Circular buffer DMA mode in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the size of the circular buffer.

Embedded C

volatile uint8_t dma_buffer[[1]];

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

A256

B512

C1024

D128

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing a size that is not a power of two.

Using a size too large for the memory.

The circular buffer size is set to 256 bytes to match the DMA transfer size.

2fill in blank

medium

Complete the code to enable circular mode in the DMA configuration.

Embedded C

DMA_InitStructure.DMA_Mode = [1];

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

ADMA_Mode_MemoryToMemory

BDMA_Mode_PeripheralControl

CDMA_Mode_Normal

DDMA_Mode_Circular

Attempts:

3 left

💡 Hint

Common Mistakes

Using normal mode which stops after one transfer.

Choosing memory-to-memory mode which is not suitable here.

DMA_Mode_Circular enables the circular buffer mode for continuous data transfer.

3fill in blank

hard

Fix the error in the DMA interrupt flag clearing code.

Embedded C

DMA_ClearFlag(DMA1_Stream5, [1]);

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ADMA_FLAG_FEIF5

BDMA_FLAG_TEIF5

CDMA_FLAG_TCIF5

DDMA_FLAG_HTIF5

Attempts:

3 left

💡 Hint

Common Mistakes

Clearing the half-transfer flag instead of transfer complete.

Clearing error flags which do not acknowledge normal completion.

The transfer complete flag (DMA_FLAG_TCIF5) must be cleared to acknowledge the DMA transfer completion.

4fill in blank

hard

Fill both blanks to correctly configure the DMA buffer size and enable the DMA stream.

Embedded C

DMA_InitStructure.DMA_BufferSize = [1];
DMA_Cmd(DMA1_Stream5, [2]);

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

A256

BENABLE

CDISABLE

D128

Attempts:

3 left

💡 Hint

Common Mistakes

Using a buffer size that does not match the array.

Disabling the DMA stream by mistake.

The buffer size must match the circular buffer size (256), and the DMA stream must be enabled with ENABLE.

5fill in blank

hard

Fill all three blanks to correctly set the DMA peripheral address, memory address, and data direction.

Embedded C

DMA_InitStructure.DMA_PeripheralBaseAddr = [1];
DMA_InitStructure.DMA_Memory0BaseAddr = [2];
DMA_InitStructure.DMA_DIR = [3];

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

A(uint32_t)&USART1->DR

B(uint32_t)dma_buffer

CDMA_DIR_PeripheralToMemory

DDMA_DIR_MemoryToPeripheral

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping peripheral and memory addresses.

Setting direction incorrectly as memory to peripheral.

The peripheral base address is the USART data register, the memory base address is the dma_buffer, and the direction is from peripheral to memory for receiving data.

Practice

1.

What is the main advantage of using circular buffer mode in DMA?

easy

A. It reduces the size of the buffer needed.

B. It increases the CPU speed automatically.

C. It disables interrupts during data transfer.

D. It allows continuous data transfer without stopping when the buffer is full.

Solution

1. Step 1: Understand circular buffer mode behavior

   Circular buffer mode lets DMA wrap around to the start of the buffer after reaching the end, enabling continuous data transfer.

2. Step 2: Identify the advantage

   This means data can be streamed continuously without stopping or manual buffer reset.

3. Final Answer:

   It allows continuous data transfer without stopping when the buffer is full. -> Option D

4. Quick Check:

   Circular buffer = continuous transfer [OK]

Hint: Circular buffer means wrap-around for continuous data [OK]

Common Mistakes:

• Thinking CPU speed changes with DMA mode
• Confusing interrupt behavior with buffer mode
• Assuming buffer size is reduced automatically

2.

Which of the following is the correct way to enable circular mode in a DMA configuration struct in embedded C?

DMA_InitTypeDef DMA_InitStructure;
// ... other initializations
DMA_InitStructure.Mode = ?;

easy

A. DMA_Mode_Circular

B. DMA_Mode_Interrupt

C. DMA_Mode_Normal

D. DMA_Mode_OneShot

Solution

1. Step 1: Recall DMA mode options

   DMA_Mode_Circular is the standard mode to enable circular buffer operation in DMA configurations.

