eForth for cheap STM8S Value Line gadgets

Turn cheap stuff from AliExpress into interactive development kits!

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Turn "Value Line" STM8S µC boards into Forth development kits! The code is based on Dr. C.H. Ting's eForth for the STM8S Discovery, an interactive Forth with kernel, interpreter, and compiler in 5.5K Flash, too much for very-low-cost STM8 Value Line µCs with 8K Flash. After a lot of tweaking, the demo-case fits in just 3.7K Flash.

New features are: compile Forth to Flash with variables in RAM, Forth interrupt handlers, background task, vectored I/O, board drivers for 7S-LED displays, analog and digital I/O, and enhanced vocabulary (DO..LOOP and CREATE..DOES>).

A simple configuration framework provides feature selection and support for new boards. The SDCC tool chain allows mixing C with Forth, e.g. as a shell for testing, setting parameters, or scripting.

Check out the docs in the Wiki, get cheap hardware, and enjoy interactive µC hacking!

What is it good for?

The project delivers configurable board support code for selected targets, and docs. Besides its modest size, the TG9541/STM8EF code has a long feature list. Using the code for embedded control applications is subject to new projects.

The code on GitHub can be used in many ways:

  • for writing alternative firmware Chinese commodity boards (e.g. thermostats, DCDC converters, or relay boards)
  • for embedded systems with an interactive shell (scriptable and extensible)
  • for creating smart SPI, I2C, or RS232 smart sensors with a scripting shell, e.g. for RaspberryPi, Arduino, or ESP8266
  • as an interactive environment for exploring the STM8 architecture
  • for learning Forth. It's easy and fun - find out why in the text below!
  • ...

Why a Forth for Cheap Chinese boards?

Because it's fun: cheap mass-produced imperfection is a playground for creativity :-)

Right now, the W1209 is my favorite target: it's a rather complete embedded control board with a UI at a very good price. It's as cheap as it looks, and the challenge is in it's imperfections: the guy who designed the board clearly didn't have a connected application in mind, and I had a lot of fun making it do things it was never intended to do.

There are challenges, like the lack of communication ports. The "sensor connector" can either be used for communicating, or for sensing. What if you need sensing and communication at the same time? Maybe the "update connector" can be used as a home brew field bus interface? A lot is possible with the right idea, and the right software!

Which target boards are supported?

Besides generic CORE target for STM8S003F3P6, there is currently support for the following boards:

I also ordered the following SmartClima control boards for tests:

@Elliot Williams worked on using the ESP-14 as an IoT deviced (the ESP-14 is an ESP8266 with an STM8S003F3P6 in a ESP-12 package).

Programmable power supplies based on the XH-M188, and a cheap DC/DC converter are work in progress. There are also several types of STM8S003F3 based voltmeters that can be supported.

Read more about likely future targets below.

Why Forth?

Again, because it's fun!

Consider this:

  • compared to other programming environments the core of Forth is easy to fully understand
  • like Lisp, Forth has a REPL (Read-Evaluate-Print-Loop) which enables software testing in a way impossible with "Edit-Compile-Run-Debug" (e.g. Arduino)
  • it's easy to build Domain Specific Languages (you can literally program the compiler!)
  • the stack-centered "factoring" approach provides implicit data flow which leads to maintainable code
  • Interpreter-compiler, basic OS functions fit in just 4K code :-)

Forth starts out as a stack machine with a tiny instruction set and minimal hardware requirements. It fits in a few KiB, and the target, even a lowly µC, can even be used as the development system. The Forth stack machine is a minimalistic VM on a standard CPU, but there are also hardware implementations (e.g. for FPGAs, or a 144 core Forth processor). The VM is ideal for fast context switching and Forth easily meets hard-real-time requirements. It's no surprise that Forth was used in many NASA projects.

A Forth programmer is in control of all levels of problem abstraction, a unique advantage in a world where layer on layer of 2nd hand solutions leads to ever growing complexity (compilers, libraries, operating systems, drivers,frameworks, IDEs... ). I'm convinced that "Thinking Forth" will make anybody a better programmer, not just in the domain of embedded control!

Why STM8S003F3 or STM8S103F3?


Read more »

The latest release is on GitHub. Release v2.2.0

Zip Archive - 18.94 kB - 12/04/2016 at 23:22


Original version of STM8EFalong with docs as received from Dr. C.H. Ting on 21/Nov/2016. The docs are worth reading, the eForth binary will run on the STM8S Discovery.

Zip Archive - 21.97 kB - 11/21/2016 at 20:13


View all 2 files

  • 1 × ST-Link V2 ICP adapter (e.g. $2.00 from AliExpress) The ST-Link on an STM8S Discovery Board can be used, too
  • 1 × serial interface /w 3.3V level (e.g. a $0.60 CH340 USB adapter) e.g a CH340 USB interface
  • 1 × STM8S target device as listed in the GitHub Wiki (e.g. a $0.70 "STM8S103F3P6 minimal system board") e.g. "STM8S103F3P6 STM8S development board" from your favorite China source
  • 1 × Some headers, patchwires, breadboard etc (about $2.00)

  • Bump to v2.2.8: new features, more boards, better

    Thomas4 days ago 0 comments

    ... and, of course, same binary size. Here is the text from the release page on GitHub:

    This release contains an important feature: VARIABLE and ALLOT allocate RAM in NVM mode, and support for a new board, or rather a nice little hack: a $1.60 programmable power supply.

    VARIABLE and ALLOT allocate RAM in NVM mode

    This feature adds transparent handling of Forth variables in for dictionaries in Flash, which means that also RAM allocation is handled automatically in the background. Please refer to the documentation in the Wiki section Forth VARIABLE in NVM mode, and to Issue #16 for technical details.

    New board DCDC

    Some cheap Chinese boards have surprising properties: the CN2596-2 "DCDC converter with voltmeter" can be turned into a programmable power supply (in the sense of the word). Please refer to the Hackaday project, and to Board CN2596 in the Wiki for details.

