Pull-up and pull-down resistor configuration in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable the pull-up resistor on the input pin.

Embedded C

GPIO_PORT->PULLUP_EN = [1];

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

A0x10

B0xFF

C0x00

D0x01

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0x00 disables the resistor.

Using 0xFF enables all pins, not just the input pin.

The pull-up resistor is enabled by setting the correct bit mask. Here, 0x10 corresponds to the input pin.

2fill in blank

medium

Complete the code to configure the pin as input with pull-down resistor enabled.

Embedded C

GPIO_PORT->DIR &= ~[1];  // Set pin as input
GPIO_PORT->PULLDOWN_EN = [1];

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

A0x04

B0x08

C0x02

D0x01

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the pin as output by mistake.

Using different masks for DIR and PULLDOWN_EN.

The pin mask 0x04 is used to clear the direction bit (input) and enable the pull-down resistor on that pin.

3fill in blank

hard

Fix the error in the code to correctly enable pull-up resistor on pin 5.

Embedded C

GPIO_PORT->PULLUP_EN |= [1];  // Enable pull-up on pin 5

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

A0x10

B0x40

C0x08

D0x20

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0x10 which is bit 4.

Using 0x40 which is bit 6.

Pin 5 corresponds to bit 5, which is 0x20 in hexadecimal.

4fill in blank

hard

Fill both blanks to configure pin 3 as input.

Embedded C

GPIO_PORT->DIR [1]= [2]0x08;  // Configure pin 3 as input

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

A&

B|

C~

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using OR (|) instead of AND (&).

Not inverting the pin mask (~).

To set pin 3 as input, clear bit 3 using bitwise AND with the complement (~) of the pin mask.

5fill in blank

hard

Fill all three blanks to create a dictionary mapping pin numbers to their pull-up status (1 for enabled, 0 for disabled) for pins 1, 2, and 3.

Embedded C

int pullup_status[] = { [1], [2], [3] };

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

AGPIO_PORT->PULLUP_EN & 0x02 ? 1 : 0

BGPIO_PORT->PULLUP_EN & 0x04 ? 1 : 0

CGPIO_PORT->PULLUP_EN & 0x08 ? 1 : 0

DGPIO_PORT->PULLUP_EN & 0x10 ? 1 : 0

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong bit masks for pins.

Not using ternary operator for boolean to int conversion.

Each expression checks if the pull-up resistor is enabled on pins 1, 2, and 3 respectively by masking the correct bit.

Practice

1. What is the main purpose of a pull-up resistor in embedded C microcontroller input pins?

easy

A. To set the input pin to a default LOW voltage level

B. To set the input pin to a default HIGH voltage level

C. To increase the input pin's voltage beyond supply voltage

D. To disconnect the input pin from the circuit

Solution

1. Step 1: Understand pull-up resistor function

   A pull-up resistor connects the input pin to a positive voltage (usually Vcc) to ensure it reads HIGH when not driven.

2. Step 2: Compare pull-up and pull-down roles

   Pull-down resistors connect to ground to set LOW by default, so pull-up sets HIGH by default.

3. Final Answer:

   To set the input pin to a default HIGH voltage level -> Option B

4. Quick Check:

   Pull-up resistor = default HIGH [OK]

Hint: Pull-up means default HIGH, pull-down means default LOW [OK]

Common Mistakes:

• Confusing pull-up with pull-down resistor function
• Thinking pull-up increases voltage beyond supply
• Assuming pull-up disconnects the pin

2. Which of the following is the correct way to enable an internal pull-up resistor on a microcontroller input pin in embedded C?

easy

A. Set the pin as output and write HIGH

B. Configure the pin as input and set the pull-down bit in the port register

C. Set the pin as output and write LOW

D. Configure the pin as input and set the pull-up bit in the port register

Solution

1. Step 1: Identify pin direction for input

   To use pull-up, the pin must be configured as input to read signals.

2. Step 2: Enable internal pull-up resistor

   Setting the pull-up bit in the port register activates the internal pull-up resistor for that input pin.