2. Step 2: Match the correct mode

   DMA_Mode_Normal is single transfer, others like Interrupt or OneShot are not standard DMA modes for circular buffer.

3. Final Answer:

   DMA_Mode_Circular -> Option A

4. Quick Check:

   Enable circular mode = DMA_Mode_Circular [OK]

Hint: Look for 'Circular' keyword in DMA mode options [OK]

Common Mistakes:

• Using DMA_Mode_Normal instead of circular
• Confusing interrupt mode with circular mode
• Using non-existent DMA modes

3.

Given the following code snippet configuring DMA in circular mode, what will be the value of buffer[0] after 5 DMA transfers if the peripheral sends values 10, 20, 30, 40, 50 continuously?

#define BUFFER_SIZE 4
uint8_t buffer[BUFFER_SIZE] = {0};
// DMA configured in circular mode with buffer size 4
// Peripheral sends: 10, 20, 30, 40, 50
// DMA writes each value to buffer sequentially
// After 5 transfers:

medium

A. 10

B. 50

C. 20

D. 40

Solution

1. Step 1: Understand circular buffer write pattern

   Buffer size is 4, so DMA writes values to buffer[0..3] in order, then wraps around to buffer[0].

2. Step 2: Track each transfer

   Transfers: 1->buffer[0]=10, 2->buffer[1]=20, 3->buffer[2]=30, 4->buffer[3]=40, 5->buffer[0]=50 (overwrite).

3. Final Answer:

   50 -> Option B

4. Quick Check:

   5th write wraps to buffer[0] = 50 [OK]

Hint: Count writes modulo buffer size for circular buffers [OK]

Common Mistakes:

• Assuming buffer[0] keeps first value
• Ignoring wrap-around overwrite
• Confusing peripheral data order

4.

Identify the error in this DMA circular buffer setup snippet:

DMA_InitTypeDef DMA_InitStructure;
uint8_t buffer[8];
DMA_InitStructure.Mode = DMA_Mode_Circular;
DMA_InitStructure.BufferSize = 10;
DMA_InitStructure.MemBaseAddr = (uint32_t)buffer;
// Start DMA

medium

A. Memory address is not cast to correct type.

B. DMA_Mode_Circular is not set correctly.

C. Buffer size is larger than actual buffer, causing overflow.

D. Buffer should be declared as volatile.

Solution

1. Step 1: Compare buffer size and BufferSize parameter

   Buffer array has 8 elements, but BufferSize is set to 10, which is larger than buffer length.

2. Step 2: Understand consequences

   This mismatch can cause DMA to write beyond buffer bounds, leading to overflow and undefined behavior.

3. Final Answer:

   Buffer size is larger than actual buffer, causing overflow. -> Option C

4. Quick Check:

   BufferSize must not exceed buffer length [OK]

Hint: BufferSize must match actual buffer length [OK]

Common Mistakes:

• Setting BufferSize larger than buffer array
• Ignoring buffer overflow risks
• Assuming casting memory address fixes size mismatch

5.

You want to use DMA in circular mode to continuously read sensor data into a buffer of size 16. However, you notice data loss when the CPU reads the buffer slower than DMA writes. What is the best way to avoid data loss?

hard

A. Increase buffer size and use double buffering technique.

B. Disable circular mode and use normal mode.

C. Reduce DMA transfer speed to match CPU speed.

D. Use interrupts to stop DMA when CPU is busy.

Solution

1. Step 1: Understand cause of data loss

   If CPU reads slower than DMA writes, new data overwrites unread data in circular buffer causing loss.

2. Step 2: Apply solution with double buffering

   Increasing buffer size and using double buffering lets CPU read one buffer while DMA writes to another, preventing overwrite.

3. Final Answer:

   Increase buffer size and use double buffering technique. -> Option A

4. Quick Check:

   Double buffering prevents overwrite in circular DMA [OK]

Hint: Use double buffering to separate read/write buffers [OK]

Common Mistakes:

• Disabling circular mode loses continuous transfer benefit
• Trying to slow DMA speed which may not be possible
• Stopping DMA interrupts causes data gaps