An Embedded System simple Operating System Framework that allows rapid development
of applications build for AVR family but can be ported to other architectures easily
enough. In this article I will describe the concepts and the structure of this OS
and also I will provide an example application in order for the reader to understand
the simplicity of building new stuff easily.
Embedded Systems is a very interesting field. You can do things using hardware
and software that everybody gets impressed when they see them, unless the beauty
is well hidden. Along with their problems, limitations and special requirements,
if you repetitively build such systems, you have some common denominator on each
design. I always used a system tick timer and a UART for example. This is why I
choose to build an OS core framework that would allow me to build faster applications
without being too complex. This OS does not do preemptive multitasking. Instead
it is a round-robin co-operative system i.e. Each task either does something or
exits if it is waiting new data (no blocking). It is very simple with very low memory
footprint and also you can remove or add components very easily, ready to use it
in your next project. With this OS in hand I could develop my small applications
very fast because when I already had the basics ready to run the only piece missing
was my pure application: What I needed to do, which many times was a one or two
pages program. I could write it and have it up and running in a few days…
When I started to work with embedded systems I wrote assembly language and we
had microcontrollers with EPROM for program memory and very little RAM to use of.
In every project I almost had to use a system tick timer and not a few times a UART
to communicate with the host PC. Around the end of the 90's with the introduction
of AVR I switched to ISP (In-System Programming). No more bulky EPROM erasers, easy
programming in a few seconds etc. However back then, memory continued to be limited.
During my porting of 8051 codes to AVR I needed a few things. A system tick timer
and a UART. My next problem was debugging. Although there was an AVR simulator from
Atmel I could not test my application when I had to take inputs from external environment.
So in order to debug more easily I build a very small footprint monitor: just read/write
ports, memory and exercise external peripherals. This debugger was an integral part
of any new build. Later on I moved to C and thus I re-write once more most of my
code to C. Also as I added more and more peripherals and program memory was precious
I began configuring my tiny kernel to add or remove components with #defines. The
end result was to have a platform that allowed me to build fast my applications.
I just have all the infrastructure support and I was focused on building the actual
application. I only used simulator for particular pieces of codes (more to see what
the actual C statement do). During debugging I use my monitor,
printf to serial
port and of course multimeters, oscilloscopes and … LEDs(!) from the hardware side
if I need to.
Also as I am using GCC for compiling I do not use an IDE for AVR development.
I have make files for building and configuring my builds and I use my favorite editor
to do my coding. So this framework can be ported to any microcontroller theoretically
(ie. PIC, ARM etc). In fact I have ported variants to ARM and ColdFire processors/controllers.
Of course you are free to use your favorite IDE.
As memory was limited and I did not need the complexity of preemptive multitasking
the philosophy of this OS is that each task checks if it has any input to process,
if there is something to do it just executes otherwise it returns to the round-robin
main. The advantage is that this is very scalable and you don't worry for complex
things, it is very efficient for RAM and also there is no context switch so you
save execution time. The drawback of course is that the worst case execution (all
tasks execute) should be small enough (preferably less than the system tick period),
but this depend from your application! You might be able to break this rule for
once in a while. However I am avoiding doing that and I believe for most projects
the timing of this loop should not be problem.
Aim of the project
Aim of the project was to have a platform that was scalable and allowed rapid
application development. I finally wrote it in C so it is portable in a way. You
have to write the main peripherals for each new processor which is the main pain.
However after having the core up and running you can benefit from this structure
to have it as a base project where you can build your new applications. As I build
this for AVR initially I name it "AVRILOS": AVR ILias Operating System. I am assuming
that you have an AVR hardware ready. For your reference and because various I/O
are mapped to specific ports (although you can easily change them) I provide my
basic schematic which again is replicated (like AVRILOS) over projects more or less
with additions for my particular problem.
The tools I use for AVRILOS are:
- (SW) WinAVR (AVR GCC for Windows).
