Reading digital input pin state in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does reading a digital input pin state mean in embedded C?

It means checking if the pin is at a HIGH (1) or LOW (0) voltage level to know if a button or sensor is ON or OFF.

beginner

Which function is commonly used to read a digital input pin state in embedded C?

The function digitalRead(pin) is used to read the state of a digital input pin, returning HIGH or LOW.

beginner

Why do we need to set a pin as INPUT before reading its state?

Setting a pin as INPUT tells the microcontroller to listen to the pin's voltage level instead of sending voltage out.

beginner

What values can a digital input pin return when read?

It can return either HIGH (usually 1 or voltage level) or LOW (0 volts), representing ON or OFF states.

intermediate

How can you avoid reading a floating input pin state?

Use a pull-up or pull-down resistor to fix the pin state when no button is pressed, preventing random readings.

What does digitalRead(pin) return when the input pin is connected to ground?

A An error

B HIGH

C LOW

D Pin number

When the pin is connected to ground, the voltage is 0, so digitalRead returns LOW.

Before reading a digital input pin, what must you do?

A Set the pin as INPUT

B Nothing, just read it

C Call <code>digitalWrite</code>

D Set the pin as OUTPUT

You must set the pin as INPUT to read its state correctly.

What problem does a floating input pin cause?

A Pin reads random HIGH or LOW

B Pin always reads HIGH

C Pin always reads LOW

D Pin gets damaged

A floating pin can randomly read HIGH or LOW because it is not connected to a fixed voltage.

Which resistor helps stabilize a digital input pin state?

A Pull-down resistor

B Either pull-up or pull-down resistor

C Pull-up resistor

D No resistor needed

Both pull-up and pull-down resistors can stabilize the pin by fixing it to HIGH or LOW when not driven.

If digitalRead(pin) returns HIGH, what does it mean?

A Pin voltage is near 0V

B Pin is damaged

C Pin is disconnected

D Pin voltage is near supply voltage

HIGH means the pin voltage is close to the supply voltage, usually 3.3V or 5V.

Explain how to read a digital input pin state in embedded C and why setting pin mode is important.

Think about how the microcontroller knows if a button is pressed.

Describe what a floating input pin is and how to prevent it when reading digital inputs.

Imagine a wire not connected to anything and how it behaves.

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