Printf debugging over UART in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to send a character over UART using printf.

Embedded C

#include <stdio.h>

int main() {
    char c = 'A';
    printf([1]);
    return 0;
}

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

A"%c", c

B"%d", c

C"%s", c

D"%f", c

Attempts:

3 left

💡 Hint

Common Mistakes

Using %d prints the ASCII code, not the character.

Using %s expects a string, not a character.

Use "%c" to print a single character with printf.

2fill in blank

medium

Complete the code to initialize UART for printf debugging.

Embedded C

void UART_Init() {
    // Set baud rate
    UBRR0 = [1];
    // Enable transmitter
    UCSR0B = (1 << TXEN0);
    // Set frame format: 8 data bits, 1 stop bit
    UCSR0C = (1 << UCSZ01) | (1 << UCSZ00);
}

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

A8

B9600

C16

D103

Attempts:

3 left

💡 Hint

Common Mistakes

Setting UBRR0 directly to baud rate value causes wrong speed.

Confusing baud rate with register value.

UBRR0 value 103 sets baud rate to 9600 for 16MHz clock.

3fill in blank

hard

Fix the error in the UART transmit function to send a character correctly.

Embedded C

void UART_Transmit(char data) {
    while (!(UCSR0A & (1 << [1]))) {
        ; // wait for empty transmit buffer
    }
    UDR0 = data;
}

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

AUDRE0

BTXC0

CRXC0

DRXEN0

Attempts:

3 left

💡 Hint

Common Mistakes

Waiting for RXC0 waits for receive complete, not transmit buffer.

Using TXC0 waits for transmit complete, not buffer empty.

UDRE0 bit indicates the transmit buffer is empty and ready for new data.

4fill in blank

hard

Fill both blanks to print a number in decimal and padded uppercase hex using printf for debugging.

Embedded C

int val = 255;
 printf("val = %d (0x%02[1])\n", val, [2]);

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

AX

Bval

Cx

D255

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'x' prints lowercase hex letters.

Passing literal value '255' instead of variable 'val'.

Use 'X' for uppercase padded hex (%02X), and 'val' as the argument.

5fill in blank

hard

Fill all three blanks to create a FILE stream that redirects printf to UART.

Embedded C

[1] uartout = FDEV_SETUP_STREAM(UART_Transmit, NULL, [2]);

int main() {
    [3] = &uartout;
    printf("Debug: UART printf ready!\n");
    return 0;
}

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

Astatic FILE

B_FDEV_SETUP_WRITE

Cstdout

Dstdin

Attempts:

3 left

💡 Hint

Common Mistakes

Using stdin instead of stdout redirects input, not output.

Missing 'static' or wrong type for the stream declaration.

'static FILE' declares the output stream, '_FDEV_SETUP_WRITE' for write-only, 'stdout = &uartout' redirects printf.

Practice

1. What is the main purpose of using printf debugging over UART in embedded C?

easy

A. To store data permanently in memory

B. To increase the speed of the embedded device

C. To send debug messages from the embedded device to a computer or terminal

D. To control hardware pins directly

Solution

1. Step 1: Understand UART communication

   UART allows serial communication between the embedded device and external devices like a PC.

2. Step 2: Role of printf debugging

   Using printf over UART sends text messages to the PC for debugging and monitoring program behavior.

3. Final Answer:

   To send debug messages from the embedded device to a computer or terminal -> Option C

4. Quick Check:

   UART + printf = debug messages [OK]

Hint: Printf over UART sends messages to PC for debugging [OK]

Common Mistakes:

• Thinking printf speeds up the device
• Confusing UART with memory storage
• Assuming printf controls hardware pins

2. Which of the following is the correct way to redirect printf output to UART in embedded C?

easy

A. Call printf directly without UART initialization

B. Set a GPIO pin high to enable printf

C. Use scanf to read UART data

D. Implement a custom _write function that sends characters via UART

Solution

1. Step 1: Understand printf redirection

   Standard printf sends output to stdout, which must be redirected to UART by implementing a low-level write function.

2. Step 2: Custom _write function role

   Implementing _write sends each character to UART transmit, enabling printf output over UART.

3. Final Answer:

   Implement a custom _write function that sends characters via UART -> Option D

4. Quick Check:

   Redirect printf by custom _write [OK]

Hint: Redirect printf by writing _write function sending UART chars [OK]

Common Mistakes:

• Calling printf without UART setup
• Using scanf instead of printf for output
• Trying to enable printf by GPIO pin

3. Given the code below, what will be the output sent over UART?

void UART_SendChar(char c);

int _write(int file, char *ptr, int len) {
  for (int i = 0; i < len; i++) {
    UART_SendChar(ptr[i]);
  }
  return len;
}

int main() {
  printf("Temp: %d C\n", 25);
  return 0;
}

medium

A. Temp: 25 C\n

B. Temp: %d C\n

C. Temp: 25 C

D. No output because UART_SendChar is not defined

Solution

1. Step 1: Understand _write function

   The _write function sends each character of the printf string to UART_SendChar, which transmits it over UART.

2. Step 2: Analyze printf output

   printf formats the string "Temp: 25 C\n" and _write sends it character by character.

3. Final Answer:

   Temp: 25 C\n -> Option A

4. Quick Check:

   Formatted printf string sent via UART [OK]

Hint: Printf formats string, _write sends chars via UART [OK]

Common Mistakes:

• Thinking %d prints literally
• Ignoring newline character
• Assuming UART_SendChar undefined means no output

4. What is the main problem in this code snippet for printf debugging over UART?

void UART_SendChar(char c) {
  while (!(UART->SR & UART_SR_TXE));
  UART->DR = c;
}

int _write(int file, char *ptr, int len) {
  for (int i = 0; i < len; i++) {
    UART_SendChar(ptr[i]);
  }
  return len;
}

int main() {
  printf("Hello UART\n");
  return 0;
}

medium

A. UART peripheral is not initialized before use

B. UART_SendChar does not wait for TXE flag

C. _write function returns wrong length

D. printf is not called in main

Solution

1. Step 1: Check UART_SendChar function

   UART_SendChar waits for TXE flag and writes data, so it is correct.

2. Step 2: Check UART initialization

   The code does not show UART initialization, which is required before sending data.

3. Final Answer:

   UART peripheral is not initialized before use -> Option A

4. Quick Check:

   Missing UART init causes no transmission [OK]

Hint: Always initialize UART before sending data [OK]

Common Mistakes:

• Ignoring UART initialization step
• Thinking TXE flag wait is missing
• Believing _write return value is incorrect

5. You want to debug a sensor reading using printf over UART. The sensor value is an integer that can be zero or negative. Which printf format specifier and UART setup is best to correctly display the value including negatives?

hard

A. Use %f in printf without UART redirection

B. Use %d in printf and ensure UART is initialized and _write redirects output

C. Use %x in printf and send data directly to GPIO pins

D. Use %u in printf and skip UART initialization

Solution

1. Step 1: Choose correct format specifier

   %d prints signed integers including zero and negatives correctly, unlike %u (unsigned) or %x (hex).

2. Step 2: Ensure UART setup

   UART must be initialized and printf output redirected via _write to UART for messages to appear on terminal.

3. Final Answer:

   Use %d in printf and ensure UART is initialized and _write redirects output -> Option B

4. Quick Check:

   Signed int format + UART init + _write = correct debug [OK]

Hint: Use %d and proper UART init with _write for signed values [OK]

Common Mistakes:

• Using %u which can't show negatives
• Skipping UART initialization
• Sending data to GPIO instead of UART