Debug Output in Terminal via USART

1 Output

To display (debug) output to a terminal, on your host, there are 3 functions defined in the file: \startup-code\syscalls.c

1. USART_Init(...)
2. _write_impl_USART_SendChar(...)
3. _write(...)

When USART2_Init(...) is called during the initialization phase of your program, you can use standard output functions such as:

printf("Debug this variable: %s\r\n",var1);     // Note the \r\n instead of just \n
puts("Enter function: HelloWorld()\r\n ");

The output will appear in a terminal emulator like PuTTY, provided the hardware and serial connection are correctly configured.

📍 More documentation can be found in the above meantioned syscalls.c file.

2 Input

Just like output above the syscalls.c also implements the system function: int _read(...) which makes sure you can ask for input in your PuTTy terminal with scanf function or by calling the function direct.

char name[50];   
unsigned  len = 50;    
_read(0, name, len);        // CR Return will end the input

3 Configure PuTTY

Assuming the files are already part of your project, and you’re using the STM32F4-Discovery board (or a compatible one):

1. Hardware Requirements:
   • Make sure you have a USB-to-TTL Serial Cable (USB connector on one side and four lose connectors on the otherside, see remark below)

2. Wiring: Connect the USB-to-TTL cable between your host and the STM32 board:

   • USB-to-TTL TX → STM32 PA3 (USART2 RX) (White)
   • USB-to-TTL RX → STM32 PA2 (USART2 TX) (Green)
   • GND → GND (black)
   • Power → NOT needed (red)

3. Make the connection

   • On the Host (Windows)
     • Locate the used COM port number using Windows Device manager
     • In the Host start Putty and configure it:
     • set the connection type to: serial
     • enter the COM Port number
     • specify the speed: 9600

Once everything is connected and the program runs, output from the device will appear in the terminal.

Remark

I used a cable like: Exactly this ( alternative 1 alternative 2 )

In Device Manager it shows up like: Silicon Labs CP210x usa to UART Bridge (COM3)

License
This file is part of: STM32 Template Stack Copyright (c) 2025-2026 Nico Jan Eelhart.This repository is MIT licensed and free to use. For optional commercial support, customization, training, or long-term maintenance, see COMMERCIAL.md.

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