Baud rate configuration in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Calculate UBRR value for 16 MHz clock and 9600 baud

Given a CPU clock of 16 MHz and a desired baud rate of 9600, what is the UBRR register value calculated by the formula UBRR = (F_CPU / (16 * BAUD)) - 1?

Embedded C

unsigned int ubrr = (16000000 / (16 * 9600)) - 1;
printf("%u", ubrr);

A 16

B 104

C 103

D 0

💡 Hint

Use integer division carefully and subtract 1 after dividing.

The formula for UBRR is (F_CPU / (16 * BAUD)) - 1. Plugging in 16,000,000 and 9600 gives 103.1666, which truncates to 103.

❓ Predict Output

intermediate

Effect of double speed mode on UBRR calculation

If the UART double speed mode (U2X) is enabled, the baud rate formula changes to UBRR = (F_CPU / (8 * BAUD)) - 1. What is the UBRR value for 16 MHz clock and 115200 baud with U2X enabled?

Embedded C

unsigned int ubrr = (16000000 / (8 * 115200)) - 1;
printf("%u", ubrr);

A 0

B 16

C 8

D 103

💡 Hint

Remember to divide by 8 instead of 16 when U2X is set.

With U2X enabled, the divisor is 8 instead of 16. So UBRR = (16000000 / (8 * 115200)) - 1 = 16.36, truncated to 16.

🔧 Debug

advanced

Identify the error in baud rate calculation code

What error does the following code produce when compiled and run? unsigned int ubrr = 16000000 / 16 * 9600 - 1; printf("%u", ubrr);

Embedded C

unsigned int ubrr = 16000000 / 16 * 9600 - 1;
printf("%u", ubrr);

A Produces a very large incorrect number due to wrong operator precedence

B SyntaxError due to missing semicolon

C Runtime error: division by zero

D Correct output: 103

💡 Hint

Check the order of operations in the expression carefully.

The expression divides 16000000 by 16, then multiplies by 9600, which is not the intended formula. The correct formula divides by (16 * 9600).

🧠 Conceptual

advanced

Why is the UBRR value subtracted by 1 in the baud rate formula?

In the baud rate calculation formula UBRR = (F_CPU / (16 * BAUD)) - 1, why do we subtract 1 from the division result?

A To reduce the baud rate by one unit for safety

B To compensate for clock drift in the microcontroller

C Because the formula is derived from an approximation that requires subtraction

D Because the UBRR register counts from zero, so subtracting 1 aligns the count

💡 Hint

Think about how hardware counters usually start counting.

The UBRR register sets the baud rate divisor and counts from zero, so subtracting 1 adjusts the value to match the desired baud rate.

❓ Predict Output

expert

Calculate baud rate error percentage

Given F_CPU = 8 MHz, desired baud rate = 19200, and UBRR calculated as 25 using formula UBRR = (F_CPU / (16 * BAUD)) - 1, what is the baud rate error percentage? Use actual baud rate = F_CPU / (16 * (UBRR + 1)) and error% = ((actual - desired) / desired) * 100

Embedded C

unsigned int ubrr = 25;
float actual_baud = 8000000.0 / (16 * (ubrr + 1));
float error = ((actual_baud - 19200) / 19200) * 100;
printf("%.2f", error);

A 0.16

B 0.00

C 0.78

D -0.78

💡 Hint

Calculate actual baud rate first, then find percentage difference.

Actual baud = 8000000 / (16 * 26) = 19230.77. Error = ((19230.77 - 19200)/19200)*100 ≈ 0.16%. The code outputs "0.16".