DMA with ADC for continuous sampling in Embedded C - Mini Project: Build & Apply

DMA with ADC for Continuous Sampling

📖 Scenario: You are working on a microcontroller project where you need to read analog sensor data continuously without blocking the CPU. To do this efficiently, you will use the ADC (Analog-to-Digital Converter) with DMA (Direct Memory Access) to transfer data automatically to memory.This setup is common in real-time sensor monitoring systems like temperature logging or audio signal processing.

🎯 Goal: Build a program that configures the ADC and DMA to continuously sample analog data into a buffer without CPU intervention. You will set up the data buffer, configure DMA parameters, start ADC with DMA, and finally print the sampled data.

📋 What You'll Learn

Create a buffer array to store ADC samples

Define the number of samples to collect

Configure DMA to transfer ADC data to the buffer continuously

Start ADC with DMA for continuous sampling

Print the collected ADC samples

💡 Why This Matters

🌍 Real World

This technique is used in embedded systems to read sensors continuously without slowing down the main program. It helps in applications like environmental monitoring, motor control, and audio processing.

💼 Career

Embedded developers often use DMA with ADC to optimize performance and reduce CPU load in real-time systems.

1

Create ADC data buffer and sample count

Create an integer array called adc_buffer with size 10 to hold ADC samples. Also, create an integer variable called num_samples and set it to 10.

Embedded C

# Create adc_buffer array and num_samples variable
# Your code here

Hint

Think of adc_buffer as a container to hold 10 numbers from the ADC.

2

Configure DMA for ADC data transfer

Create a struct variable called dma_config and set its source_address to &ADC->DR, destination_address to adc_buffer, and transfer_size to num_samples. Assume the struct has these fields: source_address, destination_address, and transfer_size.

Embedded C

int adc_buffer[10];
int num_samples = 10;
// Configure dma_config struct with source, destination, and size
# Your code here

Hint

DMA needs to know where to read from, where to write to, and how many samples to transfer.

3

Start ADC with DMA for continuous sampling

Call the function ADC_Start_DMA with arguments adc_buffer and num_samples to start ADC with DMA in continuous mode.

Embedded C

int adc_buffer[10];
int num_samples = 10;

struct DMA_Config {
    volatile int *source_address;
    int *destination_address;
    int transfer_size;
} dma_config;

dma_config.source_address = &ADC->DR;
dma_config.destination_address = adc_buffer;
dma_config.transfer_size = num_samples;

// Start ADC with DMA
# Your code here

Hint

This function tells the ADC and DMA to begin sampling and transferring data automatically.

4

Print the ADC samples

Use a for loop with variable i from 0 to num_samples - 1 to print each value in adc_buffer using printf in the format: "Sample i: value\n".

Embedded C

int adc_buffer[10];
int num_samples = 10;

struct DMA_Config {
    volatile int *source_address;
    int *destination_address;
    int transfer_size;
} dma_config;

dma_config.source_address = &ADC->DR;
dma_config.destination_address = adc_buffer;
dma_config.transfer_size = num_samples;

ADC_Start_DMA(adc_buffer, num_samples);

// Print all samples
# Your code here

Hint

Loop through the buffer and print each sample with its index.

Practice

1. What is the main advantage of using DMA with ADC in continuous sampling mode?

easy

A. It disables ADC interrupts to save power.

B. It transfers ADC data to memory automatically without CPU intervention.

C. It allows manual control of each ADC conversion.

D. It increases the ADC resolution beyond hardware limits.

Solution

1. Step 1: Understand DMA role with ADC

   DMA (Direct Memory Access) moves data from ADC to memory automatically, reducing CPU load.

2. Step 2: Identify continuous sampling benefit

   Continuous mode means ADC keeps sampling without CPU restarting it each time.

3. Final Answer:

   It transfers ADC data to memory automatically without CPU intervention. -> Option B

4. Quick Check:

   DMA automates data transfer = It transfers ADC data to memory automatically without CPU intervention. [OK]

Hint: DMA automates ADC data transfer, no CPU needed [OK]

Common Mistakes:

• Thinking DMA increases ADC resolution
• Confusing manual and automatic data transfer
• Assuming DMA disables ADC interrupts

2. Which of the following is the correct way to configure DMA for circular mode with ADC in embedded C?

easy

A. DMA_InitStructure.DMA_Mode = DMA_Mode_Circular;

B. DMA_InitStructure.DMA_Mode = DMA_Mode_Normal;

C. DMA_InitStructure.DMA_Mode = DMA_Mode_Manual;

D. DMA_InitStructure.DMA_Mode = DMA_Mode_Interrupt;

Solution

1. Step 1: Recall DMA modes for continuous ADC sampling

   DMA_Mode_Circular allows buffer to be reused endlessly for continuous data.

