Interactive Development on Small Microcontrollers

Python on MCUs

Microcontrollers are usually programmed with C or assembler. The edit-compile-flash-debug cycle can get tedious. Hence the introduction of interactive programming environments like MicroPython and its derivative CircuitPython.

These MCU specific pythons allow you to have an interactive programming environment within the MCU you are targeting. The cost being the size of the python environment which makes it suitable mainly for higher spec’ed MCUs typically a 32-bit part with 256K of flash and at least 16K of RAM.

Python Pros and cons

1. Python is a easy to learn language.
2. Its strict identation requirements results in well-formatted code. However using tabs in place of spaces creates issues.
3. Python on an MCU requires 256K flash and 16K RAM or more.

Forth on MCUs

Python will not fit on the STM8S103F3 with 8K flash and 1K RAM. But Forth will, specifically STM8ef. That Forth takes up about 5K of flash and leaves you with most of the RAM unused.

Forth Pros and cons

1. Forth can be treated as an “interactive macro-assembler”.
2. Its post-fix, Reverse Polish Notation programming style makes it very unconventional and different. Hence a greater learning curve.
3. It can fit on 8K Flash and 1K RAM.

Getting Started with STM8ef

Hardware

1. STM8S103F3 or STM8S003F3 board. I built my own by soldering a STM8S103 TSSOP-20 chip to a breakout board. The STM8S003F3 is a slightly lower spec’ed device with the same 8K flash and 1K RAM but with lower EEPROM and flash durability.

2. A st-link V2 programmer. Clones are available from sources like AliExpress for about SGD 2.50 (USD 2). The st-link is only used to initially flash the STM8S103F3 with STM8ef. It is not required in the normal development process.

3. A USB to TTL UART. I used a CP2102 based module, but the cheaper CH340 based module should also work. This modules provides the 9600,8,N,1 serial link to the STM8S103F3 running STM8ef.

Software

1. stm8flash ( https://github.com/vdudouyt/stm8flash). Used with st-link V2 to flash STM8ef to the STM8S103F3 board.

2. STM8ef binary from https://github.com/TG9541/stm8ef/releases . Download the stem8ef-bin tgz or zip file, extract and locate the binary in out/MINDEV/MINDEV.ihx . MINDEV is short of minimal development board with is the STM8S103F3 or STM8S004P3 board. This ihx file is the binary in Intel hex format.

3. e4thcom from https://wiki.forth-ev.de/doku.php/en:projects:e4thcom This software is a terminal emulator like PuTTY (Windows) or minicom (Linux). e4thcom has additional capabilities to upload files, making it better suited to Forth development.

Step-by-step instructions

1. Build your hardware. Not necessary if you purchased a pre-assembled STM8S103F3 Minimum Development Board (eg from https://www.aliexpress.com/item/32859602851.html).

2. Connect your TTL UART to PD5 (TX, pin 2) and PD6 (RX, pin 3) on the STM8S103F3.

3. I wired an LED with a current limited resistor to ground from PA2, pin 6. The pre-assembled board will likely have an LED wired from PD3, pin 20.

4. Flash your board with STM8ef thus:

   stm8flash -p stm8s103f3 -c stlinkv2 -w MINDEV.ihx
   

   -p defines the part, -c the programmer, -w writes the file to the part.

5. Connect to the board with e4thcom thus:

   e4thcom -t stm8ef -d ttyUSB0
   

   -t is the target forth, -d is the device name of the USB to TTL UART, mine is at /dev/ttyUSB0 but other modules can also show on with device names like /dev/ttyACM0 .

6. Go through the tutorial at https://github.com/TG9541/stm8ef/wiki/STM8-eForth-Programming. Further resources at the main STM8ef github page at https://github.com/TG9541/stm8ef .