Peripheral-to-memory transfer in Embedded C - Step-by-Step Execution

Concept Flow - Peripheral-to-memory transfer

Peripheral has data ready

↓

Check if DMA enabled?

No→Wait or CPU handles

Yes↓

DMA controller reads data from peripheral

↓

DMA controller writes data to memory

↓

Update transfer count

↓

Check if transfer complete?

No→Repeat DMA read/write

Yes↓

DMA signals transfer complete interrupt

↓

CPU processes data in memory

This flow shows how data moves from a peripheral device to memory using DMA, reducing CPU load.

Execution Sample

Embedded C

volatile uint8_t *PERIPH_DATA = (uint8_t*)0x4000;
uint8_t buffer[4];
for(int i=0; i<4; i++) {
  buffer[i] = *PERIPH_DATA;
}

This code reads 4 bytes from a peripheral register into a memory buffer using CPU polling.

Execution Table

Step	i	Action	PERIPH_DATA Read	buffer[i] Value
1	0	Read from peripheral	0x12	0x12
2	1	Read from peripheral	0x34	0x34
3	2	Read from peripheral	0x56	0x56
4	3	Read from peripheral	0x78	0x78
5	4	Exit loop	-	-

💡 i reaches 4, condition i<4 is false, loop ends

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
i	0	1	2	3	4	4
buffer	[_,_,_,_]	[0x12,_,_,_]	[0x12,0x34,_,_]	[0x12,0x34,0x56,_]	[0x12,0x34,0x56,0x78]	[0x12,0x34,0x56,0x78]

Key Moments - 3 Insights

Why does the loop stop when i equals 4?

Because the loop condition i<4 becomes false at step 5 in the execution_table, so the loop exits.

Is the peripheral data read the same each time?

No, each read at steps 1 to 4 shows different values (0x12, 0x34, 0x56, 0x78) as shown in the execution_table.

Why do we use a buffer array here?

The buffer stores the data read from the peripheral so the CPU can use it later without reading the peripheral again.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of buffer[2] after step 3?

A 0x34

B 0x56

C 0x78

D 0x12

💡 Hint

Check the 'buffer[i] Value' column at step 3 where i=2.

At which step does the loop condition become false and exit?

A Step 5

B Step 4

C Step 3

D Step 2

💡 Hint

Look at the 'Action' column where it says 'Exit loop'.

If the buffer size was increased to 5, how would the variable 'i' change in variable_tracker?

A It would start at 1

B It would stay at 4

C It would go up to 5 instead of 4

D It would decrease

💡 Hint

The loop runs while i < buffer size, so increasing size increases max i.

Concept Snapshot

Peripheral-to-memory transfer:
- Data moves from peripheral to memory buffer
- CPU can read peripheral directly or use DMA
- Loop reads data step-by-step
- Loop ends when count reached
- Buffer stores data for CPU use

Full Transcript

This example shows how data is transferred from a peripheral device to memory using a simple loop in embedded C. The peripheral data register is read four times, each time storing the value into a buffer array. The loop variable i starts at 0 and increments until it reaches 4, at which point the loop stops. Each step reads a different byte from the peripheral, shown in the execution table. The buffer accumulates these bytes for later CPU processing. This method uses CPU polling, but in real embedded systems, DMA can automate this transfer to reduce CPU load.