  • STM8 eForth Wiki updated

    Thomas5 days ago 0 comments

    From time to time it's necessary to revise docs, especially as a project evolves and features get more mature. It's also a good opportunity to check whether casual readers find with they expect (e.g. simple examples and some nice illustrations). Some of the pages were likely to frighten the horses, so to say.

    There is also a new sidebar, which makes finding some of the information easier.

    Please have a look at the STM8EF Wiki. If you find mistakes, or stylistic cruelties, please drop me a note.

  • New feature: in NVM mode VARIABLE just works

    Thomas02/26/2017 at 11:37 0 comments

    Issue 16 introduces "normal" operation of VARIABLE and ALLOT when compiled to non-volatitle memory. The feature also provides transparent RAM allocation.

    This may not sound not like much, but it's an essential implementation of the memory allocation by a "linker" in a C programming environment. To give you an idea of the problem, consider the following code:

    VARIABLE abc ok
    ' abc . 168 ok
    ' abc 5 dump
      A8  CD 83 54  0  0  0  0  4 64 75 6D 70  0  0  0  0  M_T_____dump____ ok
    abc . 171 ok
    12345 abc ! ok
    abc ? 12345 ok

    VARIABLE defines a word (abc) in the dictionary, which consists of code CALL DOVAR (CD 83 54) at address 168, and a RAM "CELL" (2 bytes) of storage at address 171. When the word abc is called, DOVAR places the address of the next memory location on the data stack and returns to the caller.

    From the programmer's perspective, a variable in Forth is much like an array definition in C. The word VARIABLE, as a "defining word" is akin to a C declaration, we'll come back to this later on :-). Just like in C, access to array elements works with address arithmetics (in C, square brackets are used as "syntactical sugar"). In Forth variables are accessed with words like !, @, C!, C@, and ?.

    An array with two bytes might not serve everybody. If more memory is needed (e.g. an array) it can be allocated with ALLOT.

    VARIABLE def 8 ALLOT ok
    VARIABLE ghj ok
    ghj def - . 19 ok
    def abc - . 11 ok

    The variables abc, def, and ghj use consecutive RAM cells follow in the dictionary. Because of 8 ALLOT the variable def gets 10 byte RAM instead of the 2 bytes of abc. The overhead of 9 bytes is due to 2 link, 1 string length, 3 string, and 3 CALL DOVAR.

    So far so good. But what happens when we're in NVM mode and create a variable?

    COLD ok
    NVM ok
    VARIABLE abc ok
    HEX abc . 9BD9 ok
    ' abc . 9BD6 ok

    The variable abc is now in Flash (which starts at 0x8000). Unfortunately, the memory location it refers to is also in Flash, which is no good place for a variable, to say the least.

    @RigTig provided a prototype implementation for VARIABLE with CREATE-DOES> that works in NVM:

    : VARIABLE CREATE HERE , 2 $6E +! DOES> @ ;

    This code is a bit tricky: variable is a replacement for the defining word VARIABLE, which works just like the normal one, except that it reserves memory in the RAM dictionary area when defining a new variable, and returns the address of that memory when using the variable, and $6E is the address where the next free location in the RAM dictionary is stored while STM8EF is in NVM mode.

    Forth features like CREATE-DOES> are akin to high level language concepts like inheritance, reflection, and prototypes. Extending a 5K programming system that's embedded in a 8K µC on the fly really is something else! RigTig demonstrated the power of CREATE-DOES> by implementing a useful approach to the "Flash/RAM problem".

    VARIABLE has been on my ToDo-list for months, and now had no excuse for not to sorting it out.. After some hacking, all the examples above work as expected, no matter whether you're in RAM or in NVM mode. Check out the preliminary docs and the preview code in Issue 16 on GitHub.

    EDIT: the code size is now to below 5000 bytes for the MINDEV target (80 bytes down!). The CORE binary size target is still 21 bytes above the self-imposed limit of 4096 bytes (VARIABLE formerly wasn't part of CORE due to its limited usefulness). I pushed new code to the branch variable on GiHub.

    EDIT2: the size of the CORE binary is now below 4096 bytes, and a minimal interactive system without NVM, interrupts in Forth code, and the new VARIABLE feature |is below 3700 bytes. The code has been merged from the feature branch variable to the develop branch, and there is a pre-release STM8EF v2.2.8.1.snapshot.

    EDIT3: @RigTig did some testing with the new code, and he told me that the results are encouraging (e.g. improved code size in a quite complex application...

    Read more »

  • W1401: board support "complete" (update)

    Thomas02/18/2017 at 22:39 0 comments

    The W1401 thermostat board has now initial STM8EF board support, and it's also better documented. There are maybe still some rough edges, but the things I tested work "as expected". It's now part of the "official" release v2.2.7.

    The following features are supported:
    • serial interface through PD1/SWIM
    • 7S-LED through vectored I/O (auto-off w/ time delay while serial interface active)
    • board keys through vectored I/O
    • relay/red LED, and green LED through `OUT!`
    • buzzer with programmable pitch (`BEEP_CSR` HW-register)
    • read sensor input with `5 ADC! ADC@`

    The analog input is surprisingly stable: there is just enough noise to get some extra resolution, but one still gets a stable readout down to the LSB without filtering!

    The programmable pitch of the BEEP output isn't good enough for playing music (unless you spend extra effort to tuning the RC-oscillator, or if you don't mind annoying people).

    The price for a W1401 starts from $2.60/piece (including shipping). It offers a lot of value for the money, especially if you're looking for a hackable board for SISO (Single Input Single Output) control (e.g. thermostat, solar heating, level control, charge control), or for monitoring/safeguarding applications!

  • STM8EF on the ESP-14: Hacking on the Weirdest ESP Module!

    Thomas02/17/2017 at 18:58 0 comments

      While I was on a business trip something left a spike on the visitors graph on GitHub: @Elliot Williams wrote about his ESP-14 with MQTT-and-STM8EF project on Hackaday!

      ESP-14 is an ESP8266 with an attached STM8003F3P6 in a package very similar to an ESP-12 module. Elliot used the ESP-14 for two reasons:

      1. the ESP-14 has more ADC inputs than the ESP-12
      2. a powerful ESP8266 bundled with a minimal STM8S running Forth is a crazy thing waiting to be done!

