Printf redirect to UART in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to redirect printf output to UART by defining the correct function name.

Embedded C

int [1](char ch, FILE *f) {
    // UART transmit code here
    return ch;
}

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

Afputc

Bprintf

Cuart_send

Dputchar

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'printf' or 'putchar' instead of 'fputc' for redirection.

The fputc function is used to redirect characters output by printf to UART.

2fill in blank

medium

Complete the code to send a character over UART inside the fputc function.

Embedded C

int fputc(char ch, FILE *f) {
    while (![1]()); // Wait until UART ready
    UART_DR = ch; // Send character
    return ch;
}

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

Auart_error

Buart_rx_ready

Cuart_tx_ready

Duart_busy

Attempts:

3 left

💡 Hint

Common Mistakes

Using receive ready or busy flags instead of transmit ready.

The function uart_tx_ready() checks if UART is ready to transmit a new character.

3fill in blank

hard

Fix the error in the UART initialization code to enable UART transmit interrupt.

Embedded C

void uart_init() {
    UART_CR = 0x00;
    UART_BRR = 9600;
    UART_CR |= [1]; // Enable UART
    UART_CR |= [2]; // Enable TX interrupt
}

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

AUART_CR_RXNEIE

BUART_CR_RE

CUART_CR_TXEIE

DUART_CR_TE

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing receiver enable with transmitter enable.

To enable UART transmitter, set UART_CR_TE. To enable TX interrupt, set UART_CR_TXEIE.

4fill in blank

hard

Fill both blanks to complete the UART transmit function that waits for transmit buffer empty and sends a character.

Embedded C

void uart_send_char(char ch) {
    while (![1]());
    [2] = ch;
}

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

Auart_tx_ready

BUART_DR

Cuart_rx_ready

DUART_SR

Attempts:

3 left

💡 Hint

Common Mistakes

Using receive ready flag or status register incorrectly.

Wait for uart_tx_ready() to confirm UART is ready, then write character to UART_DR data register.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps characters to their ASCII codes if code is greater than 64.

Embedded C

ascii_map = [1]: ord([2]) for [3] in 'ABCDEF' if ord([2]) > 64}

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

Ach

Dchar

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names inconsistently.

The variable ch is used as key and in ord() to get ASCII code for each character.

Practice

1. What is the main purpose of overriding the fputc function in embedded C when redirecting printf output to UART?

easy

A. To clear the UART receive buffer

B. To change the baud rate of the UART communication

C. To initialize the UART peripheral before use

D. To send each character from printf to the UART hardware for transmission

Solution

1. Step 1: Understand printf output mechanism

   The printf function sends characters one by one to fputc for output.

2. Step 2: Role of overriding fputc

   By overriding fputc, we redirect each character to UART transmit function, sending it out via UART.

3. Final Answer:

   To send each character from printf to the UART hardware for transmission -> Option D

4. Quick Check:

   Overriding fputc redirects output to UART [OK]

Hint: Remember: printf calls fputc for output [OK]

Common Mistakes:

• Confusing fputc with UART initialization
• Trying to change UART settings inside fputc
• Ignoring that printf uses fputc internally

2. Which of the following is the correct prototype for overriding fputc to redirect printf output to UART in embedded C?

easy

A. char fputc(int ch, FILE *f);

B. void fputc(char ch);

C. int fputc(FILE *f, char ch);

D. int fputc(char ch, FILE *f);

Solution

1. Step 1: Recall standard fputc signature

   The standard fputc function prototype is int fputc(int ch, FILE *stream);.

2. Step 2: Match options with correct signature

   char fputc(int ch, FILE *f); matches the correct order and types: returns char (though standard is int), takes int and FILE pointer.

3. Final Answer:

   char fputc(int ch, FILE *f); -> Option A

4. Quick Check:

   Correct fputc prototype returns int and takes int, FILE* [OK]

Hint: Prototype must match standard fputc signature [OK]

Common Mistakes:

• Swapping parameter order
• Using void return type
• Using wrong parameter types

3. Given the following code snippet, what will be the output on the UART terminal?

int fputc(int ch, FILE *f) {
    UART_SendChar((char)ch);
    return ch;
}

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

medium

A. Only 'H' is sent to UART

B. No output because UART_SendChar is not defined

C. Hi followed by a newline on UART terminal

D. Compilation error due to missing UART initialization

Solution

1. Step 1: Understand fputc role in printf

   Each character of "Hi\n" is sent to fputc, which calls UART_SendChar to send it.

2. Step 2: Output on UART terminal

   Characters 'H', 'i', and newline '\n' are sent sequentially, so "Hi" and a newline appear on UART terminal.

3. Final Answer:

   Hi followed by a newline on UART terminal -> Option C

4. Quick Check:

   printf output redirected via fputc to UART [OK]

Hint: Each printf char calls fputc, which sends via UART [OK]

Common Mistakes:

• Assuming no output without UART initialization code shown
• Thinking only first char is sent
• Confusing compile errors with runtime behavior

4. Identify the error in this fputc override code that tries to redirect printf to UART:

int fputc(int ch, FILE *f) {
    UART_SendChar(ch);
    return 0;
}

medium

A. Returning 0 instead of the character causes incorrect printf behavior

B. Passing ch directly to UART_SendChar causes type mismatch

C. Missing UART initialization before sending characters

D. Function should return void, not int

Solution

1. Step 1: Check return value of fputc

   The fputc function must return the character written on success, or EOF on failure.

2. Step 2: Analyze the code's return statement

   Returning 0 (null char) instead of the character can cause printf to misbehave or stop output prematurely.

3. Final Answer:

   Returning 0 instead of the character causes incorrect printf behavior -> Option A

4. Quick Check:

   Return written char from fputc to keep printf working [OK]

Hint: Always return the character from fputc [OK]

Common Mistakes:

• Returning 0 instead of the character
• Ignoring return value importance
• Assuming void return type is allowed

5. You want to redirect printf output to UART and also support multiple UART ports. Which approach correctly extends fputc to select UART port dynamically?

hard

A. Create separate fputc functions for each UART port and call them manually

B. Use a global variable to store current UART port and send chars accordingly inside fputc

C. Modify printf to accept UART port as parameter

D. Use interrupts to switch UART ports automatically during fputc

Solution

1. Step 1: Understand fputc limitation

   Only one fputc function can be linked, so multiple versions are not possible.

2. Step 2: Use global variable for UART selection

   By storing the current UART port in a global variable, fputc can send characters to the selected UART dynamically.

3. Step 3: Evaluate other options

   Modifying printf is complex; interrupts do not switch ports automatically; multiple fputc functions cause conflicts.

4. Final Answer:

   Use a global variable to store current UART port and send chars accordingly inside fputc -> Option B

5. Quick Check:

   Global UART selector enables dynamic port choice in fputc [OK]

Hint: Use global UART variable to select port inside single fputc [OK]

Common Mistakes:

• Trying multiple fputc functions
• Changing printf signature
• Relying on interrupts to switch UART ports