3. Final Answer:

   Configure the pin as input and set the pull-up bit in the port register -> Option D

4. Quick Check:

   Input + pull-up bit set = internal pull-up enabled [OK]

Hint: Input pin + pull-up bit = pull-up enabled [OK]

Common Mistakes:

• Setting pin as output instead of input
• Confusing pull-up bit with pull-down bit
• Not enabling pull-up resistor in code

3. Consider the following embedded C code snippet configuring a microcontroller pin:

DDRB &= ~(1 << 2);  // Set pin 2 of PORTB as input
PORTB |= (1 << 2);  // Enable pull-up resistor on pin 2

if (PINB & (1 << 2)) {
    // Pin reads HIGH
} else {
    // Pin reads LOW
}

What will be the output if the pin is not connected to anything externally?

medium

A. The pin will read LOW due to floating input

B. The pin will read an undefined random value

C. The pin will read HIGH because pull-up resistor sets default HIGH

D. The pin will cause a short circuit

Solution

1. Step 1: Analyze pin configuration

   Pin 2 is set as input and internal pull-up resistor is enabled, connecting it internally to Vcc through resistor.

2. Step 2: Determine pin reading when floating

   Since no external connection pulls it LOW, the pull-up resistor keeps the pin voltage HIGH, so the condition reads true.

3. Final Answer:

   The pin will read HIGH because pull-up resistor sets default HIGH -> Option C

4. Quick Check:

   Pull-up enabled + floating pin = reads HIGH [OK]

Hint: Pull-up resistor keeps floating input HIGH [OK]

Common Mistakes:

• Assuming floating input reads LOW
• Thinking floating input causes short circuit
• Ignoring pull-up resistor effect

4. The following code is intended to enable a pull-down resistor on pin 5 of PORTD, but it does not work as expected:

DDRD &= ~(1 << 5);  // Set pin 5 as input
PORTD |= (1 << 5);  // Enable pull-down resistor on pin 5

What is the error in this code?

medium

A. Pull-down resistors are not enabled by setting PORTD bit HIGH; this enables pull-up instead

B. Pin 5 is set as output instead of input

C. The bit shift operator is used incorrectly

D. Pull-down resistors are enabled by clearing the PORTD bit, not setting it

Solution

1. Step 1: Review pull-up/down enabling method

   Setting PORTD bit HIGH with pin as input enables internal pull-up resistor, not pull-down.

2. Step 2: Understand pull-down resistor hardware

   Most microcontrollers do not have internal pull-down resistors; external resistors are needed, so setting PORTD bit cannot enable pull-down.

3. Final Answer:

   Pull-down resistors are not enabled by setting PORTD bit HIGH; this enables pull-up instead -> Option A

4. Quick Check:

   PORT bit HIGH + input = pull-up, not pull-down [OK]

Hint: Setting PORT bit HIGH enables pull-up, not pull-down [OK]

Common Mistakes:

• Assuming internal pull-down resistor exists
• Confusing pull-up and pull-down enabling bits
• Setting pin as output by mistake

5. You want to read a button input on pin 3 of PORTC with a default HIGH state to avoid floating. The microcontroller only supports internal pull-up resistors. Which is the best way to configure the pin and button wiring to achieve this?

hard

A. Configure pin 3 as input with internal pull-up enabled; connect button to ground to pull pin LOW when pressed

B. Configure pin 3 as input without pull-up; connect button to Vcc to pull pin HIGH when pressed

C. Configure pin 3 as output and write LOW; connect button to Vcc

D. Configure pin 3 as input with internal pull-up enabled; connect button to Vcc

Solution

1. Step 1: Understand internal pull-up limitation

   Since only pull-up resistors are available internally, enable pull-up to default pin HIGH.

2. Step 2: Wire button to ground

   Connect button to ground so pressing it pulls pin LOW, overriding pull-up and avoiding floating.

3. Step 3: Confirm logic

   Pin reads HIGH when button not pressed (due to pull-up), LOW when pressed (button connects to ground).

4. Final Answer:

   Configure pin 3 as input with internal pull-up enabled; connect button to ground to pull pin LOW when pressed -> Option A

5. Quick Check:

   Pull-up enabled + button to ground = default HIGH, pressed LOW [OK]

Hint: Pull-up + button to ground = default HIGH [OK]

Common Mistakes:

• Trying to enable internal pull-down resistor which doesn't exist
• Connecting button to Vcc with pull-up enabled causing always HIGH
• Setting pin as output instead of input