Baud rate configuration in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set the baud rate register to 9600 baud assuming F_CPU is 16MHz.

Embedded C

UBRR0H = ([1] >> 8);

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

A51

B207

C103

D16

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong UBRR value for 9600 baud at 16MHz.

The value 103 sets the baud rate to 9600 for 16MHz clock using the formula: UBRR = (F_CPU / (16 * Baud)) - 1.

2fill in blank

medium

Complete the code to enable the transmitter in the UART control register.

Embedded C

UCSR0B = (1 << [1]);

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

ARXEN0

BTXCIE0

CRXCIE0

DTXEN0

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing receiver enable (RXEN0) with transmitter enable.

TXEN0 bit enables the UART transmitter.

3fill in blank

hard

Fix the error in the code to set the baud rate low byte correctly.

Embedded C

UBRR0L = [1] & 0xFF;

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

AUBRR0L

BUBRR_VALUE

CUBRR0H

DUBRR

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong register or variable for the low byte.

The low byte of the baud rate register should be set to the lower 8 bits of the calculated UBRR_VALUE.

4fill in blank

hard

Fill both blanks to configure UART for 8 data bits and no parity.

Embedded C

UCSR0C = (1 << [1]) | (1 << [2]);

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

AUCSZ01

BUCSZ02

CUCSZ00

DUPM01

Attempts:

3 left

💡 Hint

Common Mistakes

Setting parity bits instead of data size bits.

Setting UCSZ01 and UCSZ00 bits configures 8 data bits. No parity bits are set.

5fill in blank

hard

Fill all three blanks to calculate and set the baud rate registers correctly.

Embedded C

uint16_t [1] = (F_CPU / (16 * [2])) - 1;
UBRR0H = ([1] >> 8);
UBRR0L = [1] & 0xFF;

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

Aubrr_value

Bbaud_rate

Dbaud

Attempts:

3 left

💡 Hint

Common Mistakes

Using inconsistent variable names for baud rate and UBRR value.

The variable 'ubrr_value' stores the calculated baud rate register value. 'baud' is the desired baud rate.

Practice

1. What does the baud rate setting control in UART communication?

easy

A. The type of data encoding

B. The voltage level of signals

C. The size of data packets

D. The speed of data transmission

Solution

1. Step 1: Understand baud rate meaning

   Baud rate defines how fast bits are sent per second in serial communication.

2. Step 2: Identify what baud rate affects

   It controls the speed, not voltage, size, or encoding type.

3. Final Answer:

   The speed of data transmission -> Option D

4. Quick Check:

   Baud rate = speed [OK]

Hint: Baud rate equals bits per second speed [OK]

Common Mistakes:

• Confusing baud rate with voltage levels
• Thinking baud rate controls data size
• Mixing baud rate with encoding type

2. Which of the following is the correct way to calculate the UART baud rate register value if the clock frequency is 16 MHz and desired baud rate is 9600?

easy

A. UBRR = (16000000 / (32 * 9600)) - 1

B. UBRR = (16000000 / (16 * 9600)) - 1

C. UBRR = (16000000 / (8 * 9600)) + 1

D. UBRR = (16000000 / (16 * 9600)) + 1

Solution

1. Step 1: Recall standard baud rate formula

   For normal speed UART, UBRR = (F_CPU / (16 * baud)) - 1.

2. Step 2: Apply values to formula

   Substitute F_CPU=16,000,000 and baud=9600 gives UBRR = (16000000/(16*9600))-1.

3. Final Answer:

   UBRR = (16000000 / (16 * 9600)) - 1 -> Option B

4. Quick Check:

   Standard UART formula = UBRR = (16000000 / (16 * 9600)) - 1 [OK]

Hint: Use formula UBRR = (F_CPU/(16*baud))-1 for normal speed [OK]

Common Mistakes:

• Using wrong divisor like 8 or 32 instead of 16
• Adding 1 instead of subtracting 1
• Mixing up normal and double speed formulas

3. Given the following code snippet, what is the value loaded into UBRR if F_CPU = 8 MHz and baud rate = 115200?

unsigned int ubrr_value = (8000000 / (16 * 115200)) - 1;
UBRR0H = (ubrr_value >> 8);
UBRR0L = ubrr_value;

medium

A. 5

B. 4

C. 3

D. 6

Solution

1. Step 1: Calculate UBRR value

   Calculate (8000000 / (16 * 115200)) - 1 = (8000000 / 1843200) - 1 ≈ 4.34 - 1 = 3.34, truncated to 3.

2. Step 2: Check integer truncation

   Since ubrr_value is unsigned int, decimal part is truncated, so ubrr_value = 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   Integer division truncates to 3 [OK]

Hint: Integer division truncates decimals, so round down [OK]

Common Mistakes:

• Rounding up instead of truncating
• Using wrong clock frequency
• Confusing UBRR with baud rate

4. Identify the error in this baud rate configuration code snippet:

#define F_CPU 16000000
#define BAUD 9600
unsigned int ubrr = F_CPU / 16 / BAUD;
UBRR0H = (ubrr >> 8);
UBRR0L = ubrr;

medium

A. Missing subtraction of 1 in UBRR calculation

B. Wrong bit shift for UBRR0H

C. Incorrect baud rate value

D. F_CPU should be divided by 8, not 16

Solution

1. Step 1: Review UBRR calculation formula

   The correct formula is UBRR = (F_CPU / (16 * BAUD)) - 1, but code misses the '-1'.

2. Step 2: Check other parts

   Bit shift and baud rate are correct; division by 16 is standard for normal speed.

3. Final Answer:

   Missing subtraction of 1 in UBRR calculation -> Option A

4. Quick Check:

   UBRR formula requires '-1' [OK]

Hint: Always subtract 1 in UBRR formula [OK]

Common Mistakes:

• Forgetting to subtract 1
• Shifting bits incorrectly
• Using wrong clock divisor

5. You want to configure UART for double speed mode with F_CPU = 16 MHz and baud rate 115200. Which formula correctly calculates the UBRR value?

hard

A. UBRR = (F_CPU / (8 * baud)) - 1

B. UBRR = (F_CPU / (16 * baud)) - 1

C. UBRR = (F_CPU / (32 * baud)) - 1

D. UBRR = (F_CPU / (4 * baud)) - 1

Solution

1. Step 1: Understand double speed mode formula

   In double speed mode, UART uses UBRR = (F_CPU / (8 * baud)) - 1.

2. Step 2: Compare options to formula

   Only UBRR = (F_CPU / (8 * baud)) - 1 matches the double speed formula correctly.

3. Final Answer:

   UBRR = (F_CPU / (8 * baud)) - 1 -> Option A

4. Quick Check:

   Double speed divisor is 8 [OK]

Hint: Double speed uses divisor 8, not 16 [OK]

Common Mistakes:

• Using normal speed formula for double speed
• Confusing divisor values
• Forgetting to subtract 1