Practice - 5 Tasks

Answer the questions below

1fill in blank

easy

Complete the code to get the current time in milliseconds.

Arduino

unsigned long currentTime = [1]();

Drag options to blanks, or click blank then click option'

Atime

Bmillis

Cmicros

Ddelay

Attempts:

3 left

💡 Hint

Common Mistakes

Using delay() instead of millis()

Using micros() which returns microseconds, not milliseconds

✗ Incorrect

The millis() function returns the number of milliseconds since the Arduino started running.

2fill in blank

medium

Complete the code to check if 1000 milliseconds have passed since lastTime.

Arduino

if (currentTime - lastTime [1] 1000) {

Drag options to blanks, or click blank then click option'

A==

C>=

D<=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of >=

Using == which is too strict

✗ Incorrect

We check if the difference is greater than or equal to 1000 to see if 1 second passed.

3fill in blank

hard

Fix the error in the state machine update condition.

Arduino

if (state == [1] && currentTime - lastTime >= interval) {

Drag options to blanks, or click blank then click option'

Btrue

CHIGH

DON

Attempts:

3 left

💡 Hint

Common Mistakes

Using true instead of a numeric state

Using HIGH which is for pin states

✗ Incorrect

The state variable is usually an integer representing the current state, so we compare it to a number like 1.

4fill in blank

hard

Fill both blanks to update the state and reset the timer.

Arduino

state = [1];
lastTime = [2];

Drag options to blanks, or click blank then click option'

Bmillis()

Ddelay(1000)

Attempts:

3 left

💡 Hint

Common Mistakes

Using delay() instead of millis() for lastTime

Setting state to 1 instead of 2

✗ Incorrect

We set the state to 2 and reset lastTime to the current time using millis().

5fill in blank

hard

Fill all three blanks to create a timing-based state machine that toggles an LED every second.

Arduino

unsigned long [1] = 0;
int [2] = 0;
const unsigned long [3] = 1000;

Drag options to blanks, or click blank then click option'

AlastToggle

Bstate

Cinterval

DledPin

Attempts:

3 left

💡 Hint

Common Mistakes

Using ledPin for timing variables

Mixing variable names for state and time

✗ Incorrect

lastToggle stores last time LED toggled, state tracks LED on/off, and interval is the delay time.

Practice

(1/5)

What is the main purpose of using millis() in a timing-based state machine on Arduino?

easy

A. To pause the program for a fixed time

B. To reset the Arduino board

C. To track elapsed time without stopping the program

D. To read analog sensor values

Solution

Step 1: Understand what millis() does
millis() returns the number of milliseconds since the Arduino started running. It keeps counting without stopping the program.
Step 2: Connect millis() to timing-based state machines
Using millis() lets the program check how much time passed and change states without pausing or blocking other tasks.
Final Answer:
To track elapsed time without stopping the program -> Option C
Quick Check:
millis() tracks time without delay [OK]

Hint: Remember: millis() never stops your code [OK]

Common Mistakes:

Thinking millis() pauses the program
Confusing millis() with delay()
Using millis() to reset Arduino

Which of the following is the correct way to check if 1000 milliseconds have passed using millis()?

unsigned long previousMillis = 0;
unsigned long interval = 1000;

void loop() {
  unsigned long currentMillis = millis();
  // What condition checks if interval passed?
  if (__________) {
    // do something
    previousMillis = currentMillis;
  }
}

easy

A. previousMillis + currentMillis <= interval

B. previousMillis - currentMillis >= interval

C. currentMillis + previousMillis >= interval

D. currentMillis - previousMillis >= interval

Solution

Step 1: Understand elapsed time calculation
Elapsed time is current time minus previous time: currentMillis - previousMillis.
Step 2: Check if elapsed time reached interval
We compare if elapsed time is greater or equal to the interval: currentMillis - previousMillis >= interval.
Final Answer:
currentMillis - previousMillis >= interval -> Option D
Quick Check:
Elapsed time = current - previous [OK]

Hint: Subtract previous from current time to get elapsed [OK]

Common Mistakes:

Reversing subtraction order
Adding times instead of subtracting
Using <= instead of >=

What will be the output of this Arduino code snippet that uses a timing-based state machine?

unsigned long previousMillis = 0;
const long interval = 2000;
int ledState = LOW;

void setup() {
  pinMode(13, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    if (ledState == LOW) {
      ledState = HIGH;
    } else {
      ledState = LOW;
    }
    digitalWrite(13, ledState);
    Serial.println(ledState);
  }
}

medium

A. Prints alternating 0 and 1 every 2 seconds

B. Prints 1 continuously every 2 seconds

C. Prints 0 continuously every 2 seconds

D. No output because of syntax error

Solution

Step 1: Understand the timing and state toggle
Every 2000 ms, the code toggles ledState between LOW (0) and HIGH (1).
Step 2: Check output printed
Each toggle prints the current ledState (0 or 1) to Serial, alternating every 2 seconds.
Final Answer:
Prints alternating 0 and 1 every 2 seconds -> Option A
Quick Check:
State toggles and prints 0,1 alternately [OK]

