atomthreads/ports/stm8/README

---------------------------------------------------------------------------

Library:      Atomthreads
Author:       Kelvin Lawson <info@atomthreads.com>
Website:      http://atomthreads.com
License:      BSD Revised

---------------------------------------------------------------------------

STM8 PORT

This folder contains a port of the Atomthreads real time kernel for the
STM8 processor architecture. These instructions cover compiler-agnostic
aspects of usage of Atomthreads.

All of the cross-platform kernel code is contained in the top-level 
'kernel' folder, while ports to specific CPU architectures are contained in
the 'ports' folder tree. A port to a CPU architecture can comprise just one
or two modules which provide the architecture-specific functionality, such
as the context-switch routine which saves and restores processor registers
on a thread switch. In this case, the kernel port is split into two files:

 * atomport.c: Those functions which can be written in C
 * atomport-asm-raisonance.s: Main register save/restore assembler routines

Each Atomthreads port requires also a header file which describes various
architecture-specific details such as appropriate types for 8-bit, 16-bit
etc variables, the port's system tick frequency, and macros for performing
interrupt lockouts / critical sections:

 * atomport.h: Port-specific header required by the kernel for each port

A few additional source files are also included here:

 * tests-main.c: Main application file (used for launching automated tests)
 * uart.c: UART wrapper to allow use of stdio/printf()
 * stm8s-periphs/*.*: Peripheral drivers as delivered by ST (no changes
     to distributed code).

Atomthreads includes a suite of automated tests which prove the key OS
functionality, and can be used with any architecture ports. This port
provides an easy mechanism for building, downloading and running the test
suite to prove the OS on your target.

The port was carried out and tested on an STM8S105C6 running within an
STM8S-Discovery board, and supports the SDCC, Cosmic, Raisonance and IAR compiler
tools. It is possible to use it with other processors in the STM8 range, as
well as other hardware platforms and compilers, with minimal changes.
Platform and compiler specific code has been kept to an absolute minimum.

This README covers th ecompiler-agnostic aspects of usage of Atomthreads.

Instructions for users of particular compilers are available in README-SDCC,
README-IAR, README-COSMIC and README-RASONANCE.

---------------------------------------------------------------------------

STM8S-DISCOVERY SPECIFICS

There are very minimal board-specific aspects to the STM8 port so it is
trivial to run Atomthreads on other STM8 platforms.

The test applications make use of a LED to indicate test pass/fail status.
This is currently configured to use a bit in GPIOD, which on the Discovery
board maps to the board's only LED. You may change the port and register
bit in tests-main.c to utilise a different pin on other hardware platforms.
You may also completely omit the LED flashing in the test application if
you prefer to use the UART for monitoring test status.

The test applications also make use of the UART to print out pass/fail
indications and other information. For this you should connect a serial
cable to the Discovery board via the external pin connectors. Use of
a UART is not required if you prefer to use the LED or some other method
of notifying test pass/fail status.

To connect a serial cable to the Discovery you will need to connect to
the following pins on the external connectors:
 Vcc: CN2 pin 8
 GND: CN2 pin 7
 UART RX: CN4 pin 11 (connect to TX at the PC end)
 UART TX: CN4 pin 10 (connect to RX at the PC end)
Note that the board uses TTL levels so you may need to use a level
converter. External level converters may need to be powered using
a Vdd of 5v, which can be achieved by positioning JP1 on the Discovery.

The STM8 device on the Discovery only offers UART2. If you are using a
different device or wish to use an alternative UART then you must change
the stm8s_conf.h file.

If you are using a CPU other than the STM8S105C6 you should change the
PART macro from "STM8S105" to your target CPU. This can be changed in the
raisonance.mak Makefile. If you are using the STVD project it should be
changed in the project preprocessor settings for both Debug and Release
builds. You may also wish to enable any CPU peripherals which you wish to
use in the stm8s_conf.h file.

