Why Peripheral-to-memory transfer in Embedded C? - Purpose & Use Cases

The Big Idea

What if your program could collect sensor data without lifting a finger?

The Scenario

Imagine you have a sensor sending data byte by byte, and your program must read each byte and store it in memory manually.

You write code that waits for each byte, reads it, and then saves it to an array one by one.

The Problem

This manual reading is slow because the CPU must stop what it is doing to wait for each byte.

It wastes time and can miss data if it is busy with other tasks.

Also, writing code to handle each byte is error-prone and hard to maintain.

The Solution

Peripheral-to-memory transfer lets the hardware move data directly from the sensor to memory without CPU involvement.

This frees the CPU to do other work and ensures data is captured quickly and reliably.

Before vs After

✗ Before

while(!data_ready()) {}
buffer[i++] = read_data();

✓ After

start_dma_transfer(sensor, buffer, length);

What It Enables

This concept enables efficient, fast, and reliable data collection from peripherals without slowing down the main program.

Real Life Example

In a weather station, sensor data like temperature and humidity can be transferred directly to memory while the CPU processes other tasks like display updates.

Key Takeaways

Manual data reading wastes CPU time and risks missing data.

Peripheral-to-memory transfer automates data movement using hardware.

This improves speed, reliability, and frees CPU for other tasks.

Practice

1. What is the main purpose of peripheral-to-memory transfer in embedded systems?

easy

A. To execute arithmetic operations on peripheral data

B. To send data from memory back to the peripheral device

C. To move data directly from a peripheral device to memory without CPU intervention

D. To reset the peripheral device after data transfer

Solution

1. Step 1: Understand peripheral-to-memory transfer

   This transfer moves data directly from a peripheral device to memory, bypassing the CPU to improve efficiency.

2. Step 2: Identify the main purpose

   The main goal is to reduce CPU load by letting hardware handle data movement.

3. Final Answer:

   To move data directly from a peripheral device to memory without CPU intervention -> Option C

4. Quick Check:

   Peripheral-to-memory transfer = direct data move without CPU [OK]

Hint: Peripheral-to-memory means data moves without CPU help [OK]

Common Mistakes:

• Confusing peripheral-to-memory with memory-to-peripheral transfer
• Thinking CPU processes data during transfer
• Assuming transfer resets the device

2. Which of the following is the correct way to enable DMA for peripheral-to-memory transfer in embedded C?

easy

A. DMA_Channel->CCR = DMA_CCR_EN << 1;

B. DMA_Channel->CCR |= DMA_CCR_EN;

C. DMA_Channel->CCR &= ~DMA_CCR_EN;

D. DMA_Channel->CCR = 0;

Solution

1. Step 1: Understand DMA enable bit setting

   To enable DMA channel, the enable bit (DMA_CCR_EN) must be set using bitwise OR to preserve other bits.

2. Step 2: Analyze options

   DMA_Channel->CCR |= DMA_CCR_EN; sets the enable bit correctly with |=. DMA_Channel->CCR = 0; clears all bits, disabling DMA. DMA_Channel->CCR &= ~DMA_CCR_EN; clears the enable bit. DMA_Channel->CCR = DMA_CCR_EN << 1; shifts enable bit incorrectly.

3. Final Answer:

   DMA_Channel->CCR |= DMA_CCR_EN; -> Option B

4. Quick Check:

   Enable DMA by setting bit with |= [OK]

Hint: Use |= to set enable bit without clearing others [OK]

Common Mistakes:

• Using = instead of |= clears other bits
• Clearing enable bit instead of setting it
• Shifting bits incorrectly

3. Given the following code snippet, what will be the value of memory_buffer[0] after DMA transfer completes?

uint8_t peripheral_data = 0x55;
uint8_t memory_buffer[1] = {0x00};
// DMA configured to transfer 1 byte from &peripheral_data to memory_buffer
// DMA transfer completes here

medium

A. 0x55

B. 0x00

C. 0xFF

D. Undefined (random value)

Solution

1. Step 1: Understand DMA transfer setup

   DMA is set to transfer 1 byte from peripheral_data (0x55) to memory_buffer.

2. Step 2: Determine memory_buffer[0] after transfer

   After transfer, memory_buffer[0] will hold the value 0x55 copied from peripheral_data.

3. Final Answer:

   0x55 -> Option A

4. Quick Check:

   DMA copies 0x55 to memory_buffer[0] [OK]

Hint: DMA copies source byte to destination buffer [OK]

Common Mistakes:

• Assuming memory_buffer stays unchanged
• Confusing peripheral_data value
• Thinking DMA sets memory to 0xFF

4. Identify the error in this DMA peripheral-to-memory transfer setup code snippet:

DMA_Channel->CPAR = (uint32_t)&memory_buffer;
DMA_Channel->CMAR = (uint32_t)&peripheral_data;
DMA_Channel->CNDTR = 4;
DMA_Channel->CCR &= ~DMA_CCR_EN;
DMA_Channel->CCR |= DMA_CCR_EN;

medium

A. The DMA channel is enabled before setting transfer length

B. The DMA channel is enabled without disabling first

C. The transfer length (CNDTR) is set after enabling DMA

D. The peripheral address and memory address are swapped

Solution

1. Step 1: Recall DMA register roles

   CPAR holds peripheral (source) address, CMAR holds memory (destination) address for peripheral-to-memory transfer.

2. Step 2: Check the code assignments

   The code sets CPAR = &memory_buffer (should be peripheral), CMAR = &peripheral_data (should be memory). Addresses are swapped.

3. Final Answer:

   The peripheral address and memory address are swapped -> Option D

4. Quick Check:

   CPAR=peripheral, CMAR=memory [OK]

Hint: Peripheral address = CPAR, Memory address = CMAR [OK]

Common Mistakes:

• Swapping peripheral and memory addresses
• Enabling DMA before setting parameters
• Not disabling DMA before re-enabling

5. You want to transfer 8 bytes from a UART peripheral data register to a memory buffer using DMA. Which sequence correctly sets up the DMA for peripheral-to-memory transfer?

hard

A. Set DMA peripheral address to UART data register; set memory address to buffer; set transfer length to 8; enable DMA channel

B. Set DMA memory address to UART data register; set peripheral address to buffer; set transfer length to 8; enable DMA channel

C. Set DMA peripheral address to buffer; set memory address to UART data register; set transfer length to 8; enable DMA channel

D. Set DMA transfer length to 8; enable DMA channel; set peripheral and memory addresses afterwards

Solution

1. Step 1: Identify source and destination

   Source is UART data register (peripheral), destination is memory buffer.

2. Step 2: Set DMA registers correctly

   Peripheral address register (CPAR) must point to UART data register, memory address register (CMAR) to buffer, transfer length to 8 bytes, then enable DMA channel.

3. Final Answer:

   Set DMA peripheral address to UART data register; set memory address to buffer; set transfer length to 8; enable DMA channel -> Option A

4. Quick Check:

   Peripheral = source, Memory = destination, length set before enable [OK]

Hint: Peripheral address = source, memory address = destination [OK]

Common Mistakes:

• Swapping peripheral and memory addresses
• Enabling DMA before setting addresses and length
• Setting transfer length after enabling DMA