2.1

Structure and function of a microcontroller

Generally a microcontroller is a single chip that contains an entire small computer. Hence it is also called “computer on a chip”. Usually they are 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:

Figure 2:

Microcontroller system, drawn by author

The ALU loads its data from memory and also stores it there afterwards. The control unit decodes the given instructions of the program, which can be:

Subsequently the ALU executes these instructions and manages the data in- and output. To get in contact with the outside world you need several peripheral units in addition to the microprocessor, such as digital input/output registers (often called “ports”), data memory, program memory and timers. The in/out registers are there to control and read the peripheral devices. They are the ports to all external devices.

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 absolutely essential for our educational purpose: With this condition our students can freely test their written programs or program fragments by trial and error.

2.2

Architecture of the microcontroller “ATmega2561”

For our development board we use the microcontroller ATmega2561 from the 8-bit AVR family of Atmel Corporation. Building our microcontroller systems based on this device provides us a reasonable and useful hardware development environment.

In Figure 3 you find the system architecture of the Atmega2561 in detail. The microcontroller is equipped with ten input/output ports, each with eight pins that are switched as digital I/O ports by default. To enable further functions the configuration can be changed with the help of controller-specific register entries.

Figure 3:

Atmega2561 system architecture [3]

3.1

The development board (hardware)

The development board is structured modularly. At the bottom right you find the microcontroller board with its LCD (1).

Figure 4:

Realized application setup with Atmega2561 system architecture.

3.2

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 which makes their functions easy to understand and allows a wide range of applications. With this approach it is possible to cover the demands of a good entry into the world of microcontrollers.

Our board contains three input devices:

… and three output devices:

Out of these boards we will take a closer look at the LED matrix, as this board is particularly interesting for optical and mathematical uses:

When designing the LED matrix board (Figure 5) we intentionally avoided the use of a multiplex control, to make it easier for the students to understand the close correlation between the 8 data lines for the columns of the matrix (port 1) and the eight data lines for the rows (port 2).

Figure 5:

Circuit diagram of the LED matrix, drawn by author

Example: A logical “1” at row 1 and a logical “0” at column 1 will enlighten the LED11.

On our 8x8 LED matrix we can display the complete ASCII character set, as well as simple graphics.

3.3

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. We use the following sensors:

Figure 6:

External sensors

…of which the last two sensors are particular interesting for uses in optics:

Function of the light sensor BPW42

The light sensor BPW42 shows a linear relationship between the irradiance and the phototransistor current. When using the displayed circuit (Figure 7) 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).

Figure 7:

BPW42, drawn by author

If required, the irradiance has to be converted into the illuminance E_v (in lux), while having to pay attention to the correct wavelength. Afterwards the calculated voltage U_L has to be adapted to the 10-bit A/D converter. [4]

Function of the infrared light sensor TSOP 1730

The light sensor TSOP 1730 is an infrared receiver with a digital output signal, whose spectral maximum sensitivity peaks at a wavelength of λ = 950 nm. It works with a carrier frequency of 30 kHz and a maximum data rate of 2400 bps.

An infrared transmitter sends a carrier-keyed signal, i.e. the infrared signal is being switched on and off with a frequency of 30 kHz. Such signal sequence (“burst”) has a transmission period of circa 600 μs. By sending this burst at specific times it becomes possible to transmit data. The receiver’s output signal S_OUT can be passed directly to the microcontroller.

Figure 8:

TSOP 1730, drawn by author

Figure 9 illustrates the relationship between the optical signal and the output signal SOUT at the receiver. [5]

For the development of appropriate algorithms for measuring illuminance or temperature the students have to possess a basic knowledge of optics and electrical engineering.

Figure 9:

Optical signal and digital output signal [5]

3.4

Software

When it comes to the software, an important part of our microcontroller project – along with electrical engineering, optics and mathematics – is the subject of information technology. 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 IDE (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. This has the advantage for the developer that he can focus completely on his real task: the development of microcontroller applications [6].

To transfer the written source code to the microcontroller, the code first is being compiled into machine code in hex format, then the controller has to be connected to the computer via a programmer to load the code to the controller’s memory.

At this project we are using the development platform AVR Studio 4 by Atmel in combination with the compiler WinAVR. This IDE offers:

Triggering an LED

Task: The LED connected to Pin 4 of Port A shall be switched on and off periodically every second. Source code (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.