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Embedded Cprogramming~30 mins

DMA with UART for bulk transfer in Embedded C - Mini Project: Build & Apply

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DMA with UART for Bulk Transfer
📖 Scenario: You are working on an embedded system that needs to send a large amount of data over UART efficiently. To avoid slowing down the CPU, you will use DMA (Direct Memory Access) to transfer data from a memory buffer to the UART peripheral automatically.
🎯 Goal: Build a simple embedded C program that sets up a data buffer, configures DMA to transfer this buffer over UART, and starts the transfer. This will demonstrate how DMA can handle bulk data transfer without CPU intervention.
📋 What You'll Learn
Create a data buffer array with exact values
Define a DMA transfer size variable
Configure DMA to transfer the buffer to UART data register
Start the DMA transfer and print a confirmation message
💡 Why This Matters
🌍 Real World
DMA with UART is used in embedded devices like sensors, communication modules, and IoT devices to send large data efficiently without blocking the CPU.
💼 Career
Embedded software engineers often configure DMA and UART peripherals to optimize data transfer and improve system performance in real-time applications.
Progress0 / 4 steps
1
Create the data buffer
Create a uint8_t array called data_buffer with these exact 8 values: 0x48, 0x65, 0x6C, 0x6C, 0x6F, 0x21, 0x0A, 0x00.
Embedded C
Hint

Use uint8_t data_buffer[8] = { ... }; to create the array with the exact bytes.

2
Define DMA transfer size
Create an int variable called transfer_size and set it to the size of data_buffer (which is 8).
Embedded C
Hint

Use int transfer_size = 8; to set the transfer size.

3
Configure DMA for UART transfer
Write code to configure DMA to transfer transfer_size bytes from data_buffer to the UART data register at address 0x40013804. Use a pointer volatile uint8_t* called UART_DR for the UART data register. Assume DMA registers are DMA_SRC, DMA_DST, and DMA_CNT. Assign DMA_SRC to data_buffer, DMA_DST to UART_DR, and DMA_CNT to transfer_size. Also, set DMA_START to 1 to start the transfer.
Embedded C
Hint

Use pointers and assign DMA registers as described. Remember to cast the UART address to volatile uint8_t*.

4
Print confirmation of DMA start
Write a printf statement to print exactly: "DMA transfer started for 8 bytes over UART.\n".
Embedded C
Hint

Use printf("DMA transfer started for 8 bytes over UART.\n"); inside main().

Practice

(1/5)
1. What is the main advantage of using DMA with UART for bulk data transfer?
easy
A. It reduces CPU load by handling data transfer automatically.
B. It increases the UART baud rate beyond hardware limits.
C. It encrypts data during transmission for security.
D. It allows UART to work without any clock signal.

Solution

  1. Step 1: Understand DMA role in UART transfer

    DMA transfers data directly between memory and UART without CPU intervention for each byte.

  2. Step 2: Identify the benefit of reduced CPU load

    This frees CPU to perform other tasks while data moves in background.

  3. Final Answer:

    It reduces CPU load by handling data transfer automatically. -> Option A

  4. Quick Check:

    DMA offloads CPU [OK]
Hint: DMA moves data without CPU, so CPU load drops [OK]
Common Mistakes:
  • Thinking DMA increases UART speed beyond hardware limits
  • Confusing DMA with encryption or clock control
  • Assuming UART can work without clock signal
2. Which of the following code snippets correctly enables UART DMA transmission in embedded C?
easy
A. UART->CR3 |= USART_CR3_DMAT;
B. UART->CR1 |= USART_CR1_DMAT;
C. DMA->CR |= DMA_CR_EN;
D. UART->BRR = USART_BRR_DMAT;

Solution

  1. Step 1: Identify UART register for DMA transmit enable

    UART control register 3 (CR3) contains the DMAT bit to enable DMA for transmission.

  2. Step 2: Check correct bit and register usage

    Setting USART_CR3_DMAT in UART->CR3 enables DMA transmit requests.