- (SW) Atmel AVR Studio (for Simulation).
- (SW) Your preferable Editor.
- (SW) Terminal Program (ie. Terminal, PuTTY).
- (SW) Programmer software (AVRDude is already included in WinAVR
but you might use AVREAL32 if you want).
- (HW) Hardware Board where you controller lives!
- (HW) Programmer Dongle.
- (HW) USB/RS232-TTL Serial Level Converter for connecting to
Optionally I have used some extra tools:
- (SW) CVTFPGA (for integrating serial bitstreams of Xilinx Spartan
FPGAs to my code, more on this later)
- (SW) Hexbin3
- GNUWIN32 (for makefiles if I don't use the WinAVR ie. Other
compiler packages like MPLAB)
- (SW) Python and Python Wx for building host applications.
- (HW) Oscilloscope (recommended)
- (HW) Multimeter (nothing less than that!)
- Anything else you can imagine and fits.
The directory structure is as follows:
There are two major directories, HW and SW.
- HW is the directory where all my hardware development is done.
This includes board schematic & PCB files as well as FPGA design.
- SW is the software directory which contains directory name of
the processor used, so I know which processor is used in each project
after many years. Also I might put here host software (in another
directory called host).
Let's concentrate on this directory structure of avr16. AVR16 refers to ATMega16
AVR. You could name it as you wish. Because the compilers have a tendency to generate
many intermediate files I don't want these to interfere with my source files. So
I have included three directories specifically for this purpose:
- build.dep: Here the dependencies of the C files are placed.
These are auto-generated by the makefile.
- build.err: Here I instruct the compiler to put any error files
in order to track them if I need them.
- build.obj: Here I place the object files of each module. Also
the final .elf file. From the MAP file I can find the memory location
of any variable and use it in my monitor for inspection.
- build.lst: Listing file of each module.
- build.rom: final programming files ready to be used for the
The root makefile (named makefile) resides on the root of avr16.
The cfg directory contains all the satellite makefiles. These files contains
the configuration options, compiler commands, etc.
The src directory is what you are expecting for. The source files. There we have:
- Applic: The main file Kernel.c which contains the "scheduler".
Initialization and main loop are here. Also my specific application
c files are placed here as well.
- peripheint: Internal peripherals of the microcontrollers. These
are timer, UAR etc.
- periphext: External peripherals outside of the microcontroller.
These might be smart cards, LPC flash, SPI devices etc.
- utils: Contains many type conversion tools (hex2bin, bin2hex),
delays etc. Of course you might use the stdlib
sprintf instead if
you have enough memory space available.
- debug: Here I have my monitor debugger and in addition an extended
debug file which I have functions individual enabled or disabled
for external peripherals that I might use. If I don't use them then
I just disable the corresponding functions and I save some memory
- include: Here I have global definitions and settings. Also pin/port
allocation for each peripheral used if this is programmable.
Description of Kernel
The Kernel.c contains the initialization code and the main loop. During then
startup the Kernel executes the various initializations of each module/peripheral.
There is no one initialization file for all modules. Instead each module modifies
its own bits in its I/O registers with Read & Modify instructions. This way each
peripheral does not interferes with the others unless there is a port conflict.
This allows more modular design and easy addition/removal of modules.
The scheduler does a round-robin execution of the tasks we need to. Each task
checks if should be triggered due to some flag like the SysTick timer (In periphint/Timer0.c):
if ( ( v_SysStat & (1 << b_SysTick) ) != 0)
Checks to see that the timer interrupt have flagged
b_SysTick (Bit) in
(Variable), if no just exits. If yes executes all its timer functions it needs to.
Another case would be to test if there are any new data in the serial port (like
the applic/serial/SerApp.c) does and exit if nothing is there.
