Design of an embedded experimental platform based on ARM7

    1 Introduction

    Embedded system is the product of the combination of advanced computer technology, semiconductor technology, electronic technology and specific applications in various industries. It is a system in which hardware and software are closely tied together. This feature determines that it must be a technology-intensive, highly decentralized, and constantly innovative knowledge integration system. Most of the current embedded system talents are trained on the job. Domestic universities rarely set up undergraduate majors in embedded systems. It is only to meet the needs of society that relevant courses are opened for students of certain majors to learn. Embedded system design is a very practical course, and its theory is still in constant development. Without a good practice platform, embedded system technology is difficult to master, students' skills cannot be improved, and trained students cannot adapt to market needs.

    Some existing embedded system experimental platforms are already very mature, and their products cover chips from major companies from ARM7 to ARM9. These platforms not only realize all the functions of ARM chips on an experimental board, but also carry out relevant expansions, and provide almost all interfaces on the experimental board, so that students can fully access the programming of various types of interfaces, and can well conduct various software experiments and development on the experimental board. However, this type of platform also has considerable shortcomings. On the experimental platform provided by it, students can only complete the exercises of the software part, but cannot get a good exercise in the analysis and design of the hardware part. ARM development is a technology that combines software and hardware. In terms of ARM hardware design, students generally still have considerable deficiencies, and educational resources in this area are currently scarce. A suitable ARM experimental platform is needed to comprehensively improve students' software and hardware levels. This design proposes some reference opinions on the construction of the ARM experimental platform.

    2 Hardware Implementation of LPC2368 Embedded Experiment Platform

    In order to allow students to fully access the hardware design of the ARM board and better achieve the teaching purpose, the experimental platform adopts a hardware separation design of the core board and the expansion board (as shown in Figure 1). Only one ARM minimum system is designed on the core board, and the port is led out for expansion, while the relevant interfaces are expanded on the expansion board as needed. This simplifies the complex, so that students can not only have a deeper understanding of the simple ARM minimum system during the experiment, but also design the peripheral expansion circuit by hand , and finally perform programming. The software and hardware capabilities are improved at the same time, deepening the impression and interest of learning.

    According to the design, considering the teaching requirements, cost issues and the difficulty of development, this system uses the 32-bit processor LPC2368 based on the ARM7TDMI processor core produced by NXP (formerly PHILIPS Semiconductor). It has built-in ISP/IAP Flash up to 512KB and SRAM up to 58KB, and can run uclinux and uc/OS-II without expanding storage and memory, simplifying the design and reducing costs while meeting the requirements of the experimental platform. At the same time, LPC2368 also supports 10/100 Ethernet, full-speed (12Mbps) USB 2.0 and CAN 2.0B, multiple serial communication interfaces (3 II C, 1 IIS, 4 UART and 3 SPI/SSP) to enhance the flexibility of the design, provide a larger buffer, and have higher processing power; 10-bit A/D and D/A converters and an IRC oscillator, and an SD memory card interface is available. It is powerful and has rich interfaces, making it very suitable for building an experimental platform.

    2.1 Design of the core board

    To make an embedded processor work, it must have a power supply, a clock signal, and a reset signal. If the chip does not have on-chip memory or the internal memory capacity is insufficient, a memory system must be added before the embedded processor chip can work properly. These circuits that provide the necessary conditions for the operation of the embedded processor together with the chip constitute the minimum system. The core board leads out all the I/O interfaces of LPC2368 to facilitate hardware expansion and interface design on the main board. Therefore, the core board part of this design includes ARM chip, power supply, clock, reset, and I/O lead-out.

    2.2 Expansion Board Design

    In this experimental platform, the expansion board mainly improves students' hardware hands-on ability by allowing them to design related peripheral interface hardware by themselves. The I/O port of the mainboard is led out through the slot, which can easily expand the peripheral hardware interface required for the circuit design experiment. LPC2368 is powerful and has rich interfaces. It has multiple serial communication interfaces, USB devices, network interfaces and SD card interfaces.

