Using Interrupts Effectively in Embedded Systems

What is an Interrupt? 

An Interrupt is a signal to the processor that halts the normal execution of the code on the processor and transfers control to a special routine called an Interrupt Service Routine (ISR) in response to an event. These events can be external or internal. After servicing, the CPU resumes its previous task. This mechanism is used to avoid CPU time waste in continuously polling the hardware.

External Interrupts

• Triggered by external pins (Button presses, sensor signals)

Internal Interrupts

• Watchdog Timeouts, ADC completion

Using interrupts effectively in embedded systems is critical for optimising performance, reducing latency, and managing real-time tasks. 

Best Practices for Using Interrupts Effectively

Minimises ISR execution time

• Keep the ISR short and fast, as long ISRs can delay other interrupts or the main program, causing latency or missed events.
• Example: We can store the received data byte in a UART transmission in the main program’s buffer instead of processing the byte in the ISR.

Prioritize Interrupts

• Assign higher priorities to critical events
• Example: A timer interrupt for motor control has a higher priority compared to a UART interrupt 

Use Interrupt Flags and Buffers

• Use flags or buffers to communicate between ISRs and the main program
• Use volatile variables for flags to prevent compiler optimization issues

Avoid Blocking Operations in ISRs

• It’s a good practice to avoid delays, loops in ISRs, as they can lock up the system
• We can offload complex operations to a task in the main program.

Enable and Disable Interrupts Properly

It’s recommended to disable interrupts only when necessary, i.e. during critical sections, to prevent data corruption. 

__disable_irq();    // Disable interrupts

shared_resource ++; // Critical section

enable_irq();    // Re-enable interrupts 

Clear Interrupt Flags Properly

Always clear the interrupt flag in the ISR to prevent re-triggering. Failing to do so can cause infinite interrupt loops.