Circular buffer DMA mode in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of DMA circular buffer index update

Consider a DMA circular buffer of size 8. The DMA current index is updated after each transfer. What is the value of current_index after 10 transfers?

Embedded C

int buffer_size = 8;
int current_index = 0;
for (int i = 0; i < 10; i++) {
    current_index = (current_index + 1) % buffer_size;
}
printf("%d", current_index);

A 10

B 8

C 0

D 2

💡 Hint

Think about how modulo (%) operator wraps the index around the buffer size.

The index increments 10 times, but since the buffer size is 8, it wraps around using modulo 8. So, 10 % 8 = 2.

🧠 Conceptual

intermediate

Purpose of circular buffer in DMA mode

What is the main advantage of using a circular buffer in DMA mode for data reception?

A It allows continuous data reception without CPU intervention by wrapping the buffer index automatically.

B It stores data in a linked list to manage variable sizes.

C It prevents data loss by disabling interrupts during transfer.

D It increases the data transfer speed by doubling the buffer size.

💡 Hint

Think about how circular buffers help with continuous streaming data.

Circular buffers allow DMA to write data continuously by wrapping the write pointer to the start when the end is reached, reducing CPU load.

🔧 Debug

advanced

Identify the error in DMA circular buffer setup

What error will occur when running this DMA circular buffer initialization code?

Embedded C

uint8_t dma_buffer[16];
DMA_InitTypeDef dma_init;
dma_init.Mode = DMA_NORMAL;
dma_init.BufferSize = 16;
dma_init.MemInc = ENABLE;
DMA_Init(&dma_init);
// Start DMA transfer
DMA_Start(dma_buffer, 16);

A Buffer overflow error due to wrong buffer size.

B DMA will stop after filling buffer once, no circular mode enabled.

C Compilation error: DMA_InitTypeDef has no member 'Mode'.

D Runtime crash due to uninitialized dma_buffer.

💡 Hint

Check the DMA mode setting for continuous circular operation.

The code sets DMA mode to NORMAL, so DMA stops after one buffer fill. Circular mode must be enabled for continuous operation.

📝 Syntax

advanced

Correct syntax for enabling DMA circular mode

Which option correctly enables circular mode in DMA configuration?

Embedded C

DMA_InitTypeDef dma_init;
dma_init.Mode = ???;
// rest of initialization

A DMA_CIRCULAR_MODE

B DMA_MODE_CIRCULAR

C DMA_CIRCULAR

D CIRCULAR_DMA_MODE

💡 Hint

Look for the standard DMA mode constant for circular mode.

The correct constant to enable circular mode is usually DMA_CIRCULAR in embedded DMA libraries.

🚀 Application

expert

Calculate remaining data in DMA circular buffer

You have a DMA circular buffer of size 32 bytes. The DMA current data counter shows 10 bytes remaining to transfer. How many bytes have been transferred so far?

A 22 bytes

B 10 bytes

C 42 bytes

D 32 bytes

💡 Hint

Subtract the remaining bytes from the total buffer size.

The total buffer size is 32 bytes. If 10 bytes remain, then 32 - 10 = 22 bytes have been transferred.