Hint: Toggle state and print inside timed if-block [OK]

Common Mistakes:

Assuming constant output without toggle
Confusing HIGH/LOW with 1/0
Missing update of previousMillis

Identify the bug in this timing-based state machine code and choose the fix.

unsigned long previousMillis = 0;
const long interval = 1000;
int ledState = LOW;

void setup() {
  pinMode(13, OUTPUT);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis > interval) {
    ledState = !ledState;
    digitalWrite(13, ledState);
  }
}

medium

A. Add previousMillis = currentMillis; inside the if-block

B. Change int ledState to bool ledState

C. Replace ! with ~ in toggle

D. Remove the if condition to toggle every loop

Solution

Step 1: Check timing update logic
The code never updates previousMillis, so the condition stays true forever after first pass.
Step 2: Fix by updating previousMillis
Adding previousMillis = currentMillis; inside the if-block resets the timer for the next interval.
Final Answer:
Add previousMillis = currentMillis; inside the if-block -> Option A
Quick Check:
Update previousMillis to reset timer [OK]

Hint: Always update previousMillis after interval check [OK]

Common Mistakes:

Forgetting to update previousMillis
Using bitwise NOT (~) instead of logical NOT (!)
Removing timing check causes fast toggling

You want to create a state machine that cycles through three LED states: OFF, RED, GREEN. Each state lasts 3 seconds. Which code snippet correctly implements this using millis()?

unsigned long previousMillis = 0;
const long interval = 3000;
int state = 0;

void setup() {
  pinMode(RED_PIN, OUTPUT);
  pinMode(GREEN_PIN, OUTPUT);
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - previousMillis >= interval) {
    previousMillis = currentMillis;
    state = (state + 1) % 3;
    switch(state) {
      case 0:
        digitalWrite(RED_PIN, LOW);
        digitalWrite(GREEN_PIN, LOW);
        break;
      case 1:
        digitalWrite(RED_PIN, HIGH);
        digitalWrite(GREEN_PIN, LOW);
        break;
      case 2:
        digitalWrite(RED_PIN, LOW);
        digitalWrite(GREEN_PIN, HIGH);
        break;
    }
  }
}

hard

A. Does not change states due to missing update

B. Correctly cycles OFF, RED, GREEN every 3 seconds

C. Cycles states every 1 second instead of 3

D. Cycles only RED and GREEN, skipping OFF

Solution

Step 1: Check timing and state update
The code uses millis() to check 3 seconds passed, then updates state cycling 0,1,2 with modulo 3.
Step 2: Verify LED outputs per state
State 0 turns both LEDs off, 1 turns RED on, 2 turns GREEN on. This matches the required cycle.
Final Answer:
Correctly cycles OFF, RED, GREEN every 3 seconds -> Option B
Quick Check:
State cycles with modulo and timing [OK]

Hint: Use modulo (%) to cycle states smoothly [OK]

Common Mistakes:

Forgetting to update previousMillis
Incorrect modulo causing wrong cycles
Not turning off LEDs in OFF state

Timing-based state machines in Arduino - Interactive Code Practice

Start learning this pattern below

Practice

Solution

Step 1: Understand what `millis()` does

Step 2: Connect `millis()` to timing-based state machines

Final Answer:

Quick Check:

Solution

Step 1: Understand elapsed time calculation

Step 2: Check if elapsed time reached interval

Final Answer:

Quick Check:

Solution

Step 1: Understand the timing and state toggle

Step 2: Check output printed

Final Answer:

Quick Check:

Solution

Step 1: Check timing update logic

Step 2: Fix by updating `previousMillis`

Final Answer:

Quick Check:

Solution

Step 1: Check timing and state update

Step 2: Verify LED outputs per state

Final Answer:

Quick Check:

Start learning this pattern below

Practice

Solution

Step 1: Understand what millis() does

Step 2: Connect millis() to timing-based state machines

Final Answer:

Quick Check:

Solution

Step 1: Understand elapsed time calculation

Step 2: Check if elapsed time reached interval

Final Answer:

Quick Check:

Solution

Step 1: Understand the timing and state toggle

Step 2: Check output printed

Final Answer:

Quick Check:

Solution

Step 1: Check timing update logic

Step 2: Fix by updating previousMillis

Final Answer:

Quick Check:

Solution

Step 1: Check timing and state update

Step 2: Verify LED outputs per state

Final Answer:

Quick Check:

Step 1: Understand what `millis()` does

Step 2: Connect `millis()` to timing-based state machines

Step 2: Fix by updating `previousMillis`