  3. Final Answer:

    UART->CR3 |= USART_CR3_DMAT; -> Option A

  4. Quick Check:

    DMA transmit enable bit is in CR3 [OK]
Hint: DMA transmit enable is in UART CR3 register [OK]
Common Mistakes:
  • Using CR1 instead of CR3 for DMA transmit enable
  • Confusing DMA channel enable with UART DMA request enable
  • Setting baud rate register instead of control bits
3. Given this DMA setup code for UART RX bulk transfer, what will be the value of DMA->NDTR after starting the transfer? 
DMA->CCR = DMA_CCR_MINC | DMA_CCR_TCIE;
DMA->CNDTR = 100;
DMA->CPAR = (uint32_t)&UART->RDR;
DMA->CMAR = (uint32_t)rx_buffer;
DMA->CCR |= DMA_CCR_EN;
medium
A. 0
B. 99
C. 100
D. Undefined

Solution

  1. Step 1: Understand DMA CNDTR register role

    CNDTR holds the number of data items to transfer; initially set to 100.

  2. Step 2: Check when CNDTR changes

    CNDTR decrements only after each data transfer, so right after enabling DMA, it remains 100.

  3. Final Answer:

    100 -> Option C

  4. Quick Check:

    DMA transfer count starts at set value = 100 [OK]
Hint: DMA count decrements after transfer, not immediately [OK]
Common Mistakes:
  • Assuming CNDTR decrements immediately on enable
  • Confusing CNDTR with current data register
  • Thinking CNDTR resets to zero on start
4. Identify the error in this DMA UART transmit setup code snippet: 
DMA->CCR = DMA_CCR_MINC | DMA_CCR_DIR;
DMA->CNDTR = 50;
DMA->CPAR = (uint32_t)&UART->TDR;
DMA->CMAR = (uint32_t)tx_buffer;
DMA->CCR |= DMA_CCR_EN;
UART->CR3 &= ~USART_CR3_DMAT;
medium
A. DMA direction bit is set incorrectly for UART transmit.
B. UART DMA transmit request is disabled after enabling DMA.
C. Memory increment mode is not enabled.
D. Peripheral address points to wrong UART register.

Solution

  1. Step 1: Check UART DMA transmit enable bit

    UART->CR3 must have DMAT bit set to enable DMA requests for transmit.

  2. Step 2: Identify disabling of DMAT bit

    Code clears DMAT bit after DMA enable, disabling UART DMA transmit requests, so transfer won't start.

  3. Final Answer:

    UART DMA transmit request is disabled after enabling DMA. -> Option B

  4. Quick Check:

    DMAT bit must be set for DMA transmit [OK]
Hint: Keep UART DMAT bit set to allow DMA transmit [OK]
Common Mistakes:
  • Disabling UART DMA transmit request after DMA enable
  • Confusing DMA direction bit meaning
  • Forgetting to enable memory increment mode
5. You want to transfer 256 bytes from a memory buffer to UART using DMA. Which sequence correctly sets up the DMA and UART for this bulk transfer?
hard
A. Set DMA source to UART RDR, destination to buffer, length 256; enable UART DMAR bit; enable DMA channel.
B. Set DMA source to UART TDR, destination to buffer, length 256; enable UART DMAR bit; enable DMA channel.
C. Set DMA source to buffer, destination to UART RDR, length 256; enable UART DMAT bit; enable DMA channel.
D. Set DMA source to buffer, destination to UART TDR, length 256; enable UART DMAT bit; enable DMA channel.

Solution

  1. Step 1: Identify direction for UART transmit DMA

    For UART transmit, DMA source is memory buffer, destination is UART TDR register.

  2. Step 2: Enable UART DMA transmit request and DMA channel

    Set UART DMAT bit to enable DMA requests on transmit, then enable DMA channel to start transfer.

  3. Final Answer:

    Set DMA source to buffer, destination to UART TDR, length 256; enable UART DMAT bit; enable DMA channel. -> Option D

  4. Quick Check:

    Memory to UART TDR + DMAT enabled [OK]
Hint: For UART TX DMA: memory source, UART TDR destination, enable DMAT [OK]
Common Mistakes:
  • Confusing UART TDR and RDR registers for source/destination
  • Using DMAR bit for transmit instead of DMAT
  • Setting DMA direction opposite to data flow
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10 questions~5 mineasy hard