The modules that I almost always have here are
SysTick (performs software timers
for mS, Led Alive etc),
debugger (monitor for debugging),
SysADC which captures
all 8 channels in sequence and thus the application just read the memory locations
for ADC data,
SerApp which is a small serial command application. Also we have the
capability to run the application either on every main loop or we might select to
run the application every n-times (which is when the LED flashes):
if ( ( v_SysStat & (1 << b_AppTick) ) != 0)
This allows the application to use a simple counter as delays or timeouts because
the action of modifying (increment/decrement) is done every SysTick. Of course with
a slight modification you can have a different pace for LedAlive and the application.
Finally for low power application I have included a sleep command at the end
of the loop. If there is no interrupt active the system will go to low power mode
until an event happens.
So in order to make a new task (application, device etc) you need to know that
the new element should not block the system waiting for too-long. Specifically the
execution delay of all the functions should not exceed the SysTick timing. If it
does you have too alternatives: Either increase the Systick interval or reduce the
In my experience I haven't had the need to trim these timing for any of my projects
Another concept I am using is that my interrupt routines are minimal. For example
timer0 interrupt just sets a flag and exits. The main loop will execute the
(deferred handler) which in turn will do all the hard work. Of course the
interrupts might be more complex like the serial interrupt which puts the data in
the FIFO. But the idea is to avoid any major processing at the interrupt level.
Thus the possibility of interrupts blocking will be minimal.
Also I use simple producer-consumer communication between interrupts and deferred
handlers. I check that each variable modification in the background is not affected
by the interrupt with atomic operations or by unidirectional actions (write/read-only
variables). These will be visible in the UART module.
Description of Modules
SysTick module does all the major timing functions. It flashes the Alive
LED, it triggers the ADC for starting a new conversion, triggers the LCM end of
delay, triggers the keyboard scan function and also has the software timers.
These triggers are simple flags (bits) residing on
v_SysStat. You can change
easily the Application time interval by modifying includes/ifc_time.h constants
in the beginning of the file:
#define c_ALIVEOK_ms 250 /* Alive LED When Ok */
#define c_ALIVESER_ms 500 /* Alive LED When Serial Error */
#define c_AppInterval_ms 8
#define c_ADCInterval_ms 32
#define c_KEYInterval_ms 16
The corresponding activation bit definitions are stated in includes/settings.h:
#define b_SysTick 0
#define b_DBG 1
#define b_AppTick 2
#define b_ADCTick 3
#define b_LCDBusy 4
I have omitted the flags that are not activated by the
Systick timer. Clearing
of the flags is done by the Kernel.c main loop. Keyboard scanning and LCD actions
are directly called by the Systick so there are no flags in
v_SysStat for these.
As these constants are referred in millisecond units you can change them at your
desire. The CPU clock frequency is defined in the cfg/hw.in makefile and all timings
should immediately comply, unless you need a different prescaler (set later on same
file as CLK/64).
#define c_T0CLK c_CLK64
#define c_T0DIV 64
Also you can provide dynamic error indications using the
function. This function modifies the flash LED (Alive) interval (in Ticks) so you
can notify the user that something is wrong/different changing the flash interval
of the LED Alive. For example in case of serial communication error you can call
from your application:
c_ALIVESER_ticks is defined later on include/ifc_time.h and is derived by the
ALIVESER_ms in the beginning of this same file.
Now what happens when do you need a timer interval to do general timeouts or
what ever? Easy! The SysTick provides a programmable (
MAXSWTIMERS) number of soft-timers
plus some additional definitions for easy reference:
#define c_MAXSWTIMERS 4
#define c_SwTimer0 0 /* Timer Activation (0: Stop, >0: Run) */
#define c_SwTimer1 1
#define c_SwTimer2 2
#define c_SwTimer3 3
These counters count in mS. In order to start a timer you write:
buf_SwTimer[SwTimer0] = 10;
This starts SW timer 0 with a timeout value of 10 mS.