      The schematics around the ESP-14 looks like this:

      The ESP8266 connects the tiny STM8S003F3P6 to a Linux machine using JeeLab’s esp-link.

      The ASCIIcinema recording of a remote log-in via WiFi to an 8bit Forth, with a demo of interactive testing on the Forth console is well worth watching!

      Check out Elliot's article, it's great!

      PS: Elliot and me discussed other use cases for the ESP-14, like battery powered IoT devices. All the tools to make this happen are now available!

  • Boards and Docs

    Thomas02/11/2017 at 10:42 0 comments

    The core code in this project has reached some degree of maturity, and I guess it's the right time to shift the attention to board support code, and to docs.

    The following needs to be done:

    • publish contents from notebooks, and also from this log, to the Wiki on GitHub
    • restructure the Wiki without breaking links on external pages
    • use custom GitHub sidebars and overview pages to make the contents accessible
    • Enrich Wiki pages with diagrams, schematics, and photos
    • Create a simple HowTo with a shopping list, and instructions that require "zero-problem-solving".

    There is a page at GitHub that presents "Best Practice" for GitHub Wikis, but "Best" seems to stand for "use it for more than leaving an outdated boring Markdown wasteland", and "Practice" appears to mean "Improvise".

    Documentation in a uniform and manageable way calls for a CMS, but a good folder structure and templates will do the job. I like the method used in this Wiki: it's based on sub-folders with local sidebars and locally stored images. It also shows that it takes quite some discipline to make it effective.

    EDIT: I'm on a business trip, so no hacking :-{ Some of the docs on GitHub received an update.
    There are some more boards in the pipeline. XH-W1401 is half-ready, and I can start with XH-W1219 or XH-1701. In order to improve accessibility to information it's maybe better to create some more HaD sub-projects.

    My idea is:

    • simple control boards (e.g. thermostats),
    • voltmeters
    • power supplies
    • other boards here (unless they require more work)

  • Yet Another Voltmeter

    Thomas02/07/2017 at 22:44 7 comments

    After a day filled with business meetings | didn't want to risk killing the DC/DC converter for good, and I decided to look a little closer at one of the new arrivals from China, small and cheap voltmeter ($1.12 including shipping).

    Repetitio non placet they say, but this voltmeter turned out to be really nice:

    The LED display 3361BS is "0.36", red, common anode". The internal supply voltage is a bit unusual (3.0V), but otherwise the module is very hackable.

    The 6 pads with 1.27mm pitch to the left are connected to the following signals:

    1 SWIM (7S-LED segment D)
    2 TxD
    3 RxD
    4 NRST
    5 +Ub
    6 GND
    The STM8S003F3P6 pins are assigned as follows:
    STM8S003F3P6assigned toSTM8S003F3P6assigned to
    1 PD47S-6 (NC!)20 PD37S-5 G
    2 PD5Pad2 TxD19 PD27S-4 C
    3 PD6Pad3 RxD18 PD1/SWIM7S-3 DP, Pad1 SWIM
    4 NRSTPad4 NRST17 PC77S-2 D
    5 PA1
    7S-9 Digit 216 PC67S-1 E
    6 PA2(7S-9 Digit 2)15 PC57S-7 B
    7 VssGND14 PC4/Ain2TP (535k-Ain-8k26-GND)
    8 VcapC13 PC37S-12 Digit 1
    9 Vdd+3.0V12 PB47S-10 F
    10 PA37S-8 Digit 311 PB57S-11 A

    I guess that the designer was bored by making yet another cheap Chinese voltmeter, and he decided to make a hackable voltmeter instead. If not, why would one expose not only SWIM/NRST but also RxD and TxD? Why route a spare GPIO to the (unpopulated) NC pad of the LED display?

    Hat-tip to China!

  • First STM8EF application project on HaD

    Thomas01/30/2017 at 10:29 0 comments

    In this project log I wrote several times about cheap Chinese DC/DC converters with a STM8S based voltmeter that can be converted into something much more interesting: a programmable power supply.

    Yesterday I started a "HaD Downstream Project" to track the progress with this board. The new project isn't fully independent, as it will contribute requirements to this project (downstream is a bit misleading - in a development process it's actually one integration level up).

    The first requirements are:

    • all aspects of LED_MUX should be handled by the board support code
    • assigning RAM to board support code should not not require changing forth.asm

    The second requirement is trivial to solve.

    I solved the second requirement with a "RAM pool" and a set of macros, and now I wonder why it took so long to do such an obvious thing. The disadvantage of automatic RAM allocation is of course that addresses now depend on the configuration (for the application there is no problem since a Forth word can export the address). For debugging a look into the list file is necessary. I guess I sub-conciously re-applied 80s embedded control practice: the RAM address sheet. Meh.

  • Bump to v2.2.6: new features, same binary size, more user code

    Thomas01/28/2017 at 22:49 0 comments

    I just released STM8EF v2.2.6 with some really nice features from the development branch:

    There was also an important bug fix for DO LEAVE LOOP/+LOOP (LEAVE actually works now, and it's possible to use negative increments).

    Thanks to new coding techniques, e.g. TRAP for literals, the binary size still is within the same bounds as before:

    • interactive Forth (the use case of the original STM8EF code) < 3800 bytes,
    • CORE ("compile to Flash", interrupts in Forth) < 4096 bytes
    • MINDEV (DO-LOOP, CREATE-DOES>, board I/O, background tasks) < 5000 bytes
    • W1209 (7S-LED display, COM-simulation, board keys) < 5500 bytes

    The code generator improvements (relative calls, STM8 opcodes for literals, branch and exit) reduce the size of compiled user code by 20-25%. This should be sufficient for 5 to 10 screens of Forth, enough for non-trivial applications.

    Edit: with this release some of my personal goals were reached, and it's some kind of milestone. I took the opportunity to update the project's description and details. Sorry for the update spam :-)

  • Coding Interrupt Handlers in Forth

    Thomas01/26/2017 at 21:29 0 comments

    @Elliot Williams convinced me that writing interrupt handlers in Forth instead of assembly (or C) is a Cool Thing. I had pondered a lot about how to do that with the least amount of overhead but it took a lengthy discussion with Elliot to get it right!

