Printf debugging over UART in Embedded C

Introduction

Printf debugging over UART helps you see what your embedded program is doing by sending messages to a computer. It is like talking to your program to understand its actions.

You want to check if a sensor value is read correctly in your embedded device.

You need to find out why your program is stuck or not working as expected.

You want to monitor variables or program flow without a screen.

You want to log events or errors during device operation.

You are developing firmware and want quick feedback from your device.

Syntax

Embedded C

int printf(const char *format, ...);

// To send printf output over UART, you usually:
// 1. Initialize UART hardware.
// 2. Redirect printf output to UART transmit function.
// 3. Use printf as usual to send messages.

UART must be set up before using printf for debugging.

Redirecting printf output depends on your microcontroller and compiler.

Examples

This example shows how to redirect printf output to UART by implementing the _write function.

Embedded C

// Example UART initialization (pseudo code)
UART_Init(9600); // Set baud rate to 9600

// Redirect printf output to UART
int _write(int file, char *ptr, int len) {
    for (int i = 0; i < len; i++) {
        UART_SendChar(ptr[i]);
    }
    return len;
}

Prints the temperature value over UART so you can see it on your computer terminal.

Embedded C

printf("Temperature: %d C\n", temperature);

Sample Program

This program simulates sending printf messages over UART by redirecting output to UART_SendChar. It prints a start message and a sensor value.

Embedded C

#include <stdio.h>
#include <stdint.h>

// Mock UART send function
void UART_SendChar(char c) {
    // In real hardware, this sends a char over UART
    // Here we just print to standard output for simulation
    putchar(c);
}

// Redirect printf output to UART
int _write(int file, char *ptr, int len) {
    for (int i = 0; i < len; i++) {
        UART_SendChar(ptr[i]);
    }
    return len;
}

int main() {
    int sensor_value = 42;
    printf("Starting UART debug...\n");
    printf("Sensor value is %d\n", sensor_value);
    return 0;
}

Important Notes

Make sure UART baud rate matches your terminal program settings.

Some compilers require different functions to redirect printf output.

Using printf debugging can slow down your program, so remove it in final code.

Summary

Printf debugging over UART lets you see messages from your embedded device.

You must initialize UART and redirect printf output to UART transmit.

This method helps find bugs and monitor program behavior without a screen.

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