To see if the timer expired:
If (buf_SwTimer[SwTimer0] == 0) Action_Timer0_finished();
To prematurely stop the timer:
buf_SwTimer[SwTimer0] = 0;
Additionally you may need small delay functions (just a few microseconds). In
these cases you can find some blocking functions in utils/delay.c. These can be
used during initialization where you waiting for a peripheral to startup. For example
I use such a delay during FPGA code configuration at startup. Or alternatively you
can use a microsecond delay on an SPI component.
The UART is my favorite peripheral. As most of my applications do not directly
need the UART, in my hardware I do not add the level shifter from TTL-RS232 level
to connect to my PC. Given that many times I hand made my boards I do not like to
spend components, wires and time for functionality that is useful only during development
(for my monitor or application that is). So what I do, is just put on a standard
(my standard of course) pin-header the Tx/Rx signals along with power and ground
and I use an adapter (dongle) to connect to my PC. This dongle could be a simple
level shifter (and that's why I need power at the pin-header) or now where every
computer uses USB ports I use a USB-serial converter (for example my Polulu AVR
programmer provides this additional functionality). Now every board has this pin-header
with 3-4 wires and I am done.
The UART has simple I/O functionality. For initialization you call the f_ConfigSerial.
Baudrate is defined in the makefile (hw.in). The rest of the settings are standard
8N1. If you need to change them you have to change the source of this function in
For the hardware level we support a number of processors and we use software
FIFO for both Tx/Rx. The size of the FIFO is defined in includes/settings.h:
#define c_RXBUFLEN 16
#define c_TXBUFLEN 64
As you see the Tx FIFO is larger. I do this in order to be able to respond with
a large Tx string from my application without blocking the system. If the FIFO was
smaller than my larger string sent to the host then the Put String function would
wait (and block there) until it could write all the data to the FIFO.
The main interface functions are:
bool f_Uart_PutChar(INT8 c);
bool f_Uart_PutStr(INT8 s);
f_Uart_PutChar: Put a character to Tx in the TxFIFO unless the FIFO is full.
Return code signifies that (1: success, 0: Fail). It does not block the system.
f_Uart_GetChar: Checks if a character is available and if yes it removes it from
the Rx FIFO and returns it to the application (-1 [0xFFFF]: Fail, 0x00XX Character
received). It does not block the system.
f_Uart_PutStr: Send a null terminated string to serial port. Always successful.
This function may block the system if the Tx FIFO is smaller than the string.
If you need a
printf function you can use an
sprintf function to a buffer that
would be sent to
f_Uart_PutStr. Or you can build your own
printf function as well.
In order to minimize footprint I use simple conversion functions that resides
in utils/typeconv.c. A
Although I refer to the "debugger" in this article what I actually mean is a
monitor. The debugger/monitor I have built does not take execution control, does
not step instructions or does any other fancy things that you would do with a normal
debugger. It is more like a monitor. You can change variables at run time (without
interrupting program execution), you can inspect memory, port, I/O or write these
peripherals. Also you can exercise other devices like SPI, LPC or FPGA if you have
added this functionality in the monitor.
The basic functionality is summarized below:
Command & Format
Read Byte at Addr XXXX
W XXXX YY
Write Byte YY at XXXX
See hex 4 bytes starting at XXXX
see ASCII 4 bytes starting at XXXX
B XX YY
Write Port PortX(01-04), DDRX(11-14) YY
Read Port PortX(01-04), DDRX(11-14)
Read Analog Port 0X (X: 0-7)
Inspect Data in EEPROM at addr XXXX
P XXXX YY
Write byte YY in EEPROM at addr XXXX
Read LCM addr XX
Write LCM command XX
Write LCM data XX
Read 4 bytes at LPC Bus Addr. XXXXXXXX-X+4 (32
s XXXXXXXX YY
Write LPC Byte at address XXXXXXXX (32 Bits)
t XXXXXXXX YY
Write a byte at LPC FLASH SST49F020/A (with write
Revert To Serial APP. Disable Debugger
Read SPI DR, SR
Read FPGA reg XX (Custom Commands, depend on
f 00XX YY
Write FPGA Byte YY at reg XX (Custom Commands,
depend on FPGA Code)
Note: All numbers are in Hex format and should be in Capital (case sensitive).
