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STM8 eForth (stm8ef)


STM8 eForth is an interactive Forth system for the full range of STM8 8-bit MCUs including the low-power families. The Forth console, with its interpreter and native-code compiler, turns a $0.20 device into a "computer" that even provides a simple multi-tasking feature. The Forth system allows interactive exploration of peripherals, parameter tuning or programming (including compiling code into running applications). Code examples can be used as a starting point (e.g. for using STM8 peripherals like I2C, ADC, PWM, RTC, etc).

The original STM8 eForth was written by Dr. C.H. Ting's eForth for the STM8S Discovery. With the kind permission of Dr. Ting the code presented here is under MIT license. Bugs were fixed, the code size reduced, standards compatibility improved and many features were added (e.g. compilation to Flash memory, autostart code, interrupt handling - see overview).

The binary release provides ready-to-run Forth binaries for a range of devices and target boards, STM8 register definitions and a library. Build- and test-automation uses the uCsim STM8 simulator in Travis-CI. The release also contains source code and libraries so that downstream projects can configure the STM8 eForth core, e.g. using the modular build support, to add new targets with specific hardware support, memory layout or a tailored vocabulary.


About Forth

Forth works by defining new words with "phrases" consisting of existing words - "Hello World" in Forth is this:

: hello ." Hello World!" ;

Forth is a "low level" language that offers a high level of abstraction:

Forth has no real syntax but good phrases look like Yoda-speak (e.g. 15 deg left servo turn-by or center right servo turn-to).

Data flows on the stack from one word to the next and in most cases there is no need for temporary variables. This leads to very good code density and it also simplifies testing. Even control structures in the compiler are just Forth words.

The best feature of Forth is that it allows interactive testing of words and phrases. This turns the embedded system into an application oriented test environment!

Forth is so simple that you can learn the basics in a snap, e.g. in the STM8 eForth Walk-Through.

About STM8 eForth

The STM8 eForth core is written in STM8 assembler but it uses the SDCC tool chain. Combining Forth with C is possible.

STM8 eForth is highly configurable: a Forth binary that allows compiling new words to Flash ROM or RAM requires less than 4K. A binary with an extended vocabulary needs no more than about 5.5K. Due to the extraordinary code density very-low-cost 8K devices, e.g. STM8S003F3P6 or STM8L051F3P6, are sufficient for non-trivial applications. If more space is needed a low-cost 32K device can be used, e.g. STM8S005C6 or STM8L052C6.

The Forth console uses the STM8 U(S)ART or a simulated serial interface for communicating with the console (3-wire full-duplex or 2-wire half-duplex are supported). For console access and programming e4thcom is recommended but any serial terminal will work. The console can be configured, even at runtime, to use other character I/O channels, e.g. keyboard and display.

The Wiki on GitHub covers various topics, e.g. converting low-cost Chinese thermostats, voltmeters or DC/DC-converters into Forth powered embedded control boards.

Release and development cycle

The Github Releases section contains binary releases. As STM8 eForth is based on eForth V2 (an embedded STC Forth), and it improves on STM8EF V2.1, releases follow the naming scheme "STM8 eForth 2.2.x".

Using a target binary requires setting a symlink target to the desired board target folder, e.g. ln -s out/STM8S105K4/target target (a make BOARD=<target> will do that for you).

The git master branch contains the current development version (releases are tagged). After cloning the repository make will build all targets.

Running make BOARD=STM8S105K4 will set a symlink. The Forth console prompt will show the release version (e.g. STM8eForth 2.2.27) or the next pre-release (e.g. STM8EF2.2.28.pre1).

Generic targets

STM8 eForth provides configurations and binaries for typical STM8S and STM8L devices. The binaries for selected "Low", "Medium" or "High density" can be expected to work for all of the listed packaging and memory "specs variants". For details please refer to the in the configuration folders referenced below.

Generic STMS targets

Support for STM8S devices in the RM0016 family is stable. Peripherals aren't as advanced as those of e.g. STM8L devices but that makes them easy to master. Automotive grade STM8AF devices in the same family can be expected to work. Various STM8 Discovery boards and breakout boards for "Low", "Medium", and "High density" devices can be used.

