Porting the most important features of the Arduino API to the STM8S.

Within a few minutes you are ready to compile and upload your first STM8S-based project while still retaining the flexibility to use ST's SPL functions.

All you need to get started is a simple STM8S103F breakout board for 70 cents and a ST-Link V2 compatible flash programmer for $2.50. Three boards and one flash programmer together are available for well under five dollars including shipping on aliexpress.



If you have ever used the Arduino environment before you will feel at home right away, despite this project beeing based on a makefile rather than the full Arduino IDE. But don't be afraid, it is based on the amazing makefile by Sudar to control the build process, that makes everything very easy.

Let's blink an LED using the Blink example from Arduino:

  Turns on an LED on for one second, then off for one second, repeatedly.

  This example code is in the public domain.

#include <Arduino.h>

// Pin 13 has an LED connected on most Arduino boards.
// Pin 3 for the STM8S103 break out board
// give it a name:
int led = LED_BUILDIN;

// the setup routine runs once when you press reset:
void setup() {
  // initialize the digital pin as an output.
  pinMode(led, OUTPUT);

// the loop routine runs over and over again forever:
void loop() {
  digitalWrite(led, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);               // wait for a second
  digitalWrite(led, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);               // wait for a second

All we need for a full build is this very basic Makefile:

BOARD_TAG   = stm8sblue

include ../../sduino/

Compile and upload it:

make upload

Done! Your first STM8 based project is up and running!

Tools used

This project is based on free tools that are available for Linux, MacOS, and Windows. Installation instructions

It uses the small devices C compiler (SDCC) for compiling, stm8flash for uploading the binary to the CPU, and simple Makefiles for the build process.

More information on using SDCC

Support for the Cosmic compiler under Windows and integration into the ST visual developer IDE might be possible, but is not done (yet?).

The build process is controlled by a makefile based on the amazing makefile by Sudar.

Supported hardware

Compatibility with the Arduino world

I adopted the Arduino core functionality for the STM8S to set up a simple programming environment. But unfortunatly there is no free C++ compiler for these CPUs. This makes it impossible to do a full port of the whole enviroment and integrate it with the Arduino IDE and build system as is has been done for the STM32 and the ESP8266.

This is not a drop-in replacement for an AVR, but the programming API is still very, very similar. Adopting existing libraries from C++ to C for use with the simplified C API is often easy and can be done quite fast, depending on the degree of dependency on specific hardware features.

The whole Arduino build system is deeply based on the assumption of processing C++ source files. I am not sure if it would be even possible to configure a build process based only on C files without modifing the IDE sources. This makes a full IDE integration very unlikely.

Using a converter/compiler like cfront to translate from C++ to C might be an option.

Included libraries

Some Arduino libraries are already ported to C-syntax. The resulting API is still very close to the C++ version and porting an existing application is not hard. Check out the API migration guidelines for details.



Motor control

Float arithmetics

Floating point arithmetics is supported by the SDCC standard library, but it comes at a pretty high cost in terms of code space and CPU load. This is how much the generated code grows by using a single float operation compared to using a long int:

Floating point operation approx. code size
addition 736 Bytes
subtraction 754 Bytes
division 673 Bytes
multiplication 907 Bytes
sinf() or cosf() 3346 Bytes
log10f() 3437 Bytes

The Arduino standard example '01. Basics/ReadAnalogVoltage' is a very simple simple program with only very little floating point arithmetics. But it already uses 7336 bytes of flash. A similar sketch using integer arithmetics results in much more compact code occuping only 3791 bytes.

Float does work, but is better to be avoided and replaced by fixed point arithmetics whenever possible.

Current status and to-do list

tested and working

WMath: map()
serialEvent() Print
SPI: working, no interrupt support
LiquidCrystal: Text LCD based on the HD44780 controller
PCD8544: Nokia 5110 type displays
Mini_SSD1306: Monochrome OLED displays based on the SSD1306 controller Stepper: Multi-instance design for more than one stepper at a time
Servo: Multi-instance design for more than one servo at a time)

implemented and partly working


tested, but not working


not tested


not implemented

module WCharacter
module WString

Unresolved problems

The compile environment needs to detect which interrupts are actively used and link only the needed ones into the binary. See test/digitalWrite: Compiling with the straight Makefile.classic does not add UART interrupt routines. But when using the Makefile the two UART interrupt routines are pulled into the binary by the interrupt table in main.c.

Why use a STM8 instead of an ATmega?

The fairly new ESP-14 module includes a STM8S003F3P6. Wifi and a programmable I/O-CPU for just over two dollars - that might be the most compelling reason to get started on the STM8S series. Apart from pure curiosity and eagerness to learn something new, of course.

The simple STM8S103F breakout boards are powerful and dirt cheap. They cost well under one dollar. You can get three boards and one flash programmer together for well under five dollars on , including shipping from China.

The major downside of this CPU series is the lack of information and community support for the STM8. The community support and the sheer number of existing libraries for all kinds of sensors and hardware is outstanding in the Arduino world. If you just want to get something done, go for an Arduino board. Nothing will give you faster and easier results.

For commercial use the STM8S offers some interesting advantages:

Motor control: The STM8 has a strong focus on motor and position control systems. Things you need to handle yourself on an ATmega are implemented in hardware and work independently of the state of the software. There is even hardware support for quadrature encoders as used in position sensors and rotary encoders.

Low power modes: The numbers in the datasheets don't look that different, but in real life the STM8 can be powered two or three times longer using the same battery capacity due to the finer control on the power modes (very, very careful programming required).

Value for the money: 40 to 60 cents for a STM8 with 14 I/O pins compared to $1.60-$3.00 for an ATmega8.

Upgrade path: The peripheral units of the STM8 are identical or at least very, very similar to the ones used with the STM32 family of 32 bit ARM-Cortex CPUs. This makes it is relatively easy to migrate existing software between the 8- and the 32-bit world. This is quite unique among the other CPUs.

Further reading and application notes

project documentation files More in detail information about supported boards, tools and the API.

[Quick introduction to the makefile] (

PM0051: STM8AF Flash programming manual
UM0470: STM8 SWIM protocol and debug manual
AN2658: Using the analog-to-digital converter of the STM8S microcontroller

Many examples and presentations about the STM8S:
It contains the SPL examples from ST, the most useful resource on the STM8:

Hardware and pinouts of several ST-Link compatible flash tools:

Using the ADC:

Example for RS-232 handling with SPL:

AN3139: Migration guideline within the STM8L familiy

Modifications for the sdcc example programs

blinky.c: LED pin assignment

uart.c: pin assignment (TX is at PD5, RX is at PD6).
The UART is sending at 1200 Baud => CPU clock only 2MHz instead of 16MHz. The clock divider needs to be configured or a different baud rate prescale value has to be used. Pitfall: The register address for the clock divider is different for the STM8S and the STM8L.

Performance compared with the original Arduino environment

Benchmarking the original Arduino examples from Arduino 1.0.5. The simple Blinky compiles to 57 bytes of code, the total binary including the sduino libraries is 1868 Bytes (0x74c).

So far, wiring_analog depends on wiring_digital, even when analogWrite is not used. This could be solved by compiling the sduino functions separately into a library.

Name Code Total Linked files other than main and wiring
01. Basics/
BareMinimum 2 1238 -
Blink 57 1870 wiring_digital
AnalogReadSerial 205 3452 digital, analog, serial, print
DigitalReadSerial 57 3160 digital, serial, print
Fade 226 2189 digital, analog
ReadAnalogVoltage float not yet implemented
02. Digital/
Debounce 192 2016 digital