ADC Using DMA (Direct Memory Access)

Description

This is the Third article in the series of how to use ADC in STM32 and this time we will be using DMA (Direct Memory Access). To view the other two methods, visit ADC using interrupt in STM32 and ADC in STM32 using HAL 

DMA or Direct Memory Access is a method that allows and input/output device to send or receive data directly to or from the main memory, bypassing the CPU to speedup memory operations.

The advantage of using DMA is that we can use multi channel conversion. The result of the conversion will be stored in the memory and at any instance during the code, we can access that memory and  read ADC values within interrupt service routine.

CubeMX Setup

Select Channel 0 and Internal temperature

Resolution is 12 bit, Scan conversion- enabled, Continuous conversion – enabled, number of conversions – 2

Some Insight into the code:-

uint32_t  adc_buf[2], adc_val[2];

As i am using two channels therefore adc_buf  should store two bytes and so does adc_val.

Write a callback function to read the values from the buffer whenever  the conversion is complete. This is same as writing a callback in interrupt method

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef* hadc)
{
 for (int i =0; i<2;i++)
 {
  adc_val[i] = adc_buf[i];  // store the values in adc_val from buffer
 }

}

HAL_ADC_Start_DMA (&hadc1, (uint32_t *) adc_buf, 2);

Here      HAL_ADC_Start_DMA takes following arguments
          ADC_HandleTypeDef* hadc --->   ADC handler
          uint32_t* pData  --->   The destination Buffer address
          uint32_t Length  --->  The length of data to be transferred from ADC peripheral to memory.

uint32_t display_value[2];

main ()
{
   .......
    ..........
     while (1)
        {
            display_value[0] = adc_val[0];
            display_value[1] = adc_val[1];
             HAL_Delay (1000);
         }
}

Result

Compile the code and start debugging.

Add display_val[0] (for channel 0) and display_val[1] (for internal temperature) to the watch list and run the debugger.

Initially the channel 0, which is connected to IR sensor, shows 4095 because there is no obstacle in front of it and internal temperature sensor shows 953. We still have to convert this value to Celsius scale. We will see that part in another tutorial.

display_val[0] = 4095, display_val[1] = 953

IR sensor value changes on bringing something near it but temperature remains same. As the main() function suggests, we are reading values every 1 sec, so the change is not instant but only after a delay of 1 sec.

display_val[0] = 472, display_val[1] = 953

This is how we use DMA to for two different channels without compromising the cpu time. The converted data will be stored in memory and we can access it at any instant during the entire program.

We can use DMA to do as much conversion as we want (obviously limited by the number of channels present in the microcontroller).