---------------------------------------------------------------------------

RAM FOOTPRINT & STACK USAGE

The Atomthreads kernel is written in well-structured pure C which is highly
portable and not targeted at any particular compiler or CPU architecture.
For this reason it is not highly optimised for the STM8 architecture, and
by its nature will likely have a higher text and data footprint than an
RTOS targeted at the STM8 architecture only. The emphasis here is on
C-based portable, readable and maintainable code which can run on any CPU
architecture, from the 8-bitters up.

A good rule of thumb when using Atomthreads on the STM8 architecture is
that a minimum of 1KB RAM is required in order to support an application
with 4 or 5 threads and the idle thread. If a minimum of approximately
128 bytes per thread stack is acceptable then you will benefit from the
easy-to-read, portable implementation of an RTOS herein.

The major consumer of RAM when using Atomthreads is your thread stacks.
Functionality that is shared between several kernel modules is farmed out
to separate functions, resulting in readable and maintainable code but
with some associated stack cost of calling out to subroutines. Further,
each thread stack is used for saving its own registers on a context
switch, and there is no separate interrupt stack which means that each
thread stack has to be able to cope with the maximum stack usage of the
kernel (and application) interrupt handlers.

Clearly the stack requirement for each thread depends on what your
application code does, and what memory model is used etc, but generally
you should find that 128 bytes is enough to allow for the thread to be
switched out (and thus save its registers) while deep within a kernel
or application call stack, and similarly enough to provide stack for
interrupt handlers interrupting while the thread is deep within a kernel
or application call stack. You will need to increase this depending on
what level of stack the application code in question requires.

At this time the maximum stack consumed by the test threads within the
automated test modules is 95 bytes of stack, and the main test thread has
been seen to consume 163 bytes of stack. At this time the queue9 test is
the largest consumer of test thread stack (95 bytes) and the sem1 test
consumes the largest main thread stack (137 bytes). If your applications
have large amounts of local data or call several subroutines then you may
find that you need larger than 128 bytes.

You may monitor the stack usage of your application threads during runtime
by defining the macro ATOM_STACK_CHECKING and calling
atomThreadStackCheck(). This macro is defined by default in the Makefile
so that the automated test modules can check for stack overflows, but you
may wish to undefine this in your application Makefiles when you are happy
that the stack usage is acceptable. Enabling ATOM_STACK_CHECKING will
increase the size of your threads' TCBs slightly, and will incur a minor
CPU cycles overhead whenever threads are created due to prefilling the
thread stack with a known value.

With careful consideration and few threads it would be possible to use
a platform with 512 bytes RAM, but not all of the automated test suite
would run on such a platform (some of the test modules use 6 threads: a
main thread together with 4 test threads and the idle thread).


---------------------------------------------------------------------------

INTERRUPT HANDLING

Interrupt handlers use the stack of the thread which was running when the
interrupt occurred. If no thread rescheduling occurs during the ISR then
on exit from the ISR any data stacked by the ISR on the thread's stack is
popped off the stack and execution of the thread resumes. If a reschedule
during the ISR causes a context switch to a new thread, then the ISR's
data will remain on the thread's stack until the thread is scheduled back
in.

Interrupt priorities (via the ITC_SPRx registers) are left in their
default power-on state, which disables interrupt nesting. Kernel changes
may be required to support interrupt nesting.

Note that the STM8 programming manual currently describes the following
feature:

 "Fast interrupt handling through alternate register files (up to 4
  contexts) with standard stack compatible mode (for real time OS
  kernels)"

This feature was implemented by ST in the core but has to date never been
included in any STM8 products. If it is included in future products then
you will need to put the device in the stack compatible mode described.


---------------------------------------------------------------------------

WRITING NEW INTERRUPT HANDLERS

All interrupt handlers which will call out to the OS kernel and potentially
cause a thread switch must call atomIntEnter() and atomIntExit(). An
example of this can be seen in the timer tick ISR in atomport.c.

You may also implement fast interrupt handlers in the system which do not
call atomIntEnter()/atomIntExit(), however these ISRs cannot perform OS
functions such as posting semaphores or effecting a thread switch.


---------------------------------------------------------------------------