1.

INTRODUCTION

The regular low-cost instruments (DMMs, scopes, generators, power supply, etc.) are currently used for laboratory education. Nevertheless, two problems can arise:

The basic instrument – simple low-cost handheld digital multimeter is mostly at an affordable price accessible for everybody. It makes possible to realize static measurements only [1]. The problem arises in the case of generating and recording time-variable signals when scope must be used. The virtual USB scopes, which are suitable for this purpose, are available at the market [2], nevertheless their obtaining has some restrictions, especially for individual work at home or in the case when many students solve the similar task parallelly.

This situation had to be solved at the CTU-FEE, Dept. of Measurement, both in the subject “Industrial electronics laboratory”, where 20–30 students solve a similar task parallelly, and in the intensive preparatory course “Practical Electronics”, which is determined especially for school-leavers of secondary schools and a similar number of students (2 x 20–30) take part in.

In both cases, it is for a lot of students the first practical meeting with electronic circuits and corresponding experiments. Therefore, for some of them, the time spent in laboratories is not enough to finish the tasks. These need to have a possibility to work at home without any fuss, too. Besides, considering a high number of participants, the capacity of special laboratories for electronic education equipped with scopes and generators is not enough and computer laboratories have to be used. The facts mentioned above led to the demand to design and develop a suitable tool for laboratory teaching based on PC and USB connection, which should serve not only for generating and recording time-variable signals but also for supplying of experimental circuits. F0-Lab can be used as a simple DC voltmeter, a digitizing oscilloscope, a waveform recorder (3-channels in all cases) and a PWM signal generator.

2.

DESIGN OF THE LOW-COST TOOL – VIRTUAL INSTRUMENT – FOR LABORATORY TEACHING

During the design of the low-cost tool for laboratory teaching, it was necessary to take into consideration besides the functions mentioned above also a possibility to acquaint students with fundamentals of microcontroller technology, interfacing, and programming. It was demanded, to be possible to realize the tool also by students without special knowledge and without using special paid software. The main instruments, which should be realized, were a virtual scope and a generator. To use a low-cost powerful microcontroller with embedded fast ADC and USB interface and develop the necessary firmware for the microcontroller and corresponding PC application was chosen as an optimal solution.

2.2

Circuit structure

The basic connection of the virtual instrument for laboratory teaching is very simple (see figure 1). Thanks to it each student can connect it on the solderless breadboard himself without a problem. The 5 V supply from the USB port is at disposal in the board and the microcontroller is supplied using the stabilizer 3.3 V. This voltage is also distributed in the board and both voltages can also be used for experiments realization.

Figure 1.

The basic connection of the virtual instrument

3.

REALIZATION OF VIRTUAL INSTRUMENT FUNCTIONS

The approach Software Defined Instruments (SDI), where all virtual instrument functions were realized by software using circuits on the chip of the microcontroller only, was applied. The possibilities and properties of the realization are depended on the software used. The function realized in the last version of the presented virtual instrument is described below.

4.

THE PROGRAMME MLI - MULTIFUNCTIONAL LABORATORY INSTRUMENT

The new software variant named MLI - Multifunctional Laboratory Instrument was solved in the diploma work [7] based on the previous experience. Its basic functions (see figure 2) are:

Figure 2.

The basic function of MLI

The 4 channels voltmeter with averaging and the 4 channels voltage recorder was also implemented (see figure 3). This record can be also exported in the file with the CSV format for the following processing using Excel. For simple experiments, the graph in the PNG format can be exported.

Figure 3.

The 4 channels voltmeter

The scope uses the 8-bit or the 12-bit resolution of ADC; the sampling rate is adjustable up to 750 kS/s in the single channel mode. The record length is again limited with the internal RAM and it is 1 kS for 8-bit resolution. The virtual control panel of the scope is presented in figure 4.

Figure 4.

The 4 channels oscilloscope

Considering a need for observation of the digital data flow the 4 channels logic analyzer was implemented -figure 5. The sampling rate is up to 8 MS/s and the record length 1 kS for 4 channels. The trigger is derived from the edge (rise/fall) in the selected channel. The primary application concerns the digital data flow analysis in the case of microcontroller interfaces as IIC, SPI, etc., which are applied in many microcontrollers and intelligent sensors e.g. Raspberry Pi, Arduino, etc. The channel UART can also be easily monitored.

Figure 5.

The 4 channels logic analyzer

The counter for frequency and pulse-width measurement was also implemented as a fully new function - figure 6. Its resolution is 20.83 ns (the system clock 48 MHz) for a period and pulse-width measurement. The averaging is also possible according to “period count” setting.

Figure 6.

The counter

The PWM generator mentioned in paragraph 3.3 was realized in this case in 2 channel version (its virtual panel see figure 7).

Figure 7.

The PWM generator

5.

CONCLUSION

The simple tool for laboratory experiments in electronics mentioned in the paper was primarily developed for education in electronics. However, it can also be used for the teaching of embedded programming in C/C++. The microcontroller STM32F042F6P6 used here is supported by online IDE Mbed [8]. There is only one difference contrary to the kit Nucleo STM32F042K6, which is supported with Mbed directly, the smaller number of pins. Therefore, it is possible to use only these pins, which are at disposal also in the TSSOP20 case. Other parameters (the flash memory, RAM, peripheries, etc.) are the same. Thanks to this fact, the kit Nucleo STM32F042K6 can be virtually selected in Mbed and only these pins, which are at disposal in the TSSOP20 case, are used by programming.

Considering the microcontroller STM32F042F6P6 contains USB interface including programming support of the boot loader, the software developed by means of Mbed can be loaded from the corresponding Mbed web site and using STM Cube programmer is uploaded directly into the microcontroller using USB. There are not necessary any special software and components, thus it is possible to realize the similar tasks like those ones, which are presented in [9].

The equipment mentioned above was realized already in several hundreds of pieces. It was used as in the intensive preparatory course “Practical Electronics” designed for school-leavers of secondary schools, so in two subjects in regular laboratory teaching. Thanks to the support of the company STMicroelectronics we can lend the describing kits not only to bachelor students but also students of secondary school, who attend our Embedded Technology Club. The number of laboratory tasks in the electronics branch was created, where the described equipment is successfully used for more than 3 years. It helps both to understand theoretical knowledge gained in lectures and to obtain practical experience by design and development of simple electronics circuits in the framework of individual projects.