2.1

Structure and function of a microcontroller

A microcontroller is a “computer on a chip” – a single chip that contains an entire simple, small computer. Usually it is constructed for a particular purpose, e.g. for controlling a traffic light system, a digital clock etc.

The microcontroller’s centerpiece is the central processing unit (CPU), which is also called microprocessor and which primarily consists of the arithmetic and logic unit (ALU) and the control unit (CU). Its task is to perform all operations on data by means of a program:

The ALU loads its data from memory and also stores it there afterwards. The control unit decodes the given instructions of the program, e.g. jump instructions or bitwise operations. Subsequently the ALU executes these instructions and manages the data in- and output. All these peripheral units are built in a microcontroller, for the aim is to integrate as much (programmable) functionality as possible into one single chassis. The microcontrollers we use are reprogrammable, which is a precondition for our educational purpose: Our students can freely test their written programs or program fragments by trial and error. At our laboratory we use the AVR Atmega 2561 by Atmel. Reasons for this decision will be discussed in the following chapter.

Figure 2:

Microcontroller system, drawn by author

2.2

Input/Output

Our microcontroller provides 64 pins which can be independently declared either as input or as output registers. This setting is controlled by software. Eight pins are grouped into one port. As the pins work digitally, they can differentiate between two levels:

Being used as an output register, a pin can be pulled to “low”, which means that is connected with ground and set to 0V. The second level, identified with “high”, connects the pin with the supply voltage of the controller. Typically this is 5V.

Working as an input register, a pin can distinguish between the two voltage levels “ground” (0V) and “Vcc” (5V), whereas the threshold lies around 2,5V.

For interpreting analog signals an analog-digital converter (ADC) is required. The AVR Atmega 2561 contains a 10-bit ADC, which determines the discrete values by using successive approximation in a range between “ground” and “Vcc”. In our seminar both digital and analog sensors are used. [2]

3.1

Decision for a self-made microcontroller

Despite of this progress, we decided not to use an “out of the box” solution, but to develop and build a microcontroller board on our own. The main reason for this decision was our request to use a board which fits perfectly to the teaching methods of our seminar: Our learning goal for the students is not to get quick programming results, but to stimulate a general comprehension of the topic and to create knowledge, that is not bound to a special type of microcontroller. So afterwards the students are able to work with microcontrollers, no matter of which brand.

The leading thought when developing our board was to give it a clear structure, so it would be easy to understand its functions and signal paths. One example is the visible 8-bit architecture, which can be found all over the board: The LED panel consists of 8 LEDs, the keyboard has 8 buttons and the LED matrix is built up of 8 columns and 8 rows. This logical structure is not only helpful for understanding the hardware, but especially for programming the microcontroller, because many calculations and addressing have to be done in binary arithmetic.

3.2

Why not Arduino?

Of course our board is not the first one to be designed especially for beginners. Boards like “Arduino” or the “Wiring board” have managed to bundle the complex hard- and software of a microcontroller in a user-friendly package. But their target audience are mainly artists in the broadest sense, who don’t want to familiarize themselves with electronics, bus protocols and hardware-near programming. Their aim is to get quick and simple results for their ideas. On Arduino instead of writing plain code you work with descriptive commands, which are easier to use, but on the other hand have limited possibilities and are much more power consuming. For example, the C code for switching one pin of a microcontroller port, which looks like this

is not intuitive at all, but works on every controller by using only one or two clock cycles. To do the same on an Arduino board you can write

which takes about 50 clock cycles and does not explain what is going on inside the machine. But it’s easy to understand.

At this time there is one other popular microcontroller board beside Arduino: Raspberry Pi. That board is also adequate for beginners and allows a wide range of application possibilities. To work with it, it has to be connected to some hardware like a monitor and keyboard and then can be programmed in the programming language Python. As our students are already familiar with the language C and as our aim is to keep it simple, the Raspberry Pi doesn’t fit our “beginners” seminar.

Apart from this seminar we could experience both Arduino and Raspberry Pi to be great platforms for students, who want to develop further projects.

4.1

The development board (hardware)

The development board consists of seven modules: Most important is the microcontroller board at the bottom. It is equipped with seven plugs for in- and output devices and an integrated LCD panel (figure 3, item 1). The microprocessor AVR Atmega 2561 from Atmel Company is situated on the back side of the board.

