DMA with ADC for continuous sampling in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this DMA ADC buffer processing code?

Consider the following embedded C code snippet that uses DMA to continuously sample ADC values into a buffer. What will be the printed output after the DMA transfer completes once?

Embedded C

#include <stdio.h>
#include <stdint.h>

#define BUFFER_SIZE 4

uint16_t adc_buffer[BUFFER_SIZE] = {0};

void DMA_TransferComplete_Callback() {
    for (int i = 0; i < BUFFER_SIZE; i++) {
        printf("%d ", adc_buffer[i]);
    }
    printf("\n");
}

int main() {
    // Simulate ADC values filled by DMA
    adc_buffer[0] = 100;
    adc_buffer[1] = 200;
    adc_buffer[2] = 300;
    adc_buffer[3] = 400;

    DMA_TransferComplete_Callback();
    return 0;
}

A 100 200 300 0

B 0 0 0 0

C 100 200 300 400

D 200 300 400 500

💡 Hint

Look at how the adc_buffer array is filled before the callback prints it.

The adc_buffer is manually filled with values 100, 200, 300, 400 before the DMA_TransferComplete_Callback is called. The callback prints all four values in order, so the output is '100 200 300 400 '.

🧠 Conceptual

intermediate

Which statement best describes DMA's role in continuous ADC sampling?

In embedded systems, what is the main advantage of using DMA with ADC for continuous sampling?

A DMA allows ADC to sample data without CPU intervention, reducing CPU load.

B DMA increases the ADC sampling rate beyond hardware limits.

C DMA converts analog signals to digital automatically.

D DMA disables ADC interrupts to save power.

💡 Hint

Think about how DMA helps with data transfer in the background.

DMA transfers ADC data directly to memory without CPU involvement, allowing the CPU to perform other tasks and reducing overhead.

🔧 Debug

advanced

Identify the error causing incorrect ADC data in DMA buffer

Given the code below, the DMA buffer does not contain the expected ADC values after sampling. What is the most likely cause?

Embedded C

volatile uint16_t adc_buffer[4];

void ADC_DMA_Init() {
    // DMA configuration omitted for brevity
    // Start ADC with DMA
    HAL_ADC_Start_DMA(&hadc1, (uint32_t*)adc_buffer, 4);
}

int main() {
    ADC_DMA_Init();
    // Wait for DMA transfer complete
    while(!dma_transfer_complete_flag) {}
    // Process adc_buffer here
    return 0;
}

A The DMA transfer size is set incorrectly to 4 instead of 8.

B The dma_transfer_complete_flag is never set to true.

C The ADC is not started before DMA initialization.

D The adc_buffer is not declared as volatile, so compiler optimizations may cause stale data.

💡 Hint

Consider how the compiler treats variables modified outside the main flow.

Since adc_buffer is modified by DMA hardware, it should be declared volatile to prevent the compiler from caching its value and causing stale reads.

📝 Syntax

advanced

Which option correctly configures DMA for circular ADC sampling?

Which of the following DMA initialization snippets correctly sets up circular mode for continuous ADC sampling?

A hdma_adc.Init.Mode = DMA_SINGLE;

B hdma_adc.Init.Mode = DMA_CIRCULAR;

C hdma_adc.Init.Mode = DMA_PINGPONG;

D hdma_adc.Init.Mode = DMA_NORMAL;

💡 Hint

Continuous sampling requires the DMA to restart automatically after finishing.

DMA_CIRCULAR mode allows the DMA to continuously cycle through the buffer, ideal for continuous ADC sampling.

🚀 Application

expert

How many ADC samples are stored after 3 full DMA buffer cycles?

An ADC is configured with DMA in circular mode with a buffer size of 8 samples. The DMA continuously fills the buffer. After 3 full cycles of the DMA buffer, how many unique ADC samples have been stored in total?

A 24

B 8

C 3

D 16

💡 Hint

Think about how many samples fit in one buffer and multiply by the number of cycles.

Each DMA buffer cycle stores 8 samples. After 3 cycles, total samples stored = 8 * 3 = 24.