    There was a problem to solve: the STM8 register X is the Data Stack pointer, and it's also needed for implementing certain core words. The assumption that "X is TOS" isn't always justified. There is no way around this, except by blocking interrupts, or by rewriting code so that X always represents a valid stack pointer (with undesired effects like increased code size or longer runtime).

    My "the most simple thing that works" solution assigns a small clean data stack to user-defined interrupts. The data stack is just 8 cells deep, but that should be more than sufficient (please read below why).

    Interrupt handlers in Forth code are now based on the following words:

    • SAVEC to save the context
    • IRET to restore the context and return from the interrupt
    • IVEC! to set an interrupt vector

    Writing an interrupt handler is easy - here is an AWU (Auto Wake Up) handler as an example:

    : IVEC! ( a n - -   ) 2* 2* $800A + ! ;
    : HALT ( -- ) [ $8E C, ] ;
    : awuint SAVEC awu_csr1 c@ IRET ;
    : initawu 38 awu_apr c! 1 awu_tbr c! 16 awu_csr1 c! ;
    ' awuint 1 IVEC!

    The interrupt handler awuint first does a Forth VM context switch with SAVEC. Reading awu_csr1 clears the AWU interrupt flag, and IRET restores the Forth VM context and returns to the interrupted code with IRET. It's not necessary to leave the stack balanced (otherwise DROP would be required). The word IVEC! stores the address of our new hander to interrupt vector 1 (the AWU interrupt in Flash memory). Since IVEC! is only needed at "compile time" it can be compiled to RAM and doesn't need to be part of the Forth image.

    The word HALT encodes the STM8 HALT instruction that shuts down the CPU clock until an interrupt occurs (I first added a word HALT to the Forth core but this user code implementation is just as good). When I run HALT with this code, it returns because of the Auto Wake Up interrupt. Note that I didn't find the time to make sense of the AWU configuration, and I simply took the AWU timing values from this page.

    When writing interrupt handlers in Forth, I would like to recommend the following practice:

    • interrupts should only be used for low-level code, e.g. for using µC peripherals that require interrupts to work
    • be careful about data- and return stack use - 5 levels deep should be plenty!
    • the code should be fast - if in doubt use pin-debugging with a scope or a simple logic analyzer for testing the timing
    • only do the data processing that's absolutely required for meeting your application's timing constraints. The rest should be done in a low-priority task (e.g. background)
    • simplicity is important: don't use any fancy high-level words (e.g, output string formatting shouldn't be used)
    • one should be carefully assess potential side effects of character I/O (if required!)
    • in many cases you'll need the µC manual to understand what your code is doing, and fancy abstractions won't make your code more readable

    Working code with SAVEC and IRET is in the develop branch on GitHub. I'm still working on some details but it will be part of the next release.

View all 59 project logs

  • 1

    Get some cheap hardware (e.g. a STM8S103F3P6 breakout board for $0.65 and a ST-Link V2 dongle for $2). download the binary release, flash it, and have fun!

    If you like it, and you want to hack board support code for your favorite STM8China gadget, you need:

  • 2

    a Linux SDCC tool chain installation (installation instructions for SDCC & stm8flash are in the Wiki)

  • 3

    Clone the project on GitHub

View all 4 instructions

Enjoy this project?



RigTig wrote 02/27/2017 at 21:59 point

Thomas mentioned that a better file loader would be nice. Here is my attempt. Simple to start with, but obviously capable of being expanded with features later. It is in Python2 and runs from the command line of the host machine (mine is LinuxMint).


#!/usr/bin/env python2

import serial
import sys
import time

port = serial.Serial(

if len(sys.argv) < 2:
    print('Usage %s ... [fileN]' % (sys.argv[0]))

def upload(path):
    with open(path) as source:
        for line in source.readlines():
            line = line.strip()
            if not line: continue
            if len(line) > 64:
                raise 'Line is too long: %s' % (line)
            print('\n\rsending: ' + line)
            chin = ''
            response_buffer = []
            while chin <> '\v':
                while port.inWaiting() > 0:
                    chin =
            response = ''.join(response_buffer)

for path in sys.argv[1:]:
    print('Uploading %s' % path)


Usage: Save this code as a file (say named and change its permissions to be executable (just the lines in between the code tags). I put in my local /bin folder. Edit so the port matches what you use when using a terminal console to connect to STM8 machine.

WARNING: I've just noticed that the indentation was inconsistently displayed, and python is indentation sensitive. So be very careful with just copy-and-paste. I'll put a copy of it up on RigTig's Big 3d Printer project here on

Either put FILE on first line of the file to be sent, or type it into a terminal console and close it, then use a local command line interface thus: <code>  filename file2send </code>. Enjoy!

  Are you sure? yes | no

Thomas wrote 02/27/2017 at 22:26 point

Hi RigTig, this is cool! Now the last reason not to learn Python (the haploid language, as I use to say) has gone. I'm going to need it in my job, anyway ;-)

PS: I just added the tag v2.2.8.1.snapshot, and this means that the binary size of CORE is below 4096 bytes, complete with the new "transparent" VARIABLE feature

  Are you sure? yes | no

Thomas wrote 03/04/2017 at 11:07 point

I just tried your script - the handshake seems to work, and compiling code to NVM is very fast compared to the "worst case delay" method!

However, I had to interrupt the script with ctrl-c after the transfer was finished. The reason was that my Forth code ended with "HAND", after which waited in vain for the handshake signal.

I can imagine that an improved uploader does the following:

* handle FILE and HAND (no need to include those in source file), or

* terminate transfer when the response is anything but the handshake character

The next thing on a programmer's wish list is a way for including source files (nested, of course). I can also imagine testing if "base code" has already been transferred, e.g. using some query-response between the Forth system and the uploader.

