Rising, falling, and change triggers in Arduino - Step-by-Step Execution

Concept Flow - Rising, falling, and change triggers

Start

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Read Pin State

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Compare Current and Previous State

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If Rising Edge?

Yes→Trigger Rising Event

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If Falling Edge?

Yes→Trigger Falling Event

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If Change?

Yes→Trigger Change Event

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Update Previous State

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Repeat

The program repeatedly reads a pin's state, compares it to the previous state, and triggers events on rising, falling, or any change.

Execution Sample

Arduino

const int buttonPin = 2;
int lastButtonState = LOW;

void loop() {
  int reading = digitalRead(buttonPin);

This code reads a button pin state and compares it to the last state to detect rising, falling, or any change.

Execution Table

Step	reading	lastButtonState	Condition	Action	Output
1	LOW	LOW	reading != lastButtonState? LOW != LOW	No action	None
2	HIGH	LOW	reading != lastButtonState? HIGH != LOW	Rising edge detected	Print 'Rising edge'
3	HIGH	HIGH	reading != lastButtonState? HIGH != HIGH	No action	None
4	LOW	HIGH	reading != lastButtonState? LOW != HIGH	Falling edge detected	Print 'Falling edge'
5	LOW	LOW	reading != lastButtonState? LOW != LOW	No action	None
6	HIGH	LOW	reading != lastButtonState? HIGH != LOW	Rising edge detected	Print 'Rising edge'
7	LOW	HIGH	reading != lastButtonState? LOW != HIGH	Falling edge detected	Print 'Falling edge'
8	LOW	LOW	reading != lastButtonState? LOW != LOW	No action	None
Exit	-	-	Loop continues indefinitely	Program runs continuously	-

💡 The loop runs forever, checking pin state changes continuously.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8
reading	LOW	LOW	HIGH	HIGH	LOW	LOW	HIGH	LOW	LOW
lastButtonState	LOW	LOW	HIGH	HIGH	LOW	LOW	HIGH	LOW	LOW

Key Moments - 3 Insights

Why does the program detect a rising edge only when reading changes from LOW to HIGH?

Because the condition reading != lastButtonState is true only when the current reading differs from the last state. The rising edge is specifically when reading changes from LOW to HIGH, as shown in execution_table rows 2 and 6.

What happens if the reading stays the same as lastButtonState?

No event triggers because the condition reading != lastButtonState is false. This is shown in rows 1, 3, 5, and 8 where no action occurs.

How does the program detect a falling edge?

A falling edge is detected when reading changes from HIGH to LOW, which makes reading != lastButtonState true and reading is LOW. This is shown in rows 4 and 7 where 'Falling edge detected' is printed.

Visual Quiz - 3 Questions

Look at the execution_table at step 2. What is the value of lastButtonState?

A UNKNOWN

B HIGH

C LOW

D LOW but changed to HIGH

💡 Hint

Check the 'lastButtonState' column in row 2 of the execution_table.

At which step does the program detect a falling edge for the first time?

A Step 4

B Step 2

C Step 6

D Step 7

💡 Hint

Look for 'Falling edge detected' in the 'Action' column of the execution_table.

If the reading never changes from LOW, what will the output be according to the execution_table?

A Multiple falling edges

B No output

C Multiple rising edges

D Continuous change events

💡 Hint

Check rows where reading equals lastButtonState and see the 'Output' column.

Concept Snapshot

Rising, falling, and change triggers detect pin state changes.
Compare current reading with previous state.
Rising edge: LOW to HIGH transition.
Falling edge: HIGH to LOW transition.
Change: any difference triggers event.
Update previous state after each check.

Full Transcript

This visual execution shows how an Arduino program detects rising, falling, and change triggers on a digital input pin. The program reads the pin state repeatedly and compares it to the previous state. When the reading changes from LOW to HIGH, it triggers a rising edge event. When it changes from HIGH to LOW, it triggers a falling edge event. If the reading stays the same, no event triggers. The previous state is updated after each check to track changes. The execution table traces each step, showing variable values, conditions, and actions. This helps beginners understand how edge detection works in Arduino programming.