Figure 3:

Realized application setup with Atmega2561 system architecture. (picture taken by author)

4.2

On-board in- and output devices

The development board has been designed to provide an easy entry into the field of microcontroller technology and programming. The devices are structured in a way that makes their functions easy to understand and allows a wide range of applications.

Our board is provided with three on-board input devices:

and four output devices:

As an example the authors would like to take a closer look at the 7-segment display board (6):

This output device can display numbers and some characters with four 7-segment displays, which are controlled via two ports (16 pins). The students can use this module in two ways with different levels of difficulty.

A simple possibility is to use just one of the four displays. In this case the display can be triggered directly analog: Each pin switches on and off one segment of the number in exactly the same way as one pin would switch on and off one LED of the LED panel. Seven segments and one decimal point add up to 8 pins, so exactly one port is required.

To tap the full potential of the 7-segment display board it can be used as a four-digit display. As two ports do not suffice for an analog control, the signal has to be multiplexed. The student’s first steps into this advanced method may be accompanied by several errors, but to gain insight into time-division multiplexing (TDM) delivers important knowledge about communication and media technology.

Figure 4:

Circuit diagram of the 7-segment display board, drawn by author

4.3

External devices: sensors

To support our students in their understanding of optics we are additionally using several sensors. For technical measurement reasons they are not part of the development board, but are connected as external input devices to a port of our microcontroller.

In the matter of controlling these devices there are three types of sensors:

The hardest way: Analog sensors (light sensor BPW42)

Signals from analog light sensors like the BPW42 are difficult to handle when quick results are in the focus. They require additional components, have to be digitalized via an ADC and need to be converted to get the correct units. But from an educational point of view, they are great to demonstrate how integration and conversions can be done.

The analog light sensor shows a linear relationship between the irradiance and the phototransistor current. When using the displayed circuit (Figure 5) the sensor delivers an output voltage of U_L = I_C (R_V+R_P) which is proportional to the irradiance E_e (in W/m²). If required, the irradiance has to be converted into the illuminance E_v (in lux). Afterwards the calculated voltage has to be adapted to the 10-bit A/D converter. [6]

Figure 5:

left: light sensor BPW42 (picture taken by author), right: required circuit (drawn by author)

Figure 6:

left: light sensor TSOP 1730 (picture taken by author), right: required circuit (drawn by author)

Figure 7:

PIR module HC-SR501 (picture taken by author)

4.4

Software

Along with electrical engineering, optics and mathematics, the subject of information technology is an important part of our microcontroller project. This includes knowledge about the syntax of the programming language C, the ability to structure the own source code and the skill of operating the IDE: The integrated development environment is an application package for the PC combining editor, assembler, compiler, linker and simulator applications under one uniform surface, which can be operated more easily. For programming microcontrollers there is a variety of development environments for all different types of microcontroller hardware. All of them have in common that they are useful tools, which offer fast access to important functionalities, liberating the developer from recurring and formal tasks and helping him to manage his work results, so he can focus completely on his real task: the development of microcontroller applications. [8]

To transfer the written source code to the microcontroller, the code first is being compiled into machine code in hex format and then loaded to the controller’s memory via a programmer. At this project we are using the development platform AVR Studio 4 by Atmel in combination with the compiler WinA VR.

Controlling an LED

Task: The LED connected to Pin 0 of Port C shall be switched on by pushing a button, which is connected to Pin 0 of Port D, and off by releasing it.

Example source code for a solution:

A short and clear code snippet like this has proven to be most suitable for getting used to the syntax of the programming language C and of the control commands for our microcontroller. After a couple of similar finger exercises with a constantly increasing level of difficulty the students are finally able to manage programs with more complex structures and functionalities.

At the end of the semester the student get together in groups to work on their final programming projects – their certificates of performance. Therefor the students can rent developing boards to take them home and to delve into programming. A majority of the students takes advantage of this offer. So until now they did submit several nice and useful projects. The students are free to include using sensors into their projects (for projects like e.g. a light barrier, a lux meter, a digital watch or math games) or to get by with the on-board modules of the developing board (for projects like simple games, a stop watch, a clock and date display including leap years etc.)