    This design only completes part of the hardware unit circuit. Students can design the hardware circuit of each interface part according to the actual situation, and then draw the PCB and make the plate, and then verify it repeatedly until it succeeds. This can improve students' practical ability, prevent them from talking on paper, and provide a good opportunity to practice the design process of the hardware circuit and the specific design details. Then they can connect the peripheral interface they designed to the I/O port, transplant the operating system to the motherboard, and learn to write a driver to drive the external circuit. Hardware design and software programming are consolidated and improved at the same time.

    2.2.1 UART serial port design

    LPC2368 has four serial interfaces. In addition to standard data transmission and reception, UART1 also provides full modem control handshake signals. UART3 also has an infrared mode for infrared communication.

    This design uses the RS232 level conversion chip MAX232 to realize the serial communication of UART0. URAT1 uses the SP3243ECA conversion chip to convert the signal into RS232 level , which can be connected to the MODEM to control the MODEM dialing, communication, etc. The circuit diagram is shown in Figure 2.

    2.2.2 SD/MMC interface design

    The SD/MMC interface consists of two parts:

    (1) The multimedia memory card interface adapter module provides specific functions for the SD/MMC memory card, including clock generation unit, power management control and data transmission;

    (2) The APB interface accesses the MCI adapter register and generates interrupt and DMA request signals.

    LPC2368 has an SD/MMC memory card interface. LPC2368 has integrated SD/MMC controller inside the chip, and only needs to add a memory card adapter outside. The circuit diagram is shown in Figure 3.

    2.2.3 Debug and test interface

    JTAG debugging is the most commonly used debugging method. It is a standard test interface used for testing and downloading before the system is not working properly. It can communicate through the existing JTAG boundary scan ARM CPU core and is a completely non-intrusive debugging. The debugging host PC is responsible for compiling and connecting the ARM source program, and then using the debugger to perform JTAG debugging. LPC2368 has a built-in JTAG debugging port, through which the operation of the chip can be controlled and internal information can be obtained. The debug interface circuit diagram is shown in Figure 4.

    3 Software Design of LPC2368 Embedded Experiment Platform

    An important part of the construction of the embedded system experimental platform is the determination of the embedded operating system. At present, embedded teaching is mainly based on uC/OS-II and uCLinux with open source code. uC/OS-II has a simple structure, and it is easy to fully master the embedded operating system by reading the source code. uCLinux is one of the most popular embedded operating systems, inheriting the advantages of Linux such as good network support and multiple file systems. This experimental platform can well support the operation of the above two operating systems, and students can learn related content such as operating system transplantation and driver development on this platform.

    The underlying hardware driver is a difficult point in embedded system software development, which closely links the software and hardware together. It involves a lot of hardware-related knowledge. If students are allowed to develop it completely, it will be very difficult and not conducive to teaching. Some mature development platforms provide various device drivers, on the basis of which students can easily carry out secondary development. But at the same time, the underlying hardware details are shielded, and when encountering new devices that need to be driven, students will be helpless. In order to meet the teaching requirements of colleges and universities, this system provides the underlying drivers of the main hardware components, which is convenient for students to carry out in-depth secondary development, and students can also use this as a blueprint to deeply learn and master the writing of device drivers when designing hardware interfaces independently.

    After completing the above two steps, a good software development platform is completed. Students can develop user programs based on specific applications on this platform, which is also an important part of embedded system learning. This system provides application development examples on uCLinux and uC/OS-II. Through experiments and learning of these examples, students can fully master the application development process on embedded platforms.

    4 Conclusion

    Based on the rapid development of embedded systems and the relative backwardness of universities in embedded education, this paper designs an embedded experimental platform based on ARM7. The platform has a simple structure, rich interfaces, and complete functions, which can meet the needs of embedded education in universities. It changes the design of most current embedded platforms that integrate various interfaces on a motherboard, and adopts the design concept of core board plus expansion board, so that students of different majors can learn the corresponding knowledge, cultivate software programming technology, and improve hardware design ability, so that they can systematically and comprehensively master various technologies of embedded system development and become qualified embedded system development talents as soon as possible.