Setting a specific bit in a register in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set bit 3 of the register.

Embedded C

register |= (1 << [1]);

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

A3

B4

C2

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong bit number to shift.

Forgetting to use the bitwise OR operator.

Bit 3 means shifting 1 by 3 positions to the left.

2fill in blank

medium

Complete the code to clear bit 5 of the register.

Embedded C

register &= ~([1] << 5);

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

A1

B0

Cregister

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong value instead of 1 for the mask.

Not inverting the mask before ANDing.

To clear bit 5, shift 1 by 5 and invert it with ~.

3fill in blank

hard

Fix the error in the code to toggle bit 2 of the register.

Embedded C

register [1]= (1 << 2);

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

A&

B^

C|

D~

Attempts:

3 left

💡 Hint

Common Mistakes

Using | instead of ^ to toggle bits.

Using &= which clears bits.

Toggling a bit uses the XOR operator ^.

4fill in blank

hard

Fill both blanks to set bit 4 and clear bit 1 of the register.

Embedded C

register = (register [1] (1 << 4)) [2] ~(1 << 1);

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

A|

B&

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using + or - instead of bitwise operators.

Not inverting the mask when clearing bits.

Use | to set bit 4 and & with inverted mask to clear bit 1.

5fill in blank

hard

Fill all three blanks to set bit 7, clear bit 3, and toggle bit 0 of the register.

Embedded C

register = ((register [1] (1 << 7)) [2] ~(1 << 3)) [3] (1 << 0);

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

A|

B&

C^

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up operators for set, clear, and toggle.

Forgetting to invert the mask when clearing bits.

Set bit 7 with OR, clear bit 3 with AND and inverted mask, toggle bit 0 with XOR.

Practice

1. What does the expression REGISTER |= (1 << 3); do in embedded C?

easy

A. Clears the 3rd bit (bit position 2) of REGISTER to 0

B. Sets the 4th bit (bit position 3) of REGISTER to 1 without changing other bits

C. Toggles all bits of REGISTER

D. Resets REGISTER to zero

Solution

1. Step 1: Understand bit shifting

   The expression 1 << 3 shifts the number 1 three places to the left, resulting in binary 00001000, which targets bit position 3.

2. Step 2: Apply bitwise OR to set bit

   Using |= with this mask sets bit 3 to 1 in REGISTER without changing other bits.

3. Final Answer:

   Sets the 4th bit (bit position 3) of REGISTER to 1 without changing other bits -> Option B

4. Quick Check:

   Bit shift + OR sets specific bit = A [OK]

Hint: Use |= with (1 << bit) to set that bit only [OK]

Common Mistakes:

• Confusing bit positions starting from 1 instead of 0
• Using & instead of | to set bits
• Thinking it clears bits instead of setting

2. Which of the following is the correct syntax to set bit 5 of a register named PORTA?

easy

A. PORTA = 1 << 5;

B. PORTA ^= (1 << 5);

C. PORTA &= ~(1 << 5);

D. PORTA |= (1 << 5);

Solution

1. Step 1: Identify correct operator to set bit

   To set a bit without affecting others, use bitwise OR assignment |= with a mask.

2. Step 2: Check the bit mask and syntax

   1 << 5 creates a mask for bit 5. Combining with |= sets that bit.

3. Final Answer:

   PORTA |= (1 << 5); -> Option D

4. Quick Check:

   Use |= with shifted 1 to set bit [OK]

Hint: Use |= with (1 << bit) to set bit correctly [OK]

Common Mistakes:

• Using = instead of |= overwrites all bits
• Using &= ~ to clear bits, not set
• Using ^= toggles bits instead of setting

3. Given unsigned char REG = 0x12; (binary 00010010), what is the value of REG after executing REG |= (1 << 4);?

medium

A. 0x32

B. 0x12

C. 0x16

D. 0x10

Solution

1. Step 1: Convert initial REG value to binary

   0x12 in binary is 00010010. Bit positions (0 to 7) from right to left.

2. Step 2: Calculate mask and apply OR

   1 << 4 is 00010000 (bit 4 set). OR with 00010010 results in 00110010 (0x32).

3. Final Answer:

   0x32 -> Option A

4. Quick Check:

   0x12 | 0x10 = 0x32 [OK]

Hint: Convert hex to binary, OR with mask, convert back [OK]

Common Mistakes:

• Miscounting bit positions starting from 1
• Confusing hex and decimal values
• Using &= or ^= instead of |=

4. Identify the error in the following code snippet intended to set bit 2 of REG:

unsigned char REG = 0x00;
REG = REG & (1 << 2);

medium

A. Using & instead of | clears all bits except bit 2

B. Bit shift operator << is used incorrectly

C. REG should be declared as int, not unsigned char

D. No error, code correctly sets bit 2

Solution

1. Step 1: Analyze bitwise AND usage

   The code uses & with mask (1 << 2), which clears all bits except bit 2.

2. Step 2: Understand effect on REG

   Since REG starts at 0, AND with mask results in 0, not setting bit 2. To set bit 2, OR |= should be used.

3. Final Answer:

   Using & instead of | clears all bits except bit 2 -> Option A

4. Quick Check:

   Use |= to set bits, & clears bits [OK]

Hint: Use |= to set bits, & clears bits [OK]

Common Mistakes:

• Confusing & and | operators
• Assuming &= sets bits
• Ignoring initial value of REG

5. You have a 16-bit register REG16 initially set to 0x0000. Write the correct statement to set bits 0, 7, and 15 simultaneously without affecting other bits.

hard

A. REG16 = (1 << 0) | (1 << 7) | (1 << 15);

B. REG16 = (1 << 0) & (1 << 7) & (1 << 15);

C. REG16 |= (1 << 0) | (1 << 7) | (1 << 15);

D. REG16 |= (1 << 0) & (1 << 7) & (1 << 15);

Solution

1. Step 1: Create mask for bits 0, 7, and 15

   Use bitwise OR to combine masks: (1 << 0) | (1 << 7) | (1 << 15) sets bits 0, 7, and 15.

2. Step 2: Use |= to set bits without clearing others

   Using |= applies the mask to REG16, setting those bits while preserving others.

3. Final Answer:

   REG16 |= (1 << 0) | (1 << 7) | (1 << 15); -> Option C

4. Quick Check:

   Combine masks with | and set with |= [OK]

Hint: Combine bit masks with | and set with |= [OK]

Common Mistakes:

• Using & instead of | to combine masks
• Using = overwrites all bits, losing previous data
• Using & instead of | in assignment