2. Step 2: Match correct DMA mode setting

   DMA_Mode_Normal stops after one transfer; DMA_Mode_Circular repeats automatically.

3. Final Answer:

   DMA_InitStructure.DMA_Mode = DMA_Mode_Circular; -> Option A

4. Quick Check:

   Circular mode enables continuous buffer reuse = DMA_InitStructure.DMA_Mode = DMA_Mode_Circular; [OK]

Hint: Use DMA_Mode_Circular for endless ADC data buffering [OK]

Common Mistakes:

• Using DMA_Mode_Normal for continuous sampling
• Confusing DMA_Mode_Manual which doesn't exist
• Selecting interrupt mode instead of circular

3. Given the following code snippet, what will be the content of the buffer after 3 ADC conversions with DMA in circular mode?

uint16_t adc_buffer[4] = {0};
// DMA and ADC configured in circular mode
// ADC conversions produce values: 100, 200, 300
// DMA transfers these values into adc_buffer sequentially
// After 3 conversions, what is adc_buffer content?

medium

A. [300, 200, 100, 0]

B. [0, 100, 200, 300]

C. [100, 200, 300, 0]

D. [100, 200, 0, 300]

Solution

1. Step 1: Understand DMA circular buffer filling

   DMA fills buffer sequentially from index 0 upwards with ADC values.

2. Step 2: Map ADC values to buffer after 3 transfers

   First 3 elements get 100, 200, 300; last element remains 0 as no 4th conversion yet.

3. Final Answer:

   [100, 200, 300, 0] -> Option C

4. Quick Check:

   Buffer fills sequentially = [100, 200, 300, 0] [OK]

Hint: DMA fills buffer from start sequentially [OK]

Common Mistakes:

• Assuming buffer shifts values on each new sample
• Thinking DMA fills buffer backwards
• Confusing buffer initialization with data content

4. Identify the error in this DMA + ADC continuous sampling setup code snippet:

ADC_InitTypeDef ADC_InitStructure;
DMA_InitTypeDef DMA_InitStructure;

// ADC and DMA configuration omitted for brevity
DMA_InitStructure.DMA_Mode = DMA_Mode_Normal;
DMA_Init(DMA1_Channel1, &DMA_InitStructure);
ADC_Cmd(ADC1, ENABLE);
ADC_SoftwareStartConvCmd(ADC1, ENABLE);

medium

A. DMA_Mode_Normal should be DMA_Mode_Circular for continuous sampling.

B. ADC_SoftwareStartConvCmd should be called before DMA_Init.

C. DMA1_Channel1 is not valid for ADC1 DMA transfers.

D. ADC_Cmd should be called after starting conversion.

Solution

1. Step 1: Check DMA mode for continuous ADC sampling

   DMA_Mode_Normal stops after one transfer; continuous sampling needs DMA_Mode_Circular.

2. Step 2: Verify other calls order and parameters

   Calling ADC_SoftwareStartConvCmd after DMA_Init is correct; DMA1_Channel1 is valid for many MCUs; ADC_Cmd before conversion start is correct.

3. Final Answer:

   DMA_Mode_Normal should be DMA_Mode_Circular for continuous sampling. -> Option A

4. Quick Check:

   Continuous DMA needs circular mode = DMA_Mode_Normal should be DMA_Mode_Circular for continuous sampling. [OK]

Hint: Use circular mode for continuous DMA transfers [OK]

Common Mistakes:

• Using normal mode for continuous DMA
• Misordering ADC and DMA initialization calls
• Assuming DMA channel is invalid without checking MCU

5. You want to continuously sample 8 ADC channels using DMA in circular mode into a buffer of size 16 (uint16_t). Which approach correctly handles the buffer to process the latest samples without data loss?

hard

A. Disable circular mode and manually restart DMA after each 8 samples.

B. Process the entire buffer only after DMA transfer completes once, then restart DMA.

C. Use polling to check ADC conversion complete flag before reading buffer.

D. Use half-transfer and transfer-complete DMA interrupts to process buffer halves alternately.

Solution

1. Step 1: Understand circular DMA buffer usage

   Circular DMA fills buffer continuously; half-transfer and transfer-complete interrupts signal when half/full buffer is ready.

2. Step 2: Choose method to avoid data loss

   Processing halves alternately on interrupts prevents overwriting unprocessed data and ensures continuous handling.

3. Final Answer:

   Use half-transfer and transfer-complete DMA interrupts to process buffer halves alternately. -> Option D

4. Quick Check:

   Half/full DMA interrupts enable safe continuous processing = Use half-transfer and transfer-complete DMA interrupts to process buffer halves alternately. [OK]

Hint: Use DMA half/full interrupts to process buffer safely [OK]

Common Mistakes:

• Waiting for full buffer before processing causes data loss
• Disabling circular mode loses continuous sampling benefit
• Polling ADC flags wastes CPU and misses DMA data