  • STM8S "Low density" devices (up to 1K RAM, 8K Flash and 640 bytes EEPROM)
    • STM8S103F3 for STM8S003F3/K3, STM8S103F2/F3/K3 and STM8S903F3/K3 (not recommended for STM8S001J3)
    • STM8S001J3 for STM8S001J3 (and STM8Sx03x3) with half-duplex UART_TX-RX
  • STM8S "Medium density" devices (up to 2K RAM, 32K Flash and 1K EEPROM)
    • STM8S105K4 for STM8S005C6/K6, STM8S105C4/K4/S4 and STM8S105C6/K6/S6
  • STM8S "High density" devices (up to 6K RAM, 32K + 96K Flash and 2K EEPROM)
    • STM8S207RB for STM8S007C8, STM8S207C6/K6/R6/S6, STM8S207C8/K8/M8/R8/S8, STM8S207CB/MB/RB/SB, STM8S208C6/R6/S6, STM8S208C8/R8/S8 and STM8S208CB/MB/RB/SB

Generic STML targets

Compared to STM8S most STM8L devices provide a much richer feature set and studying the reference manual may take more time. Although support for STM8L peripherals is relatively recent the STM8 eForth core is mostly the same as for STM8S devices and thus well tested. The latest addition is support for STM8L101F3 and STM8L001J3, the only members of the RM0013 family.

For more details please refer to the in the board folders below.

  • STM8L RM0013 family "Low density" devices (1.5K RAM, 8K Flash, basic peripherals)
    • STM8L101F3 for STM8L101F1, STM8L101F2/G2, STM8L101F3/G3/K3 and STM8L001J3M3
  • RM0031 family STM8L "Low density" devices (1K RAM, 8K Flash, 256 bytes EEPROM, advanced peripherals)
    • STM8L051F3 for STM8L151C3/K3/G3/F3, STM8L151C2/K2/G2/F2, STM8L051F3 and STM8L050J3M3
  • STM8L "Medium density" devices (2K RAM, 32K Flash, 1K EEPROM)
    • STM8L151K4 for STM8L151C4/K4/G4, STM8L151C6/K6/G6, STM8L152C4/K4/G4, STM8L152C6/K6/G6 and STM8L052C6
  • STM8L "High" and "Medium+ density" devices (4K RAM, 32K + 32K Flash, 2K EEPROM)
    • STM8L152R8 for STM8L151C8/M8/R8, STM8L152C8/K8/M8/R8 and STM8L052R8

Board support:

STM8 eForth provides board support, e.g. words for relay outputs or I/O with keys and LED displays, for several common development boards and "Chinese gadgets" like thermostats, voltmeters or relay boards. There is more information in the Wiki.

  • CORE "svelte" 4K configuration for STM8S "Low density" devices, some features are disabled (no background task, DO .. LOOP or CREATE .. DOES>). Also, the dictionary search is case-sensitive.
  • SWIMCOM 2-wire communication through PD1/SWIM (i.e. the ICP pin) and a full feature set (the similar DOUBLECOM also provides UART I/O words for applications)
  • MINDEV for the STM8S103F3P6 $0.80 "minimum development board" (just like the STM8S103F3 configuration but with a word OUT! for controlling the LED)
  • STM8L-DISCOVERY for the STM8L-Discovery Board (STM8L152C6 "Medium density" with LCD)
  • C0135 the "Relay-4 Board" can be used as a Nano PLC (Forth MODBUS support is available)
  • W1209 $1.50 thermostat board w/ 3 digit 7S-LED display, full- or half-duplex RS232 (some board variants, e.g. with CA LED displays, are supported). A W1209 demo application is available.
  • W1219 low cost thermostat with 2x3 digit 7S-LED display with half-duplex RS232 through PD1/SWIM
  • W1401 (also XH-W1401) thermostat with 3x2 digit 7S-LED display with half-duplex RS232 through shared PD1/SWIM
  • DCDC hacked DCDC converter with voltmeter
  • XH-M194 Timer board with STM8S105K4T6C, 6 relays, RTC with clock display, 6 keys with half-duplex RS232 through PD1/SWIM
  • XY-PWM PWM board w/ 3 digit 7S-LED display, 3 keys, dual PWM and full-duplex RS232
  • XY-LPWM PWM board w/ 2x4 digit 7S-LCD display, 4 keys, PWM and full-duplex RS232

The Wiki lists other supported "Value Line Gadgets", e.g. voltmeters & power supplies, breakout boards, and thermostats.

Targeting other boards

The binary release contains all files required for building a configured STM8 eForth, e.g. for a custom target board. The modular build repository provides instructions, a Makefile and an example "board folder". Other examples are in the GitHub repositories W1209, STM8 eForth MODBUS, STM8L051LED or XY-LPWM.

