UART protocol fundamentals in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

UART Transmission Bit Timing Calculation

Given a UART baud rate of 9600 and a system clock of 16 MHz, what is the value of the UART bit time in microseconds calculated by the code below?

Embedded C

unsigned int baud_rate = 9600;
unsigned long clock_freq = 16000000;
float bit_time_us = (1.0 / baud_rate) * 1000000;

// Print bit time
printf("%.2f\n", bit_time_us);

A 16.00

B 9600.00

C 104.17

D 1.60

💡 Hint

Remember that bit time is the inverse of baud rate, converted to microseconds.

Bit time is the duration of one bit. It is calculated as 1 divided by baud rate. For 9600 baud, bit time = 1/9600 = 0.00010417 seconds = 104.17 microseconds.

❓ Predict Output

intermediate

UART Frame Size Calculation

What is the total number of bits transmitted in one UART frame with 1 start bit, 8 data bits, no parity, and 1 stop bit?

A 10

B 9

C 11

D 8

💡 Hint

Add start bits, data bits, parity bits (if any), and stop bits.

UART frame bits = start bit (1) + data bits (8) + parity bits (0) + stop bits (1) = 10 bits total.

🔧 Debug

advanced

Identify the UART Configuration Error

The following UART initialization code is intended to set 8 data bits, even parity, and 1 stop bit. Which option correctly identifies the error causing wrong parity configuration?

Embedded C

UART_Config config;
config.data_bits = 8;
config.parity = 0; // 0 means no parity
config.stop_bits = 1;
UART_Init(&config);

A Data bits should be set to 7 for even parity to work.

B Parity is set to 0 which disables parity; should be set to 2 for even parity.

C Stop bits must be 2 for parity to be enabled.

D UART_Init function is missing baud rate parameter.

💡 Hint

Check the parity value and what it represents.

Parity value 0 disables parity. For even parity, the code should set parity to 2 (assuming 2 means even parity in this API).

📝 Syntax

advanced

UART Interrupt Handler Syntax

Which option shows the correct syntax for a UART receive interrupt handler function in embedded C?

A void UART_RX_IRQHandler(void) { /* handle RX interrupt */ }

B UART_RX_IRQHandler(void) { /* handle RX interrupt */ }

C void UART_RX_IRQHandler(int irq) { /* handle RX interrupt */ }

D int UART_RX_IRQHandler() { /* handle RX interrupt */ }

💡 Hint

Interrupt handlers usually return void and take no parameters.

The correct syntax is a void function with no parameters for interrupt handlers in embedded C.

🚀 Application

expert

UART Data Buffer Overflow Scenario

In a UART communication system, if the receive buffer is not read fast enough and new data keeps arriving, what will happen?

A The UART will store all data indefinitely without loss.

B The UART hardware will automatically pause transmission until buffer is free.

C The system will raise a hardware exception and reset the UART module.

D New incoming data will overwrite old unread data causing data loss.

💡 Hint

Think about what happens when a fixed-size buffer is full and new data arrives.

If the receive buffer is full and not read, new data overwrites old data causing loss. UART hardware usually does not pause transmission automatically.

Practice

1. What does UART stand for in embedded systems communication?

easy

A. Universal Analog Radio Transmitter

B. Universal Automatic Register Transfer

C. Unified Asynchronous Remote Terminal

D. Universal Asynchronous Receiver/Transmitter

Solution

1. Step 1: Understand UART acronym

   UART is a communication protocol used in embedded systems for serial communication.

2. Step 2: Match full form

   UART stands for Universal Asynchronous Receiver/Transmitter, describing its function.

3. Final Answer:

   Universal Asynchronous Receiver/Transmitter -> Option D

4. Quick Check:

   UART = Universal Asynchronous Receiver/Transmitter [OK]

Hint: Remember UART is about asynchronous serial communication [OK]

Common Mistakes:

• Confusing UART with synchronous protocols
• Mixing up the acronym words
• Thinking UART is analog communication

2. Which of the following is the correct way to initialize UART in embedded C?

easy

A. uart_init(9600);

B. UART_Init(baudrate=9600);

C. uartStart(9600);

D. init_uart(9600);

Solution

1. Step 1: Identify common UART init function

   In embedded C, uart_init with baud rate parameter is a common initialization call.

2. Step 2: Check syntax correctness

   uart_init(9600); uses correct function name and parameter style without extra keywords.

3. Final Answer:

   uart_init(9600); -> Option A

4. Quick Check:

   Correct UART init syntax = uart_init(9600); [OK]

Hint: Look for simple function call with baud rate only [OK]

Common Mistakes:

• Using incorrect function names
• Adding parameter names inside call
• Using camelCase instead of snake_case

3. What will be the output of this UART send code snippet?

uart_init(9600);
uart_send_char('A');
uart_send_char('B');

medium

A. BA

B. A B

C. AB

D. Error: uart_send_char undefined

Solution

1. Step 1: Initialize UART at 9600 baud

   The uart_init(9600) sets baud rate for communication.

2. Step 2: Send characters in order

   uart_send_char('A') sends 'A', then uart_send_char('B') sends 'B' sequentially.

3. Final Answer:

   AB -> Option C

4. Quick Check:

   Characters sent in order = AB [OK]

Hint: Characters send in call order, no spaces added [OK]

Common Mistakes:

• Assuming characters reverse order
• Expecting spaces between characters
• Thinking uart_send_char is undefined

4. Identify the error in this UART initialization code:

void setup() {
  uart_init(115200);
  uart_init(9600);
}

medium

A. uart_init requires no parameters

B. Multiple uart_init calls cause baud rate conflict

C. Function name should be UART_Init

D. No error, code is correct

Solution

1. Step 1: Review uart_init calls

   Two uart_init calls with different baud rates cause conflict in configuration.

2. Step 2: Understand UART config rules

   UART baud rate must be set once before communication; multiple calls overwrite settings.

3. Final Answer:

   Multiple uart_init calls cause baud rate conflict -> Option B

4. Quick Check:

   Multiple init calls cause conflict = A [OK]

Hint: Only one uart_init call needed to set baud rate [OK]

Common Mistakes:

• Thinking multiple init calls are allowed
• Assuming uart_init has no parameters
• Ignoring case sensitivity in function names

5. You want to send the string "HELLO" over UART. Which code snippet correctly sends each character one by one?

hard

A. for(int i=0; i<5; i++) uart_send_char("HELLO"[i]);

B. for(int i=0; i<5; i++) uart_send_char('H' + i);

C. for(int i=0; i<5; i++) uart_send_char('HELLO'[i]);

D. for(int i=0; i<5; i++) uart_send_char("HELLO" + i);

Solution

1. Step 1: Understand string indexing in C

   In C, single quotes ' ' denote characters, double quotes \" \" denote strings (char arrays).

2. Step 2: Check each option for correct character access

   for(int i=0; i<5; i++) uart_send_char(\"HELLO\"[i]); correctly indexes the string literal to access each character 'H','E','L','L','O'.

3. Final Answer:

   for(int i=0; i<5; i++) uart_send_char("HELLO"[i]); -> Option A

4. Quick Check:

   Correct string indexing uses double quotes \"HELLO\"[i] [OK]

Hint: Use double quotes for string literal indexing in C [OK]

Common Mistakes:

• Using single quotes for multi-character string literals
• Adding integers to characters ('H' + i sends HIJKL)
• Using pointer arithmetic without dereferencing