Printf redirect to UART in Embedded C

Introduction

We use printf to show messages. On embedded devices, we often send these messages through UART to see them on a computer.

You want to see debug messages from your microcontroller on your PC.

You need to send text data from your device to another device using UART.

You want to check sensor values or program status during development.

You want to log information without a screen or display.

You want to test communication between your embedded device and a terminal program.

Syntax

Embedded C

int fputc(int ch, FILE *f) {
    // send character ch to UART
    UART_SendChar((char)ch);
    return ch;
}

This function overrides the low-level output used by printf.

UART_SendChar is a placeholder for your UART transmit function.

Examples

Basic example redirecting printf output to UART character send function.

Embedded C

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

Waits until UART is ready before sending each character.

Embedded C

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

Adds carriage return before newline for proper terminal formatting.

Embedded C

int fputc(int ch, FILE *f) {
    if(ch == '\n') {
        UART_SendChar('\r');
    }
    UART_SendChar((char)ch);
    return ch;
}

Sample Program

This program redirects printf output to UART_SendChar, which here prints to the console. On a real device, UART_SendChar sends data over UART.

Embedded C

#include <stdio.h>

// Simulated UART send function
void UART_SendChar(char c) {
    // For demo, print to standard output
    putchar(c);
}

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

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

Important Notes

Make sure your UART is initialized before using printf.

Redirecting printf this way helps debugging without extra hardware.

Remember to handle special characters like '\n' if your terminal needs it.

Summary

Redirect printf by overriding fputc to send characters via UART.

This lets you see printf output on a PC terminal through UART.

Useful for debugging and communication in embedded systems.

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