODE BEGIN Header */
/**
 ******************************************************************************
 * @file adc.c
 * @brief This file provides code for the configuration
 * of the ADC instances.
 ******************************************************************************
 * @attention
 *
 * Copyright (c) 2025 STMicroelectronics.
 * All rights reserved.
 *
 * This software is licensed under terms that can be found in the LICENSE file
 * in the root directory of this software component.
 * If no LICENSE file comes with this software, it is provided AS-IS.
 *
 ******************************************************************************
 */
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "adc.h"

/* USER CODE BEGIN 0 */

/* USER CODE END 0 */

ADC_HandleTypeDef hadc1;
DMA_HandleTypeDef hdma_adc1;

/* ADC1 init function */
void MX_ADC1_Init(void)
{

 /* USER CODE BEGIN ADC1_Init 0 */

 /* USER CODE END ADC1_Init 0 */

 ADC_ChannelConfTypeDef sConfig = {0};

 /* USER CODE BEGIN ADC1_Init 1 */

 /* USER CODE END ADC1_Init 1 */

 /** Configure the global features of the ADC (Clock, Resolution, Data Alignment and number of conversion)
 */
 hadc1.Instance = ADC1;
 hadc1.Init.ClockPrescaler = ADC_CLOCK_SYNC_PCLK_DIV4;
 hadc1.Init.Resolution = ADC_RESOLUTION_12B;
 hadc1.Init.ScanConvMode = ENABLE;
 hadc1.Init.ContinuousConvMode = DISABLE;
 hadc1.Init.DiscontinuousConvMode = DISABLE;
 hadc1.Init.ExternalTrigConvEdge = ADC_EXTERNALTRIGCONVEDGE_RISING;
 hadc1.Init.ExternalTrigConv = ADC_EXTERNALTRIGCONV_T2_TRGO;
 hadc1.Init.DataAlign = ADC_DATAALIGN_RIGHT;
 hadc1.Init.NbrOfConversion = 2;
 hadc1.Init.DMAContinuousRequests = ENABLE;
 hadc1.Init.EOCSelection = ADC_EOC_SEQ_CONV;
 if (HAL_ADC_Init(&hadc1) != HAL_OK)
 {
 Error_Handler();
 }

 /** Configure for the selected ADC regular channel its corresponding rank in the sequencer and its sample time.
 */
 sConfig.Channel = ADC_CHANNEL_1;
 sConfig.Rank = 1;
 sConfig.SamplingTime = ADC_SAMPLETIME_15CYCLES;
 if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)
 {
 Error_Handler();
 }

 /** Configure for the selected ADC regular channel its corresponding rank in the sequencer and its sample time.
 */
 sConfig.Channel = ADC_CHANNEL_2;
 sConfig.Rank = 2;
 if (HAL_ADC_ConfigChannel(&hadc1, &sConfig) != HAL_OK)
 {
 Error_Handler();
 }
 /* USER CODE BEGIN ADC1_Init 2 */

 /* USER CODE END ADC1_Init 2 */

}

void HAL_ADC_MspInit(ADC_HandleTypeDef* adcHandle)
{

 GPIO_InitTypeDef GPIO_InitStruct = {0};
 if(adcHandle->Instance==ADC1)
 {
 /* USER CODE BEGIN ADC1_MspInit 0 */

 /* USER CODE END ADC1_MspInit 0 */
 /* ADC1 clock enable */
 __HAL_RCC_ADC1_CLK_ENABLE();

 __HAL_RCC_GPIOA_CLK_ENABLE();
 /**ADC1 GPIO Configuration
 PA1 ------> ADC1_IN1
 PA2 ------> ADC1_IN2
 */
 GPIO_InitStruct.Pin = GPIO_PIN_1|GPIO_PIN_2;
 GPIO_InitStruct.Mode = GPIO_MODE_ANALOG;
 GPIO_InitStruct.Pull = GPIO_NOPULL;
 HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

 /* ADC1 DMA Init */
 /* ADC1 Init */
 hdma_adc1.Instance = DMA2_Stream0;
 hdma_adc1.Init.Channel = DMA_CHANNEL_0;
 hdma_adc1.Init.Direction = DMA_PERIPH_TO_MEMORY;
 hdma_adc1.Init.PeriphInc = DMA_PINC_DISABLE;
 hdma_adc1.Init.MemInc = DMA_MINC_ENABLE;
 hdma_adc1.Init.PeriphDataAlignment = DMA_PDATAALIGN_HALFWORD;
 hdma_adc1.Init.MemDataAlignment = DMA_MDATAALIGN_HALFWORD;
 hdma_adc1.Init.Mode = DMA_CIRCULAR;
 hdma_adc1.Init.Priority = DMA_PRIORITY_LOW;
 hdma_adc1.Init.FIFOMode = DMA_FIFOMODE_DISABLE;
 if (HAL_DMA_Init(&hdma_adc1) != HAL_OK)
 {
 Error_Handler();
 }

