Enabling and disabling interrupts in Embedded C - Step-by-Step Execution

Concept Flow - Enabling and disabling interrupts

Start

↓

Check interrupt flag

↓

Interrupt enabled?

No→Skip interrupt handling

Yes↓

Handle interrupt

↓

Return to main program

↓

Enable or disable interrupts as needed

↓

Loop back to Start

This flow shows how the program checks if interrupts are enabled, handles them if yes, and allows enabling or disabling interrupts to control when they occur.

Execution Sample

Embedded C

sei();
// Interrupts enabled
cli();
// Interrupts disabled

This code enables interrupts with sei() and disables them with cli() in embedded C.

Execution Table

Step	Action	Interrupt Flag	Interrupts Enabled?	Result
1	Call sei()	N/A	Yes	Interrupts enabled
2	Interrupt occurs	Set	Yes	Interrupt handled
3	Call cli()	N/A	No	Interrupts disabled
4	Interrupt occurs	Set	No	Interrupt ignored
5	End of demonstration	N/A	No	Program continues without interrupts

💡 Interrupts disabled at step 3, so further interrupts are ignored.

Variable Tracker

Variable	Start	After Step 1	After Step 3	Final
Interrupts Enabled	No	Yes	No	No
Interrupt Flag	Clear	Clear	Clear	Set

Key Moments - 2 Insights

Why does the interrupt get ignored after calling cli()?

Because cli() disables interrupts, so even if the interrupt flag is set (see step 4 in execution_table), the CPU does not handle the interrupt.

Does calling sei() immediately handle interrupts?

No, calling sei() only enables interrupts. The interrupt must occur (flag set) to be handled, as shown between steps 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of 'Interrupts Enabled?' after step 3?

A Yes

B No

C Unknown

D Depends on interrupt flag

💡 Hint

Check the 'Interrupts Enabled?' column at step 3 in the execution_table.

At which step does the interrupt get handled?

A Step 3

B Step 1

C Step 2

D Step 4

💡 Hint

Look for 'Interrupt handled' in the Result column of the execution_table.

If we remove the cli() call, what would happen at step 4?

A Interrupt handled

B Interrupt ignored

C Program crashes

D No interrupt flag set

💡 Hint

Refer to the 'Interrupts Enabled?' status and Result at step 4 in the execution_table.

Concept Snapshot

Enabling and disabling interrupts in embedded C:
- Use sei() to enable interrupts.
- Use cli() to disable interrupts.
- Interrupts only handled if enabled and interrupt flag is set.
- Control interrupts to manage when CPU responds to events.

Full Transcript

This visual execution shows how interrupts are enabled and disabled in embedded C using sei() and cli(). Initially, interrupts are disabled. Calling sei() enables them, allowing the CPU to handle interrupts when they occur. When cli() is called, interrupts are disabled again, so even if an interrupt occurs, it is ignored. The execution table traces these steps, showing the interrupt flag and enabled status. Key moments clarify why interrupts are ignored after disabling and that enabling interrupts does not immediately handle them without an interrupt event. The quiz tests understanding of interrupt states at different steps.