digitalWrite() for output control in Arduino - Time & Space Complexity

Time Complexity: digitalWrite() for output control

O(n)

Understanding Time Complexity

We want to understand how the time to run a program changes when using digitalWrite() to control outputs.

Specifically, how does the number of digitalWrite() calls affect the program's speed?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

for (int i = 0; i < n; i++) {
  digitalWrite(LED_PIN, HIGH);
  delay(1);
  digitalWrite(LED_PIN, LOW);
  delay(1);
}
    

This code turns an LED on and off n times with a small delay each time.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop running digitalWrite() twice per cycle.
• How many times: The loop runs n times, so digitalWrite() is called 2n times.

How Execution Grows With Input

As n grows, the number of digitalWrite() calls grows directly with n.

Input Size (n)	Approx. Operations
10	20 digitalWrite() calls
100	200 digitalWrite() calls
1000	2000 digitalWrite() calls

Pattern observation: The number of operations grows in a straight line as n increases.

Final Time Complexity

Time Complexity: O(n)

This means the time to run the code grows directly in proportion to how many times the loop runs.

Common Mistake

[X] Wrong: "digitalWrite() runs instantly, so the loop time does not depend on n."

[OK] Correct: Each digitalWrite() takes some time, and since it runs inside the loop n times, total time grows with n.

Interview Connect

Understanding how loops and function calls like digitalWrite() affect time helps you write efficient Arduino code and explain your reasoning clearly.

Self-Check

"What if we removed the delay() calls inside the loop? How would the time complexity change?"

Practice

1. What does the digitalWrite() function do in Arduino programming?

easy

A. It sets the analog value of a pin.

B. It reads the voltage from a digital pin.

C. It initializes the serial communication.

D. It sets a digital pin to HIGH or LOW voltage.

Solution

1. Step 1: Understand the purpose of digitalWrite()

   The function digitalWrite() is used to control the voltage level on a digital pin, setting it either HIGH (on) or LOW (off). It does not read values (that's digitalRead()), nor does it set analog values or initialize serial communication.

2. Final Answer:

   It sets a digital pin to HIGH or LOW voltage. -> Option D

3. Quick Check:

   digitalWrite() controls pin voltage = D [OK]

Hint: digitalWrite sets pin ON or OFF voltage [OK]

Common Mistakes:

• Confusing digitalWrite() with digitalRead()
• Thinking digitalWrite() reads pin values
• Mixing digitalWrite() with analogWrite()

2. Which of the following is the correct syntax to turn on an LED connected to pin 13 using digitalWrite()?

easy

A. digitalWrite(13, HIGH);

B. digitalWrite(13, LOW);

C. digitalWrite(13, ON);

D. digitalWrite(13, TRUE);

Solution

1. Step 1: Recall correct constants and check options

   digitalWrite() uses HIGH and LOW constants. To turn on, use HIGH. A uses ON (not valid), B uses LOW (turns off), D uses TRUE (not valid). Only C: digitalWrite(13, HIGH); is correct.

2. Final Answer:

   digitalWrite(13, HIGH); -> Option A

3. Quick Check:

   Use HIGH or LOW with digitalWrite() = C [OK]

Hint: Use HIGH or LOW, not ON or TRUE [OK]

Common Mistakes:

• Using ON or TRUE instead of HIGH
• Using LOW instead of HIGH
• Forgetting to set pinMode to OUTPUT first

3. What will be the output on pin 8 after running this code snippet?

pinMode(8, OUTPUT);
digitalWrite(8, LOW);
digitalWrite(8, HIGH);
digitalWrite(8, LOW);

medium

A. Pin 8 will toggle rapidly.

B. Pin 8 will be HIGH (on).

C. Pin 8 will be LOW (off).

D. Pin 8 will cause a syntax error.

Solution

1. Step 1: Trace the digitalWrite() calls

   pinMode sets OUTPUT, then LOW, HIGH, LOW. Final LOW so pin off.

2. Final Answer:

   Pin 8 will be LOW (off). -> Option C

3. Quick Check:

   Last digitalWrite sets pin LOW = A [OK]

Hint: Last digitalWrite() sets pin state [OK]

Common Mistakes:

• Assuming pin toggles automatically
• Confusing initial and final pin states
• Thinking multiple digitalWrite calls cause errors

4. Identify the error in this Arduino code snippet:

digitalWrite(12, HIGH);
pinMode(12, OUTPUT);

medium

A. No error, code runs fine.

B. pinMode() must be called before digitalWrite().

C. digitalWrite() cannot use pin 12.

D. HIGH is not a valid value for digitalWrite().

Solution

1. Step 1: Check order of pinMode() and digitalWrite()

   pinMode() must set OUTPUT before digitalWrite(), otherwise unexpected behavior. Here digitalWrite first.

2. Final Answer:

   pinMode() must be called before digitalWrite(). -> Option B

3. Quick Check:

   Set pinMode before digitalWrite = A [OK]

Hint: Always set pinMode(OUTPUT) before digitalWrite() [OK]

Common Mistakes:

• Calling digitalWrite before pinMode
• Assuming pinMode defaults to OUTPUT
• Using invalid pin numbers

5. You want to blink an LED connected to pin 9 on and off every second. Which code snippet correctly uses digitalWrite() inside the loop() function to achieve this?

hard

A. digitalWrite(9, HIGH);
delay(1000);
digitalWrite(9, LOW);
delay(1000);

B. digitalWrite(9, HIGH);
digitalWrite(9, LOW);
delay(1000);

C. pinMode(9, OUTPUT);
digitalWrite(9, HIGH);
delay(1000);

D. digitalWrite(9, HIGH);
delay(500);
digitalWrite(9, HIGH);
delay(500);

Solution

1. Step 1: Understand blinking and analyze options

   Blink: HIGH, delay(1000), LOW, delay(1000). A: HIGH immediate LOW, no on time. C: pinMode in loop inefficient, no LOW. D: HIGH twice, always on. Only B correct.

2. Final Answer:

   digitalWrite(9, HIGH); delay(1000); digitalWrite(9, LOW); delay(1000); -> Option A

3. Quick Check:

   Turn ON, delay, turn OFF, delay = B [OK]

Hint: Turn ON, delay, turn OFF, delay for blinking [OK]

Common Mistakes:

• Missing delay between ON and OFF
• Not turning LED OFF after ON
• Setting pinMode inside loop repeatedly