  Are you sure? yes | no

Thomas wrote 02/27/2017 at 22:26 point

Hi RigTig, this is cool! Now the last reason not to learn Python (the haploid language, as I use to say) has gone. I'm going to need it in my job, anyway ;-)

PS: I just added the tag v2.2.8.1.snapshot, and this means that the binary size of CORE is below 4096 bytes, complete with the new "transparent" VARIABLE feature

  Are you sure? yes | no

RigTig wrote 02/23/2017 at 05:44 point

Thanks Thomas for a great environment in which to have lots of fun. I needed VARIABLE to be defined in NVM, but to keep its data in RAM. Some variables need to change for every data line processed (megabytes of GCODE), so using NVM is just not going to cut it for real use. Besides NVM access is slow. So, here is my replacement definition for my project:

: variable create here , 2 $6e +! does> @ ;

Now, this works only in NVM mode, because it makes no sense to use it in RAM mode anyway. Besides the DOES> part would be wiped every COLD or restart. The magic address of $006E is the address of next available RAM when in NVM mode.

If you want to test after a restart or COLD, you need to adjust the address of next available RAM to be after the last used address by a variable. In my case, the last used address was $009e. Note that in RAM  mode, the address of next available RAM is at $006A. So, I typed

$a0 $6a !

and then initialised all variables and they just work. Now this hack is not for every project (obviously), but does show what can be done when a need arises. Enjoy!

  Are you sure? yes | no

Thomas wrote 02/23/2017 at 21:35 point

Hi RigTig! That's indeed a nice hack, and it is much along the lines of what I had planned.

I'd like to propose the following solution: 

1. some memory below the user dictionary in RAM shall be set aside by adjusting the reset value of USRCP
2. the next variable address for NVM routines shall be stored in the (new) variable USRVAR, which shall be initialized from USRCP
3. when switching from NVM to RAM the reset value of USRCP shall be set to the value of USRVAR

I'd like to check if it's possible to make VARIABLE work transparently in NVM and RAM mode. Most likely writing a different word is easier.

  Are you sure? yes | no

RigTig wrote 02/24/2017 at 04:09 point

Thanks for compliment. I haven't really tried to make VARIABLE work in both RAM and NVM, but I am sure it'd work. The key is just understanding that there is one level of indirection (address of value instead of value), so RAM variables use an extra 2 bytes over the non-indirect version. Nice to avoid wasting ram, but not really a show-stopper. Coding in assembler should be far more memory efficient than the Forth version, but it's the joy of Forth to be able to do these kinds of things at all (and optimise later when you find that it is really a good idea!).

A hard reset or even COLD needs to preserve the ram space needed by variables, but I prefer not to lock in a pre-determined limit on the number of variables. I also hate wasting valuable resources by committing them for just-in-case scenarios. 

So let's consider the use cases. Is there a need to support programming to NVM, then RAM, and back to NVM? If we say that all NVM variables need to be defined before RAM gets any code, is that reasonable? At least all the ram needed is in one block in this case. Probably a bit hard to communicate to programmers, and practically impossible to enforce.

Even if there is some code compiled into ram before or after NVM variables are created, the only thing needed is to set the ram space used for parsing commands to be above the last used ram for any variable. This happens anyway until COLD or hard reset. Maybe all that is needed is a persistent vector stored in NVM to be used instead of the $0080 for start of ram space for code and variables. Now the incentive is for the programmer not to waste space, so it becomes 'obvious' that defining all NVM variables before using ram for anything else is just better management of the limited ram. VARIABLE needs to update the persistent vector each time, based on current ram pointer. Variables defined in ram waste space after a COLD, but that might just be a price to pay (and is quite ok during interactive development, methinks). Mmm... and RESET needs to reset the vector to first available ram for code and variables back to its compilation default ($0080).

I am sure to have missed something in this ramble, but hey, that's what hacking is about isn't it? If I knew what I was doing, then it is not real hacking! And what other language allows you to play around with how the language itself works, so Go Forth.

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Thomas wrote 02/24/2017 at 06:41 point

RigTig, we're on the same page :-) 

If you read me previous comment carefully you'll find that the "machine" needed for implementing is mainly coded in the difference between "the reset value of USRCP" and "USRCP" (there is one error though: point 2. should be "in the new variable USRVAR which shall be initialized from the reset value of USRCP"). 

One could argue that this means waste of RAM, but actually it's just a buffer for certain use cases. You already mentioned some uses cases, and how much they would "surprise Joe Programmer".

My model for the programming workflow is this:

1. start a session with COLD, reset, or flashing the µC

2. write some test code in RAM (i.e. do the things you'd normally do with the original STM8EF)

3. run COLD and set the stage (e.g. define helper words like here:
4. run NVM, define words, variables etc as you like

5. run RAM, make pointers to the newly defined words, and also to USRVAR persistent

6. return to 4. (write more persistent code) or to 2. (test your code, preferably automated)

Of course, the casual user may miss the finer parts of the "Stage/NVM/RAM/Test/COLD" cycle, but they will notice quickly that words defined in RAM can't be compiled-in (only interpreted) in NVM, and that code compiled in NVM is lost if they forget to run RAM.

As you pointed out, a certain coding style, like defining variables first, isn't difficult to get used to. Setting aside a small buffer (e.g. 32 bytes) as a variable space wouldn't be a big deal, and it would enable the "setting the stage" use case without the risk of immediately overwriting words like IVEC. It's of course also possible to cycle through the steps 2..6 more often (also as a part of the source code) , but a buffer would add some flexibility. In the extreme case (use many variables, use a huge stack), writing test code in RAM would suffer. But hey, when testing words one tests the units, not the whole program where the stack reaches its maximum size.

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Thomas wrote 02/26/2017 at 09:35 point


Please have a look at this:

The new code has the features discussed above. The behavior of VARIABLE and ALLOT is transparent in NVM mode, and in most cases there is no need to manage RAM allocation. Only if one allocates more than 32 byte RAM in a session it's necessary to cycle through COLD before using variables newly defined in NVM mode.

Edit: preliminary documentation of the new feature is here:

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RigTig wrote 02/27/2017 at 01:08 point

Thomas, I have to say that your approach is just brilliant. I love the idea of being able to just get more variable space if needed, and not wasting any ram either. 

