Speed-up data streaming from ADC to USART in DMA mode (using FIFO?)

[GSp]

Hi everyone,

I'm using STM32G071 and my goal is to serial send data acquired by 4 ADC channels as fast as possible.

In detail: I'm using DMA in circular mode to continuously acquire data from ADCs (ideally at 2,5MHz) and fill a buffer which is then sent over USART (baudrate 460800bit/s). I want to send a package of data when I reach 36 samples (9 samples for each of the 4 ADC channels). 

My problem is that the time needed to send that amount of data over USART (~1,5ms) is way longer than the time needed to fill the buffer with new ADC values (~15us) so I loose a large amount of data (the buffer is updated 100 times faster than my capability of sending it). With a circular buffer I will never be able to implement a data stream with no losses. I was thinking about these possible solutions:

1) Use a FIFO buffer instead of a circular buffer. I'm not sure on how could I do it with ADC_DMA and also I'm afraid of going into memory overflow: since my "consumer" is way slower than my "producer", wouldn't my FIFO become huge in a small amount of time?

2) Use USB full speed instead of USART. In this case I should change my microcontroller from STM32G071 (which doesn't have USB full speed) to STM32G0B0. Would this solve the problem? in that case I wuold have a data package sent in ~52us (usb full speed has a f=12Mbit/s), so my consumer would be just slightly slower than my producer and I could be able to use a FIFO with no memory issues.

Can you give me some insight on how the FIFO mode works for DMA? Am I missing something in my reasoning? 

Thank you in advance to who will answer. 

I attach below my code:

/* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file           : main.c
  * @brief          : Main program body
  ******************************************************************************
  * @attention
  *
  * Copyright (c) 2023 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 "usart.h"
#include "gpio.h"

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include "stdio.h"
/* USER CODE END Includes */

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

/* USER CODE END PTD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */
#define TIMEOUT_TX 15
#define TIMEOUT_RX 2
#define ADC_BUF_LEN 4
/* USER CODE END PD */

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

/* USER CODE END PM */

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

/* USER CODE BEGIN PV */

 uint16_t adc_buf[ADC_BUF_LEN]; 
 unsigned short int channels[72]={0};
 uint8_t all_channels_evaluated_flag = 0;
 uint8_t float_to_string [50];
 uint8_t numChar;
 uint8_t Rx_data = 0;
 uint8_t contatore = 0;

/* 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 */

/* 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_USART2_UART_Init();
  MX_ADC1_Init();
  /* USER CODE BEGIN 2 */
  HAL_ADC_Start_DMA(&hadc1,(uint32_t*)adc_buf,ADC_BUF_LEN);
  /* USER CODE END 2 */

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

    /* USER CODE BEGIN 3 */
   if(all_channels_evaluated_flag == 1){
  numChar =sprintf((char *)float_to_string, "?");// STRING START CHARACTER
  HAL_UART_Transmit(&huart2, float_to_string, numChar,TIMEOUT_TX);
  HAL_UART_Transmit(&huart2, (uint8_t *)channels,72,TIMEOUT_TX);
  numChar =sprintf((char *)float_to_string, "@");// STRING STOP CHARACTER
  HAL_UART_Transmit(&huart2, float_to_string, numChar,TIMEOUT_TX);
  all_channels_evaluated_flag = 0;
   }
   if(all_channels_evaluated_flag == 2){
  numChar =sprintf((char *)float_to_string, "?");
  HAL_UART_Transmit(&huart2, float_to_string, numChar,TIMEOUT_TX);
  HAL_UART_Transmit(&huart2, ((uint8_t *)channels + 72),72,TIMEOUT_TX);
  numChar =sprintf((char *)float_to_string, "@");
  HAL_UART_Transmit(&huart2, float_to_string, numChar,TIMEOUT_TX);
  all_channels_evaluated_flag = 0;
   }
  }
  /* 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_PWREx_ControlVoltageScaling(PWR_REGULATOR_VOLTAGE_SCALE1);

  /** Initializes the RCC Oscillators according to the specified parameters
  * in the RCC_OscInitTypeDef structure.
  */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
  RCC_OscInitStruct.HSIDiv = RCC_HSI_DIV1;
  RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
  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_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_HSI;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK)
  {
    Error_Handler();
  }
}

/* USER CODE BEGIN 4 */

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef* hadc) {
channels[contatore] = adc_buf[0];
channels[contatore+1]= adc_buf[1];
channels[contatore+2]= adc_buf[2];
channels[contatore+3]= adc_buf[3];
contatore=contatore+4;
if(contatore==36){
all_channels_evaluated_flag=1;
}
if(contatore==72){
all_channels_evaluated_flag=2;
contatore=0;
}
}


/* 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.
  * @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 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 */

[TDK]

> My problem is that the time needed to send that amount of data over USART (~1,5ms) is way longer than the time needed to fill the buffer with new ADC values (~15us)

You've correctly identified the issue. Using a FIFO won't change or solve this issue in any way. You will either need to lower your sample rate, or find a different method to get data out, or live with gaps in the data.

USB FS will max out at around 5-8 Mbps with excellent coding. Note that USB is subject to random delays of tens of ms so you will need to buffer data.

[GSp]

thank you for your answer,

so the serial communication will always be my bottleneck? isn't there a way to bufferize data at higher speed before sending it at a slower baudrate? I thought that using USB full speed I could be able to dimension a FIFO buffer to do so. Maybe I could go slightly slower than 2,5MHz with ADCs, but not as slow as the serial transmission.

[LCE]

As TDK said, you identified the problem, but it seems you ignore your own findings... 😉

For a continuous data flow, even with ADC at 1 MHz at 8 bit per sample => 8 Mbit/s, USB FS will probably fail, no matter what kind of buffering magic you're trying.

So you need a faster interface than USB FS, or you do only some ADC measurements in intervals / bursts: measure "as long as your RAM allows", stop the ADC, send data, start ADC again.