Circular buffer DMA mode in Embedded C

Introduction

Circular buffer DMA mode helps to continuously store data in a fixed-size buffer without stopping. It automatically wraps around when it reaches the end, so you don't lose data.

When reading data from sensors continuously without missing any values.

When receiving serial data streams like UART or SPI without manual buffer management.

When audio or video data streams need to be processed in real-time.

When you want to reduce CPU load by letting DMA handle data transfers automatically.

Syntax

Embedded C

DMA_InitTypeDef dma_init;
dma_init.DMA_Mode = DMA_Mode_Circular;
// Other DMA settings like direction, buffer size, peripheral address, memory address
DMA_Init(DMAx_Streamy, &dma_init);
DMA_Cmd(DMAx_Streamy, ENABLE);

The key setting is dma_init.DMA_Mode = DMA_Mode_Circular to enable circular buffer mode.

You must configure buffer size and addresses correctly to avoid overwriting important data.

Examples

This example sets up DMA to read 128 bytes from a peripheral to memory in circular mode.

Embedded C

DMA_InitTypeDef dma_init;
dma_init.DMA_Channel = DMA_Channel_4;
dma_init.DMA_DIR = DMA_DIR_PeripheralToMemory;
dma_init.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
dma_init.DMA_MemoryInc = DMA_MemoryInc_Enable;
dma_init.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;
dma_init.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
dma_init.DMA_Mode = DMA_Mode_Circular;
dma_init.DMA_Priority = DMA_Priority_High;
dma_init.DMA_FIFOMode = DMA_FIFOMode_Disable;
dma_init.DMA_BufferSize = 128;
DMA_Init(DMA2_Stream0, &dma_init);
DMA_Cmd(DMA2_Stream0, ENABLE);

Used for continuous UART reception without stopping DMA.

Embedded C

// Start DMA in circular mode for UART RX buffer
DMA_InitTypeDef dma_uart_rx;
dma_uart_rx.DMA_Channel = DMA_Channel_4;
dma_uart_rx.DMA_DIR = DMA_DIR_PeripheralToMemory;
dma_uart_rx.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
dma_uart_rx.DMA_MemoryInc = DMA_MemoryInc_Enable;
dma_uart_rx.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;
dma_uart_rx.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
dma_uart_rx.DMA_Mode = DMA_Mode_Circular;
dma_uart_rx.DMA_BufferSize = UART_RX_BUFFER_SIZE;
// Other UART DMA settings
DMA_Init(DMA1_Stream5, &dma_uart_rx);
DMA_Cmd(DMA1_Stream5, ENABLE);

Sample Program

This program configures DMA in circular mode to fill a buffer continuously from a peripheral (simulated here). It then prints the buffer content.

Embedded C

#include "stm32f4xx.h"
#include <stdio.h>

#define BUFFER_SIZE 16
uint8_t dma_buffer[BUFFER_SIZE];

void DMA_Config(void) {
    DMA_InitTypeDef dma_init;
    
    RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_DMA2, ENABLE);

    DMA_DeInit(DMA2_Stream0);
    while (DMA_GetCmdStatus(DMA2_Stream0) != DISABLE) {}

    dma_init.DMA_Channel = DMA_Channel_0;
    dma_init.DMA_DIR = DMA_DIR_PeripheralToMemory;
    dma_init.DMA_PeripheralInc = DMA_PeripheralInc_Disable;
    dma_init.DMA_MemoryInc = DMA_MemoryInc_Enable;
    dma_init.DMA_PeripheralDataSize = DMA_PeripheralDataSize_Byte;
    dma_init.DMA_MemoryDataSize = DMA_MemoryDataSize_Byte;
    dma_init.DMA_Mode = DMA_Mode_Circular;
    dma_init.DMA_Priority = DMA_Priority_High;
    dma_init.DMA_FIFOMode = DMA_FIFOMode_Disable;
    dma_init.DMA_BufferSize = BUFFER_SIZE;
    dma_init.DMA_PeripheralBaseAddr = (uint32_t)&(USART1->DR);
    dma_init.DMA_Memory0BaseAddr = (uint32_t)dma_buffer;

    DMA_Init(DMA2_Stream0, &dma_init);

    DMA_Cmd(DMA2_Stream0, ENABLE);
}

int main(void) {
    DMA_Config();

    // Simulate main loop
    for (int i = 0; i < BUFFER_SIZE; i++) {
        dma_buffer[i] = i + 1; // Simulate data received by DMA
    }

    // Print buffer content
    for (int i = 0; i < BUFFER_SIZE; i++) {
        printf("%d ", dma_buffer[i]);
    }
    printf("\n");

    while (1) {}
}

Important Notes

Make sure the buffer size matches the DMA configuration to avoid data corruption.

In circular mode, DMA never stops; you must handle data reading carefully to avoid overwriting unread data.

Check your microcontroller's reference manual for exact DMA register and function names.

Summary

Circular buffer DMA mode lets DMA write data continuously in a loop.

It is useful for streaming data like sensors or communication peripherals.

Proper buffer and DMA setup is important to avoid data loss.

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