Why Reading digital input pin state in Embedded C? - Purpose & Use Cases

The Big Idea

What if your device could instantly know when you press a button, every single time?

The Scenario

Imagine you want to check if a button is pressed on your device. Without reading the digital input pin state, you'd have to guess or rely on slow, manual checks that don't update quickly.

The Problem

Manually guessing or using delays to check button presses can cause your program to miss quick presses or react too late. It's slow and can make your device feel unresponsive or buggy.

The Solution

Reading the digital input pin state lets your program instantly know if a button or sensor is ON or OFF. This makes your device react fast and correctly every time.

Before vs After

✗ Before

delay(1000);
if (buttonPressed) {
  // do something
}

✓ After

if (digitalRead(buttonPin) == HIGH) {
  // do something
}

What It Enables

This lets your device respond immediately to user actions or sensor changes, making it smart and interactive.

Real Life Example

Think of a doorbell button: reading the digital input pin state lets your system ring the bell the moment someone presses it, without delay.

Key Takeaways

Manual checks are slow and unreliable.

Reading digital input pins gives instant, accurate state.

It makes devices responsive and user-friendly.

Practice

1. What does the function digitalRead(pin) do in embedded C programming?

easy

A. Configures the pin as an analog input.

B. Sets the digital pin to output mode.

C. Reads the current state (HIGH or LOW) of a digital input pin.

D. Writes a HIGH signal to a digital pin.

Solution

1. Step 1: Understand the purpose of digitalRead

   The function digitalRead(pin) is used to check the current electrical state of a digital input pin, which can be either HIGH (on) or LOW (off).

2. Step 2: Differentiate from other pin functions

   Unlike functions that set pin modes or write signals, digitalRead only reads the pin's state without changing it.

3. Final Answer:

   Reads the current state (HIGH or LOW) of a digital input pin. -> Option C

4. Quick Check:

   digitalRead reads pin state = D [OK]

Hint: digitalRead always reads input pin state, not sets it [OK]

Common Mistakes:

• Confusing digitalRead with pinMode or digitalWrite
• Thinking digitalRead sets pin mode
• Assuming digitalRead writes output signals

2. Which of the following is the correct way to set a pin as input before reading its state?

easy

A. pinMode(pin, OUTPUT);

B. pinMode(pin, INPUT);

C. digitalRead(pin);

D. digitalWrite(pin, HIGH);

Solution

1. Step 1: Identify the function to set pin mode

   pinMode(pin, mode) is used to configure a pin as INPUT or OUTPUT.

2. Step 2: Choose the correct mode for reading input

   To read a digital input pin, it must be set as INPUT using pinMode(pin, INPUT).

3. Final Answer:

   pinMode(pin, INPUT); -> Option B

4. Quick Check:

   Set pin as INPUT before reading = A [OK]

Hint: Use pinMode(pin, INPUT) before digitalRead [OK]

Common Mistakes:

• Setting pin as OUTPUT before reading input
• Calling digitalRead before setting pin mode
• Using digitalWrite to set pin mode

3. What will be the output of the following code snippet if the button connected to pin 7 is pressed (assuming pressed means HIGH)?

pinMode(7, INPUT);
int buttonState = digitalRead(7);
if (buttonState == HIGH) {
    printf("Button is pressed\n");
} else {
    printf("Button is not pressed\n");
}

medium

A. Button is pressed

B. Button is not pressed

C. Compilation error

D. No output

Solution

1. Step 1: Analyze pin mode and reading

   The pin 7 is set as INPUT, so digitalRead(7) correctly reads the pin state.

2. Step 2: Check condition for pressed button

   If the button is pressed, the pin reads HIGH, so the if condition is true and prints "Button is pressed".

3. Final Answer:

   Button is pressed -> Option A

4. Quick Check:

   Pressed button = HIGH = prints pressed [OK]

Hint: Pressed button means digitalRead returns HIGH [OK]

Common Mistakes:

• Forgetting to set pinMode before reading
• Confusing HIGH and LOW states
• Assuming digitalRead returns boolean true/false

4. Identify the error in the following code that reads a digital input pin state:

int pin = 5;
int state;
state = digitalRead();
if(state == HIGH) {
    // do something
}

medium

A. Pin should be set as OUTPUT before reading.

B. The if condition syntax is incorrect.

C. digitalRead returns a float, not int.

D. Missing pin number argument in digitalRead function.

Solution

1. Step 1: Check digitalRead usage

   digitalRead requires the pin number as an argument, but here it is called without any argument.

2. Step 2: Verify other parts of code

   Pin mode setting is missing but not the main error asked; the if condition syntax is correct.

3. Final Answer:

   Missing pin number argument in digitalRead function. -> Option D

4. Quick Check:

   digitalRead needs pin argument = B [OK]

Hint: digitalRead always needs pin number inside parentheses [OK]

Common Mistakes:

• Calling digitalRead without pin argument
• Setting pin as OUTPUT before reading input
• Misunderstanding return type of digitalRead

5. You want to detect if a switch connected to pin 3 is ON and turn on an LED connected to pin 13 accordingly. Which code snippet correctly reads the switch state and controls the LED?

hard

A. pinMode(3, INPUT);
pinMode(13, OUTPUT);
if (digitalRead(3) == HIGH) {
digitalWrite(13, HIGH);
} else {
digitalWrite(13, LOW);
}

B. pinMode(3, OUTPUT);
pinMode(13, INPUT);
if (digitalRead(13) == HIGH) {
digitalWrite(3, HIGH);
}

C. pinMode(3, INPUT);
if (digitalRead(13) == HIGH) {
digitalWrite(3, HIGH);
}

D. pinMode(13, OUTPUT);
digitalWrite(3, HIGH);
if (digitalRead(3) == HIGH) {
digitalWrite(13, HIGH);
}

Solution

1. Step 1: Set correct pin modes

   Pin 3 is input for the switch, so pinMode(3, INPUT). Pin 13 is output for the LED, so pinMode(13, OUTPUT).

2. Step 2: Read switch and control LED

   If digitalRead(3) is HIGH (switch ON), turn LED on with digitalWrite(13, HIGH), else turn it off with digitalWrite(13, LOW).

3. Final Answer:

   pinMode(3, INPUT); pinMode(13, OUTPUT); if (digitalRead(3) == HIGH) { digitalWrite(13, HIGH); } else { digitalWrite(13, LOW); } -> Option A

4. Quick Check:

   Input pin read controls output pin write = A [OK]

Hint: Input pin set INPUT, output pin set OUTPUT, read then write [OK]

Common Mistakes:

• Setting input pin as OUTPUT or vice versa
• Reading wrong pin for switch state
• Not controlling LED pin properly