AN677
APPLICATION NOTE
Painless microcontroller code by
graphical application description
INTRODUCTION
Some electromechanics and automatics engineers hesitate to use microcontroller (MCU)
solutions despite their recognised advantages: High integration and flexibility for enhanced
features. Their main worry is the unpleasant aspect of MCU application development: Learning,
code writing, debugging through quite unconvivial tools.
Fortunately, it is possible today to use these tools (and to keep the genuine integrity of the code
issued) through a graphic interface. This new complete toolbox, the ST6-Realizer
graphical description of the system, automatic code generation, simulation and debugging.
®
, allows a
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1 A SMART SENSOR EXAMPLE
An application development is described here as an example which covers general items such
as analog data acquisition and treatment with peripheral activation and with operation
sequencing. The ST62 family of microcontroller is chosen as target device since it provides all
the features needed for this kind of application: A/D convertors, high output current capability
and a small package.
The application shown consists of an analog sensor input associated with data treatment
issuing two types of information: The relative position, lower or higher, versus a meanpoint or
reference value and the amplitude of the difference. This smart sensor can be used in a
positioning controller, temperature regulator or any other system requiring as parameters a
direction and amplitude.
A basic version reduced to a simple, straightforward data treatment is described. Its main
purpose is to present the methodology used and main features of the tool.
Application graphical description
Analysis and code generation
Simulation and validation
In a second step, the response of the system is modified to be compatible with non linear input
signals or non linear interfaces.
Finally a self calibration feature is implemented to compensate for imprecision on the input
sensor.
These last examples highlight the easy upgrading capability for an application.
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2 A SIMPLE SMART SENSOR
In its basic form, the application features the minimum data treatment: a subtraction is
performed between an entered analog value and a constant reference to control an output
signal generator. This last circuit drives two external I/O pins according to the subtraction
result: Whether the value is positive or negative, only one of the two outputs is activated
issuing a squarewave signal with a frequency function of the amplitude of the difference.
2.1 Graphical description
The core of this development methodology is the graphical description of the application. This
description uses a standard library allowing a system oriented description; Included are:
Input / Output symbols
Logic / Arithmetic symbols
Oscillators / Pulse generators
State machine for sequencing
With such a library, it is possible to cover most application requirements as long as the time
resolution require for the oscillators and pulse generators remains over 10ms, the value used
as the time unit. This value defines only the minimum calibrated timebase available and not the
speed of the application which is fixed by the MCU quartz crystal frequency and the complexity
of the application.
The graphical description follows a decomposition of the application into such basic
functionalities; In the present case our needs are for:
One analog input with an A/D convertor
One constant reference
One subtraction operator
One wave generator
Two output pins
No sequencing is required.
The analog input and digital outputs are described by placing the appropriate symbols
proposed in the main library of the object menu. The software proposes the MCU I/O pin
available in each case: digital or analog inputs, current sink capabilities for output pins,
insuring a full compatibility with the selected hardware (Figure 1).
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Figure 1: Pin Assignment In St6-Realizer Environment
By choosing an 8-bit resolution for the A/D convertor, any analog input value is digitalized into a
number between 0 and 255. The constant reference used as meanpoint is described by placing
a specific symbol with the value 127.
The subtraction operator and the wave generator are obtained through the same object menu
and placed into the schematic.
Eventually, the graphical description is represented in Figure 2.
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Figure 2: Simple Smart Sensor Graphical Description
2.2 Analysis / Code generation
After the schematics capture, an analysis of the graphical description must be launched with
the analyse menu. This process first checks the integrity of the description, verifies I/O
assignments and connectivity providing on-line information in the case of an error, guiding the
user in the development work.
If this first analysis step is successful, all the files related to the ST62 target are automatically
generated: The source file in macro-assembler language, the symbol table, the binary
executable code. The code generation uses a library of predefined macro-instructions
associated to the graphical symbols; thus the time needed from the graphical description to the
binary executable code is considerably reduced.
Finally a report file is provided with the Input/Output assignments and the used ROM and RAM.
This can help to choose the most suitable MCU for the application. In the present case the
report shows 27 RAM bytes and 434 ROM bytes are used, informing the developer about the
memory requirement for the MCU and the memory space available for further developments.
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2.3 Simulation and validation
When the analysis and code generation are successfully concluded, the simulation can be
launched (Simulate menu). This allows the application to be simulated and for the signals to be
monitored directly on the graphical description, without having to build hardware for debugging.