 __HAL_LINKDMA(adcHandle,DMA_Handle,hdma_adc1);

 /* USER CODE BEGIN ADC1_MspInit 1 */

 /* USER CODE END ADC1_MspInit 1 */
 }
}

void HAL_ADC_MspDeInit(ADC_HandleTypeDef* adcHandle)
{

 if(adcHandle->Instance==ADC1)
 {
 /* USER CODE BEGIN ADC1_MspDeInit 0 */

 /* USER CODE END ADC1_MspDeInit 0 */
 /* Peripheral clock disable */
 __HAL_RCC_ADC1_CLK_DISABLE();

 /**ADC1 GPIO Configuration
 PA1 ------> ADC1_IN1
 PA2 ------> ADC1_IN2
 */
 HAL_GPIO_DeInit(GPIOA, GPIO_PIN_1|GPIO_PIN_2);

 /* ADC1 DMA DeInit */
 HAL_DMA_DeInit(adcHandle->DMA_Handle);
 /* USER CODE BEGIN ADC1_MspDeInit 1 */

 /* USER CODE END ADC1_MspDeInit 1 */
 }
}

/* USER CODE BEGIN 1 */

/* USER CODE END 1 */

   

/* USER CODE BEGIN Header */
/**
 ******************************************************************************
 * @file tim.c
 * @brief This file provides code for the configuration
 * of the TIM instances.
 ******************************************************************************
 * @attention
 *
 * Copyright (c) 2025 STMicroelectronics.
 * All rights reserved.
 *
 * This software is licensed under terms that can be found in the LICENSE file
 * in the root directory of this software component.
 * If no LICENSE file comes with this software, it is provided AS-IS.
 *
 ******************************************************************************
 */
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "tim.h"

/* USER CODE BEGIN 0 */

/* USER CODE END 0 */

TIM_HandleTypeDef htim2;

/* TIM2 init function */
void MX_TIM2_Init(void)
{

 /* USER CODE BEGIN TIM2_Init 0 */

 /* USER CODE END TIM2_Init 0 */

 TIM_ClockConfigTypeDef sClockSourceConfig = {0};
 TIM_MasterConfigTypeDef sMasterConfig = {0};

 /* USER CODE BEGIN TIM2_Init 1 */

 /* USER CODE END TIM2_Init 1 */
 htim2.Instance = TIM2;
 htim2.Init.Prescaler = 10-1;
 htim2.Init.CounterMode = TIM_COUNTERMODE_UP;
 htim2.Init.Period = 960-1;
 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_UPDATE;
 sMasterConfig.MasterSlaveMode = TIM_MASTERSLAVEMODE_DISABLE;
 if (HAL_TIMEx_MasterConfigSynchronization(&htim2, &sMasterConfig) != HAL_OK)
 {
 Error_Handler();
 }
 /* USER CODE BEGIN TIM2_Init 2 */

 /* USER CODE END TIM2_Init 2 */

}

void HAL_TIM_Base_MspInit(TIM_HandleTypeDef* tim_baseHandle)
{

 if(tim_baseHandle->Instance==TIM2)
 {
 /* USER CODE BEGIN TIM2_MspInit 0 */

 /* USER CODE END TIM2_MspInit 0 */
 /* TIM2 clock enable */
 __HAL_RCC_TIM2_CLK_ENABLE();
 /* USER CODE BEGIN TIM2_MspInit 1 */

 /* USER CODE END TIM2_MspInit 1 */
 }
}

void HAL_TIM_Base_MspDeInit(TIM_HandleTypeDef* tim_baseHandle)
{

 if(tim_baseHandle->Instance==TIM2)
 {
 /* USER CODE BEGIN TIM2_MspDeInit 0 */

 /* USER CODE END TIM2_MspDeInit 0 */
 /* Peripheral clock disable */
 __HAL_RCC_TIM2_CLK_DISABLE();
 /* USER CODE BEGIN TIM2_MspDeInit 1 */

 /* USER CODE END TIM2_MspDeInit 1 */
 }
}

/* USER CODE BEGIN 1 */

/* USER CODE END 1 */

This is my logic analyzer screenshot of LED_BUILTIN gpio:

So could anyone help me learn what Im missing?

[Bogdan_ST]

This is with everything else the same, just TIM2 set to 1Hz, for experiment purpose.. So what I should conlude from this.?

[MasterT]

PIcture tells you that at the adc input present  non modulated sinewave unknown frequency,

but Freq.(Hz) = 1 x N + 1/15 = N +  0.066, where N -> 0....infinity