Overview

Purpose

Basic STDIO is a minimal stdout redirection layer for STM32. It provides quick printf output through UART, ITM (SWO), or a custom input/output interface with minimal setup and without a heavyweight logging framework. It excludes advanced logging features (no log levels, filtering mechanisms, buffering queues, or timestamp insertion) to keep the footprint small and the integration effort low, making the component easy to integrate, remove, or replace. Designed for early board startup, firmware prototyping, and quick diagnostic tracing, it also serves as a simple base that can be extended later.

Key Features

The main features of Basic STDIO are:

  • Single initialization entry point.

  • Multiple input/output interfaces: UART (HAL based), ITM (CMSIS-Core only), or a custom interface from the provided template.

  • Minimal footprint by excluding log levels, filtering, buffering, and timestamps.

  • Deterministic blocking path (each character is sent immediately).

  • Abstracts toolchain libc hook differences (_write, __write, fputc variations).

Limitations

  • Not threadsafe: Concurrent writes from multiple contexts may be interleaved unpredictably.

  • No stdin support: Only stdout redirection is provided; input via stdin is not supported.

  • UART HAL-only: The UART interface supports the STM32 HAL only.

Architecture

The following diagram illustrates the software components of the Basic STDIO module. It shows the interactions between the user application, the core layer, the input/output interfaces (ITM, UART, custom), the supporting system libraries (CMSIS-Core, HAL), and the hardware components.

@startuml top to bottom direction skinparam linetype ortho <style> componentDiagram { arrow { FontSize 8 } } </style> title Basic STDIO Architecture Overview package "Application" { component APP as "User Application\n(firmware code)" } package "Basic STDIO" { component CORE as "Core Layer\n(hook adaptation + forwarding)" package "io_interface" { component B_ITM as "ITM interface" #DAE8FC component B_UART as "UART interface" #DAE8FC component B_CUSTOM as "Custom interface\n(template)" #DAE8FC } } package "System" { component CMSIS as "CMSIS-Core" component HAL as "HAL UART" #DAE8FC component HW as "STM32_HW" component PROBE as "Debug Probe\n(ST-Link, etc.)" component TERMINAL as "UART Terminal /\nTrace Viewer" } APP --> CORE : trace request CORE --> B_ITM : trace bytes CORE --> B_UART : trace bytes CORE --> B_CUSTOM : trace bytes B_ITM --> CMSIS : ITM registers B_UART --> HAL : HAL API calls HAL --> HW : register access B_ITM --> PROBE : SWO line PROBE --> TERMINAL : trace data B_UART --> TERMINAL : UART TX B_CUSTOM --> HW : user-defined link CMSIS --> HW : core register access @enduml

Component Structure

Core ( basic_stdio_core.c / basic_stdio_core.h): Handles stdout redirection by selecting the appropriate libc hook and then forwarding each character or small buffer to a neutral forwarding layer that is independent of the physical input/output interface.

Interfaces : Software layer that connects Basic STDIO to the hardware component. Three interfaces are available:

  • UART and ITM: Ready-to-use implementations that only require enabling and initialization, with no code changes.

  • Template: Empty interface that allows users to develop their own interface. Copy the file into the user space; it contains empty functions with the prototypes required by the core.

Modules and Files

The following diagram shows the Basic STDIO module and its associated files:

@startuml Basic_STDIO_modules skinparam componentStyle rectangle skinparam defaultFontName Courier skinparam ArrowColor #2A4B8D skinparam ArrowFontSize 10 skinparam RectangleBackgroundColor #FFFFFF skinparam RectangleBorderColor #2A4B8D skinparam Shadowing false skinparam linetype ortho title Basic STDIO Modules Overview package "Application" { component APP as "User Application\n(firmware code)" } package "Basic STDIO" { component CORE_H as "basic_stdio_core.h" component CORE_C as "basic_stdio_core.c" component ITF_H as "basic_stdio_itf_io.h" component B_ITM as "basic_stdio_itfio_itm.c" #DAE8FC component B_UART as "basic_stdio_itfio_uart.c" #DAE8FC component B_TEMPLATE as "basic_stdio_itfio_template.c" #DAE8FC } APP --> CORE_C CORE_C ..> CORE_H : include CORE_C ..> ITF_H : include CORE_C --> B_ITM : interface CORE_C --> B_UART : interface CORE_C --> B_TEMPLATE : interface B_ITM ..> ITF_H : include B_UART ..> ITF_H : include B_TEMPLATE ..> ITF_H : include @enduml