P.S. We might be both on the same page, but I'm only partway down. I am still getting my head around the STM8EF code. I keep going back and changing the options for a new flash image and it is installed in a second or two. What fun to play with!

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Thomas wrote 02/27/2017 at 07:17 point

@RigTig: thanks for your support! I just pushed a "size reduced" revision to the variable branch. Since I had to do some shuffling, some "review" and "testing" by "an independent person" would be great (in a hobby project that's what's known as "playing with the code" :-)

Currently some RAM gets wasted, but that could be bettered by giving the programmer control over the headroom for RAM allocation (which would be easy).

By the way, I ordered two of the radio modules you've been working on.

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Elliot Williams wrote 01/23/2017 at 08:55 point

Hiya Thomas,

Got an ESP-14-powered device up and running and installed in our basement.  Long story, must write up.

Have you played around with power saving modes on the STM8?  I'm trying to get the part into the AWU / active-halt mode.  

For one, I need the assembler's HALT command, which I've been doing in the worst brute-force means possible: HERE $8e81 , EXECUTE.  (That's HALT and RET in machine code.)  

It halts, at least.  :)  

Coming back out of halt is messy -- it looks like the clocks aren't returned to their original states and so on. I'm probably going to need to implement some start-up code.  Heck, for my purposes, hooking into COLD for a complete reset will work too... That's what I'll try next.

Just wondering if you've worked on any of the low-power modes.  Either WFI (wait-for-interrupt) or the active-halt/AWU look tasty.

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Thomas wrote 01/23/2017 at 19:14 point

That's great :-)

The power saving modes (like the watchdog) still are on my "important things that I plan to do" list. You know, that's the list on the sheet after "new and exciting things I want to play with", which in turn comes after "bugs I must fix now".

Let's put it on the "important new features for pilot applications" list :-) 

What we need is:

* a word HALT that contains the HALT instructionknow

* a word SLEEP, that stops unnecessary interrupts (user defined, and application specific). This word should run HALT. When the execution continues right after HALT, SLEEP shall re-enable "waking" interrupts

* if required a word to restore clock settings (RM0016 mentions something in 10.2.2 and in 9.9.4 "Clock master switch register (CLK_SWR)", but right now I don't undertsand why the clock changes)

Do you plan to trigger a wake-up through console events? The simulated COM port should support this use case!

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Thomas wrote 01/23/2017 at 19:45 point

I added the HALT word, and it works better than expected. Here is a demo with a blinky:

    : g tim 40 and 0= out! ; ok
    ' g bg ! ok
The when I press enter after HALT the LED stops flashing. The "ok" after HALT appears after I press enter a second time.

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Elliot Williams wrote 01/24/2017 at 11:52 point


re: clocks: I read something somewhere sometime about them needing a reset.  I can't find that anymore.  I may be crazy.  

I saw some other STM8 code ( that runs the AWU without re-clocking, strongly suggesting that I'm crazy.

That code, though, makes it look like (if interrupts are enabled) the AWU reset lands in the AWU ISR, which is uninitialized ($0000) in the vector table at $800C.

I just ran your BG example above, and it halts, but never returns until hit with a hard reset. I wonder if your code is working b/c it NOPs off to the next ISR and you got lucky.  Or does it actually try to execute whatever's at $0000?

So: how do we set up ISRs in eForth?  (Or, how do you write bytes directly to flash?)

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Thomas wrote 01/24/2017 at 19:23 point

TL;DR: the quick-fix: an AWU "driver" that does it all but I would prefer a Forth solution and this requires some design decisions.

Long version:

Due to limitations in the SDCC tool chain any interrupt must be declared in main.c. Writing ISR vectors to Flash might work, but it requires a good approach for registering (and unregistering) interrupts to be viable (I'm thinking of RESET). Also Forth VM context switching would have to be done before executing any Forth code.

Another approach would be a "catchall" interrupt handler for several interrupts that then redirects to Forth code. This has the advantage that the context switch can be handled in a uniform way, but the dispatching won't be very efficient (or again a lookup).

This brings us to the next problem: some interrupt sources require resetting some bit in some peripherals control register. Leaving that to user code is very error prone, and a "catchall" interrupt handler would have to do it for all possible sources or leave it to user code.

What do you feel about of a middle way?
* Interrupt handler declared in main.c
* basic handler code in assembler or c to do a context switch, and to clear the trigger source
* handler code in Forth registered through something like BG

A last point: how many concurrent "Forth code interrupts" can we allow?
* Level0 we have the console
* on Level1 is the BG interrupt
* on Level2 is TIM4 (for COM simulation)

I guess that some stuff like TIM4 shouldn't have to compete with other code (the current code is efficient as it gets). Most likely it's possible to drop the interrupt level in BG code to Level0, and use Level1 for Forth handlers without character-I/O. The latency would still be in the lower µs range.

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Elliot Williams wrote 01/25/2017 at 12:43 point

How does the 'BOOT mechanism work?  If you could do placeholders for the various ISRs like that, the user could write their handler function and store its address in the right place?  That seems very Forthy to me.  <code>: awu-isr stuff ; ' awu-isr ISR_AWU ! </code> or something.  One of these functions / memory locations per IRQ and you'd be done?

On resetting the flags as you leave the ISR: I think that should be user code rather than bloating up the system with it.  Yeah, it's going to hang the system if you do it wrong.  If I could count the number of times I've pressed the reset button...

On context switching in ISRs:  I'm not sure I understand the full details.  Unlike C, there's not necessarily any context to switch?  If the ISR maintains stack balance then there's no need for any context?  Leave whatever's on the stack, and it'll still be there when the interrupt is done?

For me, personally, I'd just be stoked to have a pointer to an address that I could set to execute when the AWU IRQ fires.  The rest, I can handle in code, I hope. :)  (Assuming that the return from interrupt works right.)

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Thomas wrote 01/25/2017 at 20:20 point

'BOOT is simple: it returns the address of the "Parameter" field (like DOVAR). To safe code I used it to get the address of the whole following table of initialization values for USR variables. After switching to "NVM" it's possible to simply overwrite all these values. There is a 2nd copy to restore these values, e.g. to "forget" user vocabulary in Flash memory with RESET.

