Timer interrupts with TimerOne library in Arduino - Step-by-Step Execution

Concept Flow - Timer interrupts with TimerOne library

Setup TimerOne

↓

Set Timer Period

↓

Attach Interrupt Function

↓

Timer Counts

↓

Timer Reaches Period

↓

Interrupt Triggered

↓

Interrupt Service Routine Runs

↓

Main Loop Continues

↩Back to Timer Counts

The TimerOne library sets a timer period and attaches an interrupt function that runs automatically when the timer reaches the set period, while the main program continues running.

Execution Sample

Arduino

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  Timer1.initialize(1000000); // 1 second
  Timer1.attachInterrupt(blinkLED);
}

void loop() {
  // main code runs here
}

void blinkLED() {
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
}

This code sets a 1-second timer interrupt that toggles the built-in LED every second while the main loop runs continuously.

Execution Table

Step	Timer Count (microseconds)	Condition	Action	Output
1	0	Timer started	Timer counting begins	No output
2	500000	Timer < 1000000	Continue counting	No output
3	1000000	Timer == 1000000	Interrupt triggered	blinkLED() called, LED toggled
4	0	Timer reset after interrupt	Timer counting restarts	No output
5	500000	Timer < 1000000	Continue counting	No output
6	1000000	Timer == 1000000	Interrupt triggered	blinkLED() called, LED toggled
7	...	...	...	...
Exit	N/A	Program runs indefinitely	No exit	No exit

💡 Timer interrupts run repeatedly; main loop runs forever unless stopped

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
Timer Count (microseconds)	0	500000	1000000	0	500000	1000000
LED State	OFF	OFF	ON	OFF	OFF	ON

Key Moments - 3 Insights

Why does the LED toggle even though the main loop does not change it?

Because the interrupt service routine blinkLED() runs automatically every time the timer reaches the set period, toggling the LED independently of the main loop (see execution_table steps 3 and 6).

Does the main loop stop when the interrupt runs?

No, the main loop continues running normally; the interrupt temporarily pauses the main code to run blinkLED() and then returns (see concept_flow).

Why does the timer count reset after reaching the period?

The timer automatically resets to zero after triggering the interrupt to start counting for the next period (see execution_table step 4).

Visual Quiz - 3 Questions

Look at the execution table, what is the LED state after the first interrupt triggers at step 3?

A OFF

B ON

C Blinking rapidly

D No change

💡 Hint

Check the 'LED State' row in variable_tracker after step 3

At which timer count does the interrupt trigger according to the execution table?

A 500000 microseconds

B 0 microseconds

C 1000000 microseconds

D 1500000 microseconds

💡 Hint

Look at the 'Condition' and 'Action' columns in execution_table rows 3 and 6

If the timer period is changed to 500000 microseconds, how would the execution table change?

A Interrupt triggers at 500000 microseconds instead of 1000000

B Interrupt triggers at 1000000 microseconds as before

C Timer never triggers interrupt

D LED toggles twice as slow

💡 Hint

Timer period controls when interrupt triggers; see concept_flow and execution_table

Concept Snapshot

TimerOne library lets you set a timer period in microseconds.
Attach an interrupt function that runs automatically when timer reaches that period.
The timer resets and counts again, triggering interrupts repeatedly.
Main program runs normally while interrupts toggle or update things.
Use initialize() to set period and attachInterrupt() to set ISR.

Full Transcript

This visual execution shows how the TimerOne library works on Arduino. First, the timer is set up with a period (like 1 second). Then an interrupt function is attached. The timer counts microseconds. When it reaches the set period, it triggers the interrupt. The interrupt runs the function blinkLED, which toggles the LED state. After the interrupt, the timer resets and counts again. Meanwhile, the main loop keeps running without stopping. The LED toggles every time the interrupt runs, showing how interrupts let you do things in the background automatically.