The use of the numeric adjuster on the inputs, and numeric probes makes the validation of the
application very convivial. By activating the input adjuster from 0 to 255 (the 8-bit A/D
conversion result) any subtraction situation can be checked with its impact on the output.
Figure 3 represents such a case where an input value of 110 is compared to the reference
value of 127, resulting in a difference of -17 which produces the oscillation at the relevant
output pin.
Figure 3: Simulation Results In The St6-Realizer Environment
It is possible to get information through the Run menu about the maximum time taken by the
MCU to perform all the operations described in the application. This fixes the minimum time
required between an input change and its impact on outputs, and thus the frequency response
of the application. In the present case the maximum time is 630µs for a 8Mhz oscillator.
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3 A SMART SENSOR WITH A NON-LINEAR RESPONSE
In the former version, the system has a linear response: If the entered value is increased by 3,
the result sent to the wave generator is also increased by 3. However, many systems present
non-linear behaviour either due to a non-linear input signal or to a non-linear output stage. In
such cases, non-linear transformation capabilities are required.
In this new case a response after the subtractor, as shown in Figure 4, is obtained by only
adding two symbols: A transformation table to pass from a linear evolution to a x2-like curve
and a limiter symbol to limit the total excursion. The transformation table is a standard ASCII
file with a very simple syntax : Input value, Output value, as shown in Figure 5.
Figure 4: Response Transformation Into A Non-Linear Curve
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Figure 5: Constant Table Editing In The St6-Realizer Environment
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4 A SMART SENSOR WITH NON-LINEAR RESPONSE AND AUTOCALIBRATION
In the case of a non-calibrated sensor, or of mechanical drift of the input system, the meanpoint
value may be slightly different from the theoretical reference value and subsequently induce
some unwanted actions on the outputs. One solution is to perform an autozero process
allowing a tolerance window for the meanpoint, as represented on the flowchart in Figure 6: As
long as the initial value at reset is within a tolerance window around the theoretical value of
127, the shift is considered as irrelevant and that initial value is used as the new reference; On
the contrary, any input value out of this tolerance window is understood as an instruction
request to use 127 as the reference value.
Figure 6: Flowchart Of The Autozero Process At Reset
Output = 00
Input Value in
Tolerance Window
Yes
Store Input Value
as Reference
Output = Input - Reference
No
Store Default Value
127 as Reference
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The description of the flow-chart is very intuitive thanks to the symbols provided in the library
for state machine description. As seen in Figure 7 there is a direct correspondence between
the flow-chart and the described state machine.
Figure 7: Sequencing Description By State Machine
The description of comparison features for the tolerance window, and the reference data
memorisation is done following the methodology of decomposition into basic functions already
used in the start-up version.
Eventually, the complete schematic is represented in Figure 8, where the addition of the
autozero process and the non linearization feature is easily distinguishable from the start-up
version. The modular description and development of the application are very easy to manage.
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Figure 8: Smart Sensor With Autocalibration And Non-Linear Capabilities
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5 HARDWARE IMPLEMENTATION
The binary executable code generated during the analyse process is simply programmed into
an EPROM or OTP MCU device to validate the application in the real world. By using a
ST62E25 mounted on an ST62 Starter-kit, it has been checked that:
By turning the potentiometer to the right or to the left, the appropriate output to one of the two
connected LEDs oscillates with a frequency increasing with the potentiometer rotation.
The frequency evolution is not linear with the potentiometer rotation according to the desired
response transformation.
When a reset is performed with the potentiometer slightly uncentered, any oscillation is
turned-off due to the autozero process.
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6 CONCLUSION
The presented application demonstrates that a powerful and intuitive tool is available for data
treatment and sequencing control in microcontroller based designs. The actual limitation to
10ms of its timing features is compatible with a very wide range of applications in automatic or
electromechanics systems, for example: Smart sensors, replacement of relay-based systems,
car equipment (window lifter, body mirror) and coded keys identifiers.
Finally, the files generated by the Analyse tool (The source file in macro-assembler language,
symbolic table and binary executable code) are obtained by a hidden call to genuine tools
which combine the power and integrity of the development tools with a convivial user interface.
The generated files are available to MCU designers who wish to mix or link an ST6-Realizer
based application with other application code.
The system shown is an open system that may interest experienced designers who want to
reduce their development time as well as the engineers with system know-how who want to
easily shift to MCU solutions.
Actum Realizer is a trademark of Actum Solutions, The Netherlands.
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Revision History
Date Revision Description of changes
March-1995 1 Initial release
24-June-2008 2 Logo modified
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