Yes, the 'BOOT method can be used in for interrupts, too, but that would require one more level of indirection.

About context switching:

my first approach was to re-use the Data Stack, but I quickly learned that X isn't always a valid Data Stack Pointer: it does that at the start and the end of a word, but not always in between ("always" is a very important attribute when writing interrupt code). Before implementing the background task, I tried  to make sure that X always represents a valid stack pointer in all primitive words. However, I failed to get it working until I started using a 2nd Data Stack for the background task (which I didn't like since it appears wasteful). Later on, I applied coding techniques that use X for reducing code size. Of course, it's possible to re-factor the code. It would be interesting to compare other multi-tasking Forth implementations. 

I went in a different direction: In several refactoring rounds I removed the following variables entirely: TEMP, XTEMP, PROD1, PROD2, PROD3, CARRY, and I also made the I/O context leaner.

Now, for code without character I/O only YTEMP must be saved. Otherwise also BASE, PAD, and HLD must be taken into consideration. And, of course, we need a stack. One approach would be to have a floating "stack pad" to work around the "X!=TOS" problem.

I guess it will take some time to implement a full featured solution for Forth interrupt handlers.

A minimal solution might look like this:
* a word IVEC! to set an interrupt vector
* a word SAVEC to save the context
* a word RESTC to restore the context, ends with IRET

The application could then define a word in the following way:  

: handler SAVEC ( some stuff ) RESTC ;

 ' handler 1 IVEC! \ set the AWU interrupt handler

Now that I'm looking at it, this doesn't look too bad.

Edit: I made some corrections, added some details, and added one more option for a solution

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Thomas wrote 01/25/2017 at 22:23 point

I added the solution above for testing to the develop branch. Due to the mentioned limitations it's currently necessary to initialize the interrupt to priority low (0:1) (it shares the data stack with the ticker).

I also changed TIM4 to prio "highest", which might allow to implement all user defined interrupts with priority "high" later on. This would then require 3 data stacks with the sizes normal (console), medium (background task) and small (interrupt handler).

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Thomas wrote 01/26/2017 at 20:48 point

@Elliot Williams:

Here is a starting point for Forth code user interrupts and AWU usage:


  : awuint savec awu_csr1 c@ drop restc ;

  ' awuint 1 ivec!

  : initawu 38 awu_apr c! 1 awu_tbr c! 16 awu_csr1 c! ;


When I run HALT with this code, it returns immediately. Since I didn't find the time to make sense of the AWU configuration, I simply took the AWU timing values from the page you mentioned before.

Please not that this currently only works when I run HALT from the console (I still need a solution for the Data Stack problem). Running HALT from the background task would change the contents of the first element on the stack (which would work if the stack were empty).

A quick fix here is to assume that X represents TOS when HALT is executed (which is the case), and skip initializing the stack. Please note that this only works for HALT, and not in the general case.

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Thomas wrote 01/23/2017 at 21:51 point

Changes are in the develpp branch on GitHub. The 2.2.6.snapshot release contains new binaries :-)

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Youlian Troyanov wrote 01/26/2017 at 04:51 point

please write your long story about esp-14 :)

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Thomas wrote 01/27/2017 at 22:19 point

Elliot, in order to get a simple and practical solution, I now propose the following:

1) In RAM code IVEC! (its only used once for setting an interrupt handler

: IVEC! ( a n - -  ) 2* 2* $800A + ! ;

2) Implement HALT as a user word:

: HALT  ( -- ) [ $8E C, ] ;

3) Implement your interrupt handler using SAVEC and RESTC (make sure not to use more than 8 cells on stack)

This will work for any interrupt. Please make sure to change the interrupt down from highest to high.

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jaromir.sukuba wrote 01/23/2017 at 05:17 point

Another tip for *possible* STM8 target

I didn't buy this one, haven't seen the schematics, but to me it totally smells like it could have STM8 under the display. Googling for XK-001T-1 didn't bring much info, though.

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Thomas wrote 01/23/2017 at 06:18 point

Yes, that's possible. In most cases one won't find any schematics, and also the XH-, XK, M- or B monikers aren't always used the same. There is a small list of modules that are very likely STM8S based in the Details Section of this project (in the section "How can I spot suitable boards?"). If there is any interest, I can publish a list with advertised properties and the "street price".
Edit: here is a link with a picture showing the PCB legend:
Based on the outline of the µC I would expect it's not STM8 but a STC15 based, a µC which I've seen several times on "timer" boards (MCS51-like

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Elliot Williams wrote 12/13/2016 at 23:13 point

Got my ESP-14 up and running last week, and then got distracted.  :)

Short story: it's just a STM8 chip and an ESP8266, like it says on the package.  The TX/RX lines are internally connected, so I was running your Forth on the STM8 with the ESP8266 powered down, and running all manner of software on the ESP with the STM8 powered down.

Been thinking about how to use both at once. 

a) Jeelink is a nice transparent serial port over ESP8266, which would provide remote wireless development of the Forth system on the STM8.  The idea of telnetting over WiFi into an STM8 is funny enough that I'm definitely going to do this.

b) Since the serial port is the only way in to the ESP8266, and the STM8 has only that one hardware serial port, I suppose that bit-banged serial or I2C/SPI could be used to talk to the console. I don't know how hard/easy this is. But then you'd have an STM8 that could issue AT WiFi commands, for instance, or run routines in NodeMCU, which might be very cool.

c) The other option is to code up the ESP and STM8 to take turns based on control characters: 0xFE toggles the ESP on/off the line, and 0xFF toggles the STM8, for instance.   This requires modifying _both_ firmwares, but would allow for the console, ESP, and STM8 to share the UART lines and talk to each other.

Just brainstorming so far. No real hacking yet. 

The breakout board I made for the module just fit it onto a breadboard, because I didn't really know what to expect from the module. It will probably want a transistor so that the STM8 can turn off the ESP8266 for power-saving when necessary, and will certainly want at least a jumper for flashing the ESP.  

Thanks for the case insensitive addition, and for do loops! This is a fun system to play around with.