STM8 eForth Feature Overview

Compared to the original "stm8ef" STM8 eForth offers many new features:

  • a versatile framework for development based on Manfred Mahlow's e4thcom
    • board folder and include file structure for simple configuration
    • #include and #require for loading code into the STM8 eForth dictionary
    • mcu, target and lib folders provide a high degree of abstraction
    • ALIAS words for indirect dictionary entries (even in EEPROM!)
  • compile Forth to Flash ROM with IAP (In Application Programming)
    • switch between volatile (RAM) and non volatile (NVM) modes (remember to run RAM after NVM or else new definition will be lost after a reset)
    • autostart feature for embedded applications
    • RAM allocation for VARIABLE and ALLOT in NVM mode (basic RAM management)
  • Subroutine Threaded Code (STC) with improved code density that rivals DTC
    • native BRANCH (JP), and EXIT (RET)
    • relative CALL when possible (2 instead of 3 bytes)
    • TRAP as a pseudo-opcode for literals (3 instead of 5 bytes)
    • Forth - machine-code interface using STM8 registers
  • preemptive background tasks BG
    • INPUT-PROCESS-OUTPUT task independent of the Forth console
    • fixed cycle time (configurable, default: 5ms)
    • on supported boards ?KEY reads board keys, EMIT uses board display
    • robust context switching with "clean stack" approach
  • cooperative multitasking with 'IDLE
    • idle task execution while there is no console input with < 10µs cycle time
    • 'IDLE task code can run the interpreter with EVALUATE
    • STM8 Active-Halt can be implemented as an 'IDLE task
  • Low-level interrupts in Forth
    • lightweight context switch with SAVEC and IRET
    • example code for HALT is in the Wiki
  • configuration options for serial console or dual serial interface
    • UART: ?RX and TX! full-duplex w/ half-duplex option for STM8 "Low density" and STM8L RM0031 devices
    • GPIO w/ Port edge & Timer4 interrupts: ?RXP .. TXP!
    • half-duplex "bus style" communication using simulated COM port or UART
    • any GPIO or pair of GPIOs from ports PA to PE can be used to simulate a COM port
    • option for TX! .. ?RX on simulated COM port, and ?RXP .. TXP! on UART
  • configurable vocabulary subsets for binary size optimization
    • board dependent configuration possible down to the level of single words
    • ALIAS definitions for any unlinked words, also in the EEPROM
  • Extended vocabulary:
    • CONSTANT (missing in the original code)
    • 'KEY? and 'EMIT for I/O redirection (originally hard-coded)
    • CREATE .. DOES> for defining words (few eForth variants have it)
    • DO .. LEAVE .. LOOP, +LOOP (for better compatibility with generic Forth)
    • POSTPONE replaces COMPILE and [COMPILE] (the legacy words are available as build options)
    • ADC! and ADC@ for STM8 ADC control
    • BKEY, KEYB?, EMIT7S, OUT, OUT! for board keys, outputs or LEDs
    • LOCK, ULOCK, LOCKF, ULOCKF to lock and unlock EEPROM and Flash ROM
    • B! bit access and BF!, BF@, LEBF!, LEBF@ bitfields for little- and big-endian
    • [..]B!, [..]B?, [..]SPIN (and more) bit access words using STM8 code
    • [..]C! memory byte set using STM8 code
    • 2C@, 2C! for STM8 timer 16bit register access
    • FC!, FC@ for far memory access
    • >A, A@, A>, >Y, Y@, Y>, [..]BC, [..]CB for assembler interfacing
    • NVR, RAM, WIPE, RESET and PERSIST for compiling to Flash memory
    • 'BOOT for autostart applications
    • EVALUATE interprets Forth code in text strings (even compilation is possible!)
    • OUTER and BYE a simple debug console for foreground code
    • many words from Forth systems that were popular in the 1980s are provided in the library

Other changes to the original STM8EF code:

The code has changed a lot compared to the original code but porting back some bug fixes or features should be possible.

  • original code bugs fixed (e.g. COMPILE, DEPTH, R!, PICK, UM/MOD)
  • "ASxxxx V2.0" syntax (the free SDCC tool chain allows mixing Forth, assembly, and C)
  • hard STM8S105C6 dependencies were removed (e.g. initialization, clock, RAM layout, UART2)
  • flexible RAM layout, basic RAM memory management, meaningful symbols for RAM locations
  • a simple configuration system for new targets that gives files and settings in "target configuration folders" precedence over defaults
  • significant binary size reduction

Disclaimer, copyright

This is a hobby project! Don't use the code if you need dependable support or if correctness is required.

The license is MIT. Please refer to for details.