Baud rate configuration in Embedded C - Mini Project: Build & Apply

Baud rate configuration

📖 Scenario: You are working on a microcontroller project that communicates with other devices using UART. To ensure proper communication, you need to set the correct baud rate for the UART module.

🎯 Goal: Configure the UART baud rate register to set the baud rate to 9600 bps using a given clock frequency.

📋 What You'll Learn

Create a variable for the clock frequency

Create a variable for the desired baud rate

Calculate the baud rate register value using the formula

Assign the calculated value to the baud rate register variable

Print the baud rate register value

💡 Why This Matters

🌍 Real World

Setting the correct baud rate is essential for reliable serial communication between microcontrollers and other devices like sensors, computers, or modems.

💼 Career

Embedded systems engineers and firmware developers often configure UART settings to ensure devices communicate correctly and efficiently.

1

Set clock frequency

Create an unsigned long variable called clock_freq and set it to 16000000 (16 MHz).

Embedded C

// Create an unsigned long variable called clock_freq and set it to 16000000
// Your code here

Hint

The clock frequency is usually the system clock speed in Hertz.

2

Set desired baud rate

Create an unsigned int variable called baud_rate and set it to 9600.

Embedded C

unsigned long clock_freq = 16000000;
// Create an unsigned int variable called baud_rate and set it to 9600
// Your code here

Hint

The baud rate is the speed of communication in bits per second.

3

Calculate baud rate register value

Create an unsigned int variable called baud_reg and calculate its value using the formula: baud_reg = (clock_freq / (16 * baud_rate)) - 1;

Embedded C

unsigned long clock_freq = 16000000;
unsigned int baud_rate = 9600;
// Calculate baud_reg using the formula and assign it
// Your code here

Hint

This formula is common for UART baud rate calculation in many microcontrollers.

4

Print baud rate register value

Use printf to print the text "Baud rate register value: " followed by the value of baud_reg.

Embedded C

unsigned long clock_freq = 16000000;
unsigned int baud_rate = 9600;
unsigned int baud_reg = (clock_freq / (16 * baud_rate)) - 1;
// Print the baud_reg value
// Your code here

Hint

Use printf("Baud rate register value: %u\n", baud_reg); inside main().

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