For example in order to inspect a variable at RAM location 0x10 you type at your
If you need to modify the contents of this memory location with the value of
W 0010 55
If you need to inspect PIN A of AVR you just type '1' and the pin status
of Port A is returned.
Variables address can be found at the /build.lst/kernel.map file. For example
you can search
v_SysStat address and find something similar to the following:
COMMON 0x00800160 0x2 build.obj/kernel.o
So the RAM address for AVR is 0x160.
You can provide you own commands on dbgext.c, but you need to modify the base
debugger.c file as well to get the input and process the new commands. What I prefer
to do is to implement the extra commands to the separate dbgext.c while I add the
command recognition and parsing on the debugger.c. I then enable or disable with
the HW.in makefile module definition the corresponding command when I need it. The
functionality of the extra commands exists always in the debugger.c which calls
the dbgext.c functions.
Also there is an empty user command ('
U') which you may implement differently
on each application without making more complex dbgext.c modifications.
The debugger module does not use
sprintf or stdio libs, so they have minimum
footprint. On processors with more memory you may implement a better and more capable
monitor but a small footprint monitor can be used anywhere.
I have included the LCM and the keymat 4x4 modules. However these modules are
partially tested and may not work always properly. Especially the LCM_char module
was based from Joerg Wunsch's code for HD44780 controller. In another article we
will discuss the FPGA SPI programming and the flow in order to produce the C-code
from the .bit file.
In order to allow the make files to work you need to adjust the env.in file.
Here you need to identify the compiler executables, the programmers etc. You need
to have the executables in your path or otherwise you might need to add the absolute
path to your tools.
Cfg/Srcobj.in defines which source/object files will be used depending on definitions
of hw.in file.
Cfg/Srcdef.in translates the hw.in makefile variable definitions to the C-files
#define pre-processor variables.
Finally cfg/hw.in configures all the hardware parameters (ie crystal clock frequency,
baud rate, active modules etc).
In order to compile the code you open a command prompt at the root directory
where the makefile resides. Then you type "
make" or "
make all". In order to do a
new build or when you modify the hw.in file you need to begin a new clean build:
make clean" and then "
You can see various messages during compilation. In case of error the compiler
stops and states in which lines have problems in a particular file.
When the rom files are ready you can type either "
make prog" or "
to download the program to your target. The prog option is used with AVReal programmer
while progsp uses the avr-dude which is the standard gcc tool for programming.
make size" you can review the object size.
make list" you can see the available options.
Hardware pin-out control is all set to the includes/settings.h file. All my hardware
devices are using define macros that are referring to this file. So I have all of
my I/O in one place and for each hardware I have to change only this file. Of course
there are limitations for specific peripherals like UART or SPI where functionality
In the picture below you can see an example compilation.
Our example Application will implement a lock. The key will be a simple smart
card and will control a servo for locking. The input of the system is a valid smart
card (with proper serial number) and the output is a PWM signal to control the R/C
servo for locking/unlocking.
A more detailed information about smart cards you can find at:
In this application I am using simple PROM type smart cards used for telephone
applications (card-phones). In the pictures below you can see card-readers (passive
elements for connecting card contacts to PCB) and the pin out of the smart cards.
On the diagram below you can see the mechanical configuration of a servo lock.
The Finite State Diagram below shows how the application logic control is implemented.
There are four main states. Initially the system goes at the
IDLE state. Until
a valid smart card is inserted the system waits there. After the insertion of a
valid card, which is predefined in the code, the system activates the servo and
waits for a predetermined time until the R/C servo reaches its final position at
WaitTO2Lock state. Then it enters the ARM state and waits again a valid smart
card in order to do the opposite, activating the R/C servo to its initial position.