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Thomas wrote 12/14/2016 at 19:31 point

Options a) and b) look good to me, especially in combination. How about connecting a PNP transistor for the ESP8266 power supply to PD1/SWIM? Normally one would access the STM8 serial port through ESP-Link, and the ICP interface could be used for direct access to the ESP8266 serial interface by simply pulling down both NRST and PD1/SWIM. Direct serial access to the STM8 could be acchieved by telling it to power the ESP8266 down (this might even work using PD1/SWIM once more, e.g. by using an RC element which can be detected testing its timing).

Option c) would also be possible, but at least one of the devices would have to be able to swap RxD and TxD, and the other devices would need a "tristate" mode on TxD. The Bus approach I took for the W1209 might also work for more than two devices.

A fourth option could be to have a Forth word that issues the initialization AT commands on the STM8, and execute it with 'BOOT.

I hope to find the time for some hardware hacking in the holiday season :-)

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Elliot Williams wrote 12/16/2016 at 21:22 point

"esp-link" not jeelink.  Tried it and had a telnet / web-console controllable STM8 running your Forth.  Took like 10 minutes.

Then I spent 3 hours trying to implement something like c) in NodeMCU.

First, I thought I'd set up two TCP connections: one for the ESP to be executed locally, and one to pass through to the STM8.  Didn't work b/c NodeMCU can only do one TCP connection, it seems.

Then I thought I'd use MQTT as the transport mechanism.  But there's some glitch there with MQTT and the UART port not working right.  I'll hack more at it before I give up, but it might be time to move on to MicroPython or ESP Basic for the interactive ESP part.

Anyway, try out the esp-link for the ESP when you get around to it.  It's kinda fun.  It _does_ however leave me wanting a more capable microchip on the remote end.  For another couple bucks, I could get a lot more flash, peripherals, and etc to tether to the ESP.

All of this playing around has helped me refine what's needed in a breakout board for this thing, though.  :)

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Thomas wrote 12/16/2016 at 22:35 point

Again great news, and I'm going to test esp-link too. Multiplexing communication through MQTT topics was the first thing that came to my mind. About a year ago I tried working with MQTT and NodeMCU, but I was disappointed with the stability of the platform (though I really liked working with Lua).
I guess that the case for ESP-14 is rather thin: as I mentioned before, it looks more like proof that the ESP8266 wasn't able to meet customer requirements than like the solution the world's been waiting for. But who cares as long as it's fun hacking.
In my opinion, a decent Forth environment on the ESP8266 would be rather attractive: C.H. Ting hacked something recently, but it was just the kernel, not a complete framework with persistent vocabulary (and maybe even with source stored in the Flash memory, and maybe even a JavaScript based IDE served from an embedded web server on the chip).

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Thomas wrote 11/28/2016 at 21:33 point

Hi Elliot, it's great to hear that someone got it running, and that the docs were good for a smooth start. Anyhow, congrats for the "STM8EF Blinky"! Did you try to do that BG style, too? 

I had a look at the CAPS issue (yes, I've been thinking about that for a while ;-) ). There are some potential clashes (e.g. PARSE/parse, NEXT,next, ABORT"/abort") but the lowercase words are the hidden "implementation part", and I don't see that their name is set in stone. I decided to name them after their assembly labels (pars, donxt, and aborq). 

New code with lowercase support is on GitHub (just set CASEINSENSITIVE = 1 in If you'd like to give it a try without building, please let me know (I can drop a binary into the files section here). If there are no issues I'll make it the default.

The ESP-14 will be one of my next targets. However, I didn't find the time to make a breakout PCBs with power supply for this module. Controlling the ESP8266 supply through the STM8S003F3 would be cool. If someone with good access to PCB prototyping could do that job I'd be more than happy to contribute some ideas about the schematics.

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Elliot Williams wrote 11/29/2016 at 15:57 point

I just got an ESP-14 in the mail from ebay today.  I'll be making a breakout for it sometime in early Dec.  (Right now, I'm churning out HaD articles like mad.)  I'll share when I do.

I still have no idea if it makes any sense to run a (powerful) ESP8266 off of a (much smaller) STM8 chip.  But I'm willing to find out.  :)

I also ordered one of those LED/relay boards. Again, just for fun, but maybe I'll do something with it.

Thanks for thinking about caps.  I'll definitely rebuild and reflash. 

No, I didn't get into the multitasking / backgrounding. I just got the thing up and running, not much more.

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Thomas wrote 11/29/2016 at 19:13 point

The ESP-14 is quite strange. I can only guess that an OEM required a solution from Espressif that meets non-functional constraints (e.g. dependability, power consumption, or periphery set) that could not be met by the ESP8266. I don't think that a lack of skilled programmers was the reason. The power consumption of the STM8S003F3 in "active halt mode" is quite low, and for a data logging sensor node a battery life of a year or more with a 100mAh battery might be feasible.

The W1209 boards are really fun, especially with a background task. When you try using STM8EF with it, please let me know if the docs for the single wire half-duplex solution are sufficient.

About the case-insensitive input: you're welcome (the option has a price tag with "23 bytes" on it :-) )

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RigTig wrote 12/15/2016 at 10:44 point

I've created an adapter for ESP14 (and ESP12) to 22-pin DIL, if you haven't done anything else yet (see new project here called 'ESP-12 and ESP-14 adapter to DIL'). My ESP14s arrived today!

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Elliot Williams wrote 11/28/2016 at 13:54 point

Hiya! Been following along, finally got a few minutes to flash stuff to one of those min-dev boards.  Great fun!  I haven't done anything useful with it yet, but I've gotten the LED blinking, naturally.

One thing that's driving me nuts is the ALL CAPS commands.  Is there an easy way to either a) lower-case them all or b) make it run case insensitively?  Or would that cause namespace clashes? It makes my shift-finger hurt. 

And that's it for now.  I have to say that your directions (combined with some of the links that you list) made it very easy to get up and running with the system.  Thanks!

I'm planning a few Forth columns for HaD, and I'm still collecting chips that have working implementations.  You've added one more to the list. 

Oh, and I've ordered an ESP-14.  We'll see how that goes.  Looks like fun. 

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