DMA controller concept in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable the DMA controller.

Embedded C

DMA->[1] |= DMA_ENABLE;

Drag options to blanks, or click blank then click option'

ACONFIG

BCTRL

CSTATUS

DINTERRUPT

Attempts:

3 left

💡 Hint

Common Mistakes

Using STATUS register instead of CONFIG.

Trying to enable DMA in INTERRUPT register.

The DMA controller is enabled by setting the enable bit in the CONFIG register.

2fill in blank

medium

Complete the code to set the source address for DMA transfer.

Embedded C

DMA->SRC_ADDR = [1];

Drag options to blanks, or click blank then click option'

A0x40000000

BDMA_ENABLE

C0x20000000

D0x00000000

Attempts:

3 left

💡 Hint

Common Mistakes

Using DMA_ENABLE instead of an address.

Using zero address which is usually invalid.

The source address must be a valid memory address where data is read from.

3fill in blank

hard

Fix the error in the code to start the DMA transfer.

Embedded C

DMA->[1] = 1; // Start transfer

Drag options to blanks, or click blank then click option'

ACTRL

BCONFIG

CSTATUS

DSRC_ADDR

Attempts:

3 left

💡 Hint

Common Mistakes

Writing to STATUS register which is read-only.

Writing to SRC_ADDR which sets address, not start.

The CTRL register is used to start or stop the DMA transfer by writing 1 to the start bit.

4fill in blank

hard

Fill both blanks to configure the DMA transfer size and direction.

Embedded C

DMA->[1] = 256; // Transfer size
DMA->[2] = [2]_MEM_TO_PERIPH;

Drag options to blanks, or click blank then click option'

ASIZE

BDIR

CCTRL

DCONFIG

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing CTRL with SIZE or DIR registers.

Setting direction in CONFIG register incorrectly.

SIZE register sets the number of bytes to transfer. DIR register sets the direction of transfer.

5fill in blank

hard

Fill all three blanks to create a DMA transfer descriptor with source, destination, and size.

Embedded C

dma_desc.src_addr = [1];
dma_desc.dst_addr = [2];
dma_desc.size = [3];

Drag options to blanks, or click blank then click option'

A0x20001000

B0x40002000

C512

DDMA_ENABLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using DMA_ENABLE as an address or size.

Mixing up source and destination addresses.

The source and destination addresses must be valid memory locations. Size is the number of bytes to transfer.

Practice

1. What is the main purpose of a DMA controller in embedded systems?

easy

A. To transfer data between memory and peripherals without CPU involvement

B. To execute arithmetic operations faster

C. To manage power consumption of the CPU

D. To handle interrupts from external devices

Solution

1. Step 1: Understand DMA controller role

   A DMA controller is designed to move data automatically without CPU intervention.

2. Step 2: Compare options with DMA function

   Only To transfer data between memory and peripherals without CPU involvement describes data transfer without CPU involvement, which matches DMA's purpose.

3. Final Answer:

   To transfer data between memory and peripherals without CPU involvement -> Option A

4. Quick Check:

   DMA = data transfer without CPU [OK]

Hint: DMA moves data automatically, freeing CPU [OK]

Common Mistakes:

• Confusing DMA with CPU arithmetic operations
• Thinking DMA manages power or interrupts
• Assuming DMA executes code

2. Which of the following is the correct way to start a DMA transfer in embedded C?

easy

A. dma_start(source, destination, size);

B. start_dma(source, destination, size);

C. DMA_Transfer(source, destination, size);

D. dma_transfer(source, destination, size);

Solution

1. Step 1: Identify common DMA function naming

   In embedded C, functions often use lowercase with underscores, like dma_transfer.

2. Step 2: Check syntax correctness

   dma_transfer(source, destination, size); uses a plausible function name and correct parameter order: source, destination, size.

3. Final Answer:

   dma_transfer(source, destination, size); -> Option D

4. Quick Check:

   Correct DMA function syntax = dma_transfer(...) [OK]

Hint: DMA functions usually use lowercase with underscores [OK]

Common Mistakes:

• Using incorrect function names with uppercase letters
• Wrong parameter order
• Assuming DMA functions start with 'start_' prefix

3. Given this code snippet, what will be the value of buffer[0] after DMA transfer?

char source[3] = {10, 20, 30};
char buffer[3] = {0, 0, 0};
dma_transfer(source, buffer, 3);

medium

A. 20

B. 0

C. 10

D. 30

Solution

1. Step 1: Understand dma_transfer behavior

   The function copies data from source to buffer for the given size (3 bytes).

2. Step 2: Check first element after transfer

   buffer[0] will be assigned source[0], which is 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   buffer[0] = source[0] = 10 [OK]

Hint: DMA copies source to destination byte-by-byte [OK]

Common Mistakes:

• Assuming buffer remains zero
• Mixing up source and destination
• Using wrong index for buffer

4. Identify the error in this DMA setup code:

uint8_t src[5] = {1,2,3,4,5};
uint8_t dst[5];
dma_transfer(dst, src, 5);

medium

A. Arrays must be declared as volatile

B. Source and destination parameters are swapped

C. Size parameter should be 4, not 5

D. DMA transfer function missing return type

Solution

1. Step 1: Check parameter order in dma_transfer

   Correct order is source first, then destination.

2. Step 2: Analyze given code

   Code calls dma_transfer(dst, src, 5), swapping source and destination.

3. Final Answer:

   Source and destination parameters are swapped -> Option B

4. Quick Check:

   Correct order = source, destination [OK]

Hint: Source comes before destination in DMA calls [OK]

Common Mistakes:

• Mixing source and destination arguments
• Incorrect size parameter
• Assuming volatile needed for arrays

5. You want to transfer a large array of 1000 integers from memory to a peripheral using DMA. Which approach is best to optimize CPU usage?

hard

A. Split the transfer into multiple smaller DMA transfers of 100 integers each

B. Use a single DMA transfer for all 1000 integers at once

C. Copy data manually using a CPU loop instead of DMA

D. Disable DMA and use interrupts for each integer transfer

Solution

1. Step 1: Consider CPU load and DMA transfer size

   Very large DMA transfers may block bus or cause latency; splitting reduces impact.

2. Step 2: Evaluate options for CPU efficiency

   Splitting into smaller chunks allows CPU to handle other tasks between transfers.

3. Final Answer:

   Split the transfer into multiple smaller DMA transfers of 100 integers each -> Option A

4. Quick Check:

   Smaller DMA chunks optimize CPU usage [OK]

Hint: Split large DMA transfers to keep CPU responsive [OK]

Common Mistakes:

• Assuming one big DMA transfer is always best
• Using CPU loops wastes CPU time
• Disabling DMA loses its benefits