After that (
WaitTO2UnLock) it goes to the initial
We will use the debugger to find out which PWM values are the optimum start &
stop positions for the servo (lock/unlock). Depending of how you have implemented
the mechanical assembly of the lock you will need different PWM values for the two
terminal positions. With the use of the debugger we will change the two variables
that control these positions and thus determine with trial and error the correct
PWM values. This small example will show how to use a small portion of the debugger/monitor
to develop your application.
A boot screen in example is shown below:
The zeroes after the OS/APP statement are from the smart card module (no card
reader connected in my hardware).
The minimum and maximum PWM values are stored in two variables:
We go to the file build.lst/kernel.map and we do a search of "
v_ServoLock 0x1 build.obj/applic.o
We find three instances. The first is the size of the variable. The second is
the address (with offset 0x800000). The last is the module where the variable is
The important for us is the second. So
v_ServoLock is at address 0x171 and it
is one byte.
We enter the serial application writing at the terminal "* <Enter>". Then we
try the direct key actions 'n' & 'm' exercising the min-max values of the servo.
Then we enter the debugger by "& <Enter>".
Then we read the value of
> R 0171
The returned value 32 is in hex which is 50 dec which is the initial value of
the variable. Now let's decrease this value to 40 dec -> 0x28.
> W 0171 28
> R 0171
Here is a similar example shown below:
Re-enter the application with "* <Enter>" and retry the 'm' command. The maximum
terminal position of the servo should be changed! You can play with these and find
the best values that you can hardcode to your application. Even better you can use
the EEPROM to store and use from your program for such settings.
You can do the same with v_ServoUnlock and trim this value as well.
This is a small example of how you can do some real-time tests without re-program
your controller. You can test the pin and port I/O in order to see if the controller
"see" the proper values on its ports or if any port seems floating (ie a bit would
toggle randomly when you touch the pin).
The serial application is another example task that is used to control directly
the application. For example it works seamlessly with the debugger. Entering '*'
on the debugger it reverts to serial application and the opposite. Entering '*'
to the serial application you can revert to the debugger.
The serial application shown supports two different modes. Direct key execution
and shell execution. For simplicity the shell commands are single keys (like the
debugger commands) followed maybe by parameters and terminated by the <Enter> key
The direct key execution mode is actions that you do while the key is pressed
on your terminal. For example we have some commands that control the servo position
so you can see that your hardware works. For example pressing the 'c' key you rotate
slowly the servo to one direction while pressing 'v' you rotate the servo the other
way around. The function which controls key actions is "
The shell function which provides more shell like interface is "
For example entering "q <Enter>" you do the same with pressing 'c'. That way you
can have two-fold control in your application and have flexibility of control through
One important aspect is that upon entering the serial application main function
we check if the debugger bit is set. If it is we skip execution as the debugger
is active. At the same time "
f_ProcessAPPCMD" have to support the asterisk "*" command
in order to revert to the debugger. You can use the ampersand "&" character to return
to debugger if you want to jump to the debugger mode blindly without knowing the
previous state (toggle with '*' between debugger and application).
In this article I presented a simple framework for building embedded systems
faster and easier. Of course there are easier ways of developing embedded systems
(like using basic interpreters) but this is a very scalable system which the convenience
of easy development does not compromise performance or capabilities. I began building
this system before 15 years on 8051 assembly, I then wrote it in AVR assembly and
afterwards I re-wrote the whole framework in C. I have done various modifications
over time and here it is. I provide the source code to the public in order to help
more people explore the beauty of embedded systems. I hope also that people may
contribute their own peripherals and extensions making a larger library of components.
This will make development of new systems for the rest of the people easier.
This article, along with any associated source code and files, is licensed under
Version 1.0, Initial Release.
Version 1.1, Corrected type values from
INT16U in ifc_time.h
defines (lines 77-81). Corrected minor article typos.