How to calculate ADC Conversion Time

This is the Sixth tutorial in the STM32 ADC series. In this series will see how to use the ADC peripheral of the STM32 to read the data from the Analog devices. We will cover how to use the ADC in different modes, that includes polling mode, interrupt mode and the DMA mode. We will also see how to use the multiple channels to read multiple analog devices using the same ADC. Later in the series we will cover more advanced features like differential ADC, combined ADCs, ADC watchdogs, injected channels etc.

I am going to use the STM32H750 based development board for this series because it has advanced ADC features. But I will also cover the configuration changes required for some of the popular series that includes STM32F103C8 and STM32F446RE.

This tutorial will cover how to calculate the time taken by the ADC peripheral to sample and convert the channel. We will also compare the calculated time with the resulting time on the logic analyzer.

The Formula

As per the ST’s documentation, the formula to calculate the total conversion time is shown below.

The above formula has 3 parameters:

• @Sampling Cycles is the time in terms of ADC CYCLES, taken by a particular channel to sample the data. This parameter is configurable in the cubeMX and it can be configured separately for each channel.
• @Conversion Cycles is the time in terms of ADC CYCLES, taken by the ADC to convert the sampled data. This parameter is not configurable, but it depends on the ADC resolution we set in the cubeMX. The Higher the resolution, the higher will be this parameter. This parameter also varies across different MCUs, but we can find the value in the reference manual of the MCU itself.
• @ADC Clock is the clock at which the ADC is running.

STM32F103C8 Controller

The F103C8 MCU has 12 bit ADC resolution and it is not configurable. As per the reference manual, the conversion time for this 12 bit resolution is 12.5 Cycles.

As per the Formula, the sampling cycles and the ADC Clock is configurable in the cubeMX itself. Whereas the conversion cycles have a fixed value 12.5.

Let’s assume the ADC clock is 1MHz and the Sampling Time is configured as 239.5 Cycles. The Conversion Time has a fixed value of 12.5 Cycles. The total conversion time is calculated as shown below.

F446RE MCU

The F446RE has configurable ADC resolution. The Conversion time with respect to the resolution is mentioned in the reference manual. Below is the image from pg-368 of the reference manual of F446RE.

Here, the Conversion Cycles depends on the resolution you select for the ADC

• For 12 bit Resolution, Conversion CYCLES = 12
• For 10 bit Resolution, Conversion CYCLES = 10
• For 8 bit Resolution, Conversion CYCLES = 8
• For 6 bit Resolution, Conversion CYCLES = 6

Below is the clock configuration for F446RE.

The APB2 peripheral clock is running at 4MHz. The ADC1, 2 and 3 all are connected to the APB2 Bus, therefore the ADC1 clock is also at the same frequency. We will use the prescaler to further reduce this clock.

Below is the image showing the ADC configuration.

I am using the prescaler of 6 and this will bring the ADC clock down to 4000000/6 = 666.67 KHz. The ADC Resolution is set to 12bits, therefore the conversion cycles will be 12.

We will use the DMA to transfer the data from the ADC, therefore the DMA is enabled in circular mode. Along with DMA, the continuous conversion mode and DMA continuous request are also enabled.

The sampling time is set to 480 Cycles.

Below is the image showing the calculation for the Total Conversion Time.

We will use the pin PA1 to measure the conversion time for the ADC.

The pin PA1 is configured in the output mode. I will connect the pin PA1 to the logic analyzer, therefore the output speed of the pin is set to very high.

The Code

Inside the main function we will start the ADC in DMA mode.

uint16_t ADC_VAL;
int main()
{
  ...
  HAL_ADC_Start_DMA(&hadc1, &ADC_VAL, 1);
  while (1)
  {}
}

ADC_VAL is the variable where the converted channel data will be stored and 1 is the number of channels we are converting.

Once the channel has been converted, an interrupt will trigger and the conversion complete callback will be called. Inside the callback we will toggle the pin PA1.

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef *hadc)
{
	HAL_GPIO_TogglePin(GPIOA, GPIO_PIN_1);
}

The pin toggles each time the conversion is finished. Therefore the pin remains HIGH and LOW for same time, as taken by the ADC to finish the conversion. Hence the high or low time of the pin should be the same as the time we calculated using the formula.

Result

Below is the image showing the result in the logic analyzer.

As you can see above, the pin PA1 remained high for 738 microseconds. This is exactly the same conversion time that we calculated using the formula.

So our formula and calculation was precise, you can see that in the result.

H750VB MCU

The H750 also has configurable ADC resolution. The Conversion time with respect to the resolution is mentioned in the reference manual. Below is the image from pg-952 of the reference manual of H750VB.

Here, the Conversion Cycles depends on the resolution you select for the ADC

• For 16 bit Resolution, Conversion CYCLES = 8.5
• For 14 bit Resolution, Conversion CYCLES = 7.5
• For 12 bit Resolution, Conversion CYCLES = 6.5
• and so on..

Below is the clock configuration for H750.

I have configured the ADC clock at 2 MHz. We will use the prescaler to reduce the clock further.

Below is the image showing the ADC configuration.

I am using the prescaler of 2 and this will bring the ADC clock down to 2000000/2 = 1MHz. The ADC Resolution is set to 16bits, therefore the conversion cycles will be 8.5.

We will use the DMA to transfer the data from the ADC, therefore the DMA is enabled in circular mode. Along with DMA, the continuous conversion mode should be enabled and conversion data management should be set to circular DMA.

The sampling time is set to 810.5 Cycles.

Below is the image showing the calculation for the Total Conversion Time.

We will use the pin PD8 to measure the conversion time for the ADC.

The pin PD8 is configured in the output mode. I will connect the pin PD8 to the logic analyzer, therefore the output speed of the pin is set to very high.

The Code

Inside the main function we will start the ADC in DMA mode.

uint16_t ADC_VAL;
int main()
{
  ...
  HAL_ADC_Start_DMA(&hadc1, &ADC_VAL, 1);
  while (1)
  {}
}

ADC_VAL is the variable where the converted channel data will be stored and 1 is the number of channels we are converting.

Once the channel has been converted, an interrupt will trigger and the conversion complete callback will be called. Inside the callback we will toggle the pin PD8.

void HAL_ADC_ConvCpltCallback(ADC_HandleTypeDef *hadc)
{
	HAL_GPIO_TogglePin(GPIOD, GPIO_PIN_8);
}

The pin toggles each time the conversion is finished. Therefore the pin remains HIGH and LOW for same time, as taken by the ADC to finish the conversion. Hence the high or low time of the pin should be the same as the time we calculated using the formula.

Result

Below is the image showing the result in the logic analyzer.

As you can see above, the pin PD8 remained high for 1.638 milliseconds. This is not what we calculated using the formula. Actually this is twice the conversion time (819uS * 2) we calculated.

The reason for it is mentioned in the reference manual of H750VB. Below is the image from pg-1036 (ADC_CCR Register).

As mentioned in the manual, the devices with revision V always have an additional divide factor of 2 applied to the ADC Clock. Therefore the conversion time is twice the one which we calculated before.

Below are the images showing different revisions for the same MCU.

So you have to carefully find out the device revision for the MCU you are using. If the MCU has revision V, the formula to calculate the total conversion time will be modified as shown below.

Video

Check out the video to see the complete explanation on the topic.