UART interrupt-driven communication in Embedded C - Step-by-Step Execution

Concept Flow - UART interrupt-driven communication

UART Initialized

↓

Enable UART Interrupt

↓

Wait for Data

↓

Interrupt Occurs

↓

Interrupt Service Routine (ISR)

↓

Read Data

↓

Process/Store Data

↓

Clear Interrupt Flag

↓

Return to Main Program

↓

Repeat Waiting for Data

The UART is set up and interrupts enabled. When data arrives, an interrupt triggers the ISR, which reads and processes the data, then returns control to the main program.

Execution Sample

Embedded C

void UART_ISR(void) {
    if (UART_Receive_Interrupt_Flag) {
        char data = UART_Read();
        buffer[index++] = data;
        UART_Clear_Interrupt_Flag();
    }
}

This interrupt service routine runs when UART data is received, reads the data, stores it in a buffer, and clears the interrupt flag.

Execution Table

Step	Interrupt Flag	Action	Data Read	Buffer State	Interrupt Flag Cleared
1	Set (Data received)	Enter ISR	-	[]	No
2	Set	Read UART Data	'A'	[]	No
3	Set	Store Data in Buffer	-	['A']	No
4	Set	Clear Interrupt Flag	-	['A']	Yes
5	Cleared	Exit ISR	-	['A']	Yes
6	Cleared	Wait for next data	-	['A']	Yes

💡 Interrupt flag cleared, ISR ends, main program resumes waiting for new data.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
UART_Receive_Interrupt_Flag	Set	Set	Set	Cleared	Cleared
data	N/A	'A'	'A'	'A'	'A'
buffer	[]	[]	['A']	['A']	['A']
index	0	1	1	1	1

Key Moments - 3 Insights

Why do we need to clear the interrupt flag inside the ISR?

If the interrupt flag is not cleared (see step 4 in execution_table), the ISR will be called repeatedly for the same event, causing an infinite loop.

What happens if the buffer index is not incremented after storing data?

If index is not incremented (step 3), new data will overwrite the previous data in the buffer, losing information.

Why does the main program wait after the ISR finishes?

After ISR ends (step 5 and 6), the main program waits for the next interrupt, allowing the CPU to do other tasks until new data arrives.

Visual Quiz - 3 Questions

Look at the execution_table, what is the buffer state after step 3?

A ['B']

B []

C ['A']

D ['A', 'B']

💡 Hint

Check the 'Buffer State' column at step 3 in the execution_table.

At which step is the interrupt flag cleared?

A Step 4

B Step 2

C Step 5

D Step 1

💡 Hint

Look at the 'Interrupt Flag Cleared' column in the execution_table.

If the interrupt flag is not cleared, what will happen according to the execution flow?

A ISR will not be called again

B ISR will be called repeatedly for the same data

C Data will be lost immediately

D Main program will stop

💡 Hint

Refer to key_moments explanation about clearing the interrupt flag.

Concept Snapshot

UART interrupt-driven communication:
- Initialize UART and enable interrupts
- When data arrives, interrupt triggers ISR
- ISR reads data, stores it, clears interrupt flag
- Control returns to main program
- Main program waits for next interrupt
Key: Always clear interrupt flag inside ISR to avoid repeated calls.

Full Transcript

This visual execution trace shows how UART interrupt-driven communication works in embedded C. The UART is initialized and its interrupt enabled. When data arrives, the UART sets an interrupt flag, triggering the interrupt service routine (ISR). Inside the ISR, the program reads the incoming data, stores it in a buffer, increments the buffer index, and clears the interrupt flag to prevent repeated interrupts. After clearing the flag, the ISR exits, returning control to the main program, which waits for the next data. The variable tracker shows how the interrupt flag, data, buffer, and index change step-by-step. Key moments clarify why clearing the interrupt flag is essential and why the buffer index must be incremented. The quiz questions help reinforce understanding by referencing the execution table and variable states.