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The core computing system of an embedded system can be abstracted into a typical component model: hardware layer, middle layer, software layer, and functional layer, as shown in Figure 1-2.

1. hardware layer

Hardware layer contains embedded microprocessor, memory (such as SDRAM, ROM, Flash, etc.), general device interface and I/O interface (such as A/D, D/A, I/O, etc.). Adding power circuit, clock circuit and memory circuit on the basis of an embedded processor, an embedded core control module is formed. The operating system and application programs can be fixed in ROM.

(1)embedded microprocessor

The core of embedded system hardware layer is embedded microprocessor. The biggest difference between embedded microprocessor and general CPU is that embedded microprocessor mostly works in the system specially designed for specific user groups. It integrates many tasks completed by general CPU into the chip, which is beneficial to the miniaturization of embedded system design, and also has high efficiency and reliability.

Embedded system composition structure

Embedded microprocessors have various architectures, even in the same architecture may have different clock frequencies and data bus widths, or integrate different peripherals and interfaces. According to incomplete statistics, there are more than 1000 kinds of embedded microprocessors in the world at present, and there are more than 30 series of architecture, among which the mainstream systems are ARM, MIPS( Microprocessor without Interlocked Pipeled Stages)/Power PC, X86, SH, etc.

(2)memory

Embedded systems require memory to store executable code and data. Memory of embedded system includes Cache, internal memory and external memory.

Cache: Cache is a memory array with small capacity and fast speed, which is located between memory and embedded microprocessor core, and stores the program code and data most used by microprocessor in recent time. Cache is integrated into embedded microprocessor and can be divided into data Cache, instruction Cache and mixed Cache.

Memory: located inside the microprocessor, used to store system and user programs and data. The on-chip memory capacity is small and the speed is fast.

3 external memory: external memory is used to store large amounts of program code or information, its capacity is large, but the reading speed is much slower than memory, used to store user information for a long time.

Embedded systems commonly used in the external memory are hard disk, NAND Flash, CF card, MMC, SD card and so on.

(3)Universal Device Interface and I/O Interface

Embedded system and external interaction need a certain form of universal device interface, such as A/D, D/A, I/O, etc., peripherals through the connection with other devices or sensors outside the chip to achieve microprocessor input/output functions. Each peripheral usually has a single function, either off-chip or built-in. Peripherals range from a simple serial communications device to a very complex 802.11 wireless device.

At present, the commonly used universal device interfaces in embedded systems include A/D(analog/digital conversion interface), D/A(digital/analog conversion interface),I/O interfaces include RS-232 (serial communication interface), Ethernet(Ethernet interface), USB(universal serial bus interface), audio interface, VGA video output interface, I2C(field bus), SPI(serial peripheral interface), IrDA(infrared interface) and so on

2. intermediate layer

Between hardware layer and software layer is middle layer, also called hardware abstract layer (HAL) or board support package (BSP), which separates the upper software from the lower hardware, makes the lower driver independent of hardware, the upper software developers do not need to care about the details of the lower hardware, according to the interface provided by BSP layer can be developed. This layer generally contains initialization of the underlying hardware, input/output operations of data, and configuration functions of hardware devices.

In fact, the BSP is a software layer between the operating system and the underlying hardware, including most of the hardware-related software modules in the system. Designing a complete BSP requires two parts: hardware initialization of embedded system and BSP functions, and designing hardware related device drivers.

3. software layer

The software layer consists of embedded operation system (EOS), file system, graphic user interface (GUI), network system and general component module. EOS is the foundation and development platform of embedded application software. The first three are described below.

(1)embedded operating system

The complexity of embedded systems with different functions varies greatly. Simple embedded system has only a single function, the program in memory is designed for this function, and its system processing core is also a single task processor. Complex embedded systems are not only powerful, but also often equipped with embedded operating systems, such as powerful smart phones, which have almost the same functions as microcomputers.

Embedded Operation System (BOS) is a kind of system software with wide applications. In the past, it was mainly used in the field of industrial control and national defense system. EOS is responsible for the allocation of all software and hardware resources, task scheduling, control and coordination of concurrent activities. It must reflect the characteristics of the system in which it is located and be able to achieve the required functions of the system by loading and unloading certain modules. At present, some EOS product series with more successful applications have been launched. With the development of Internet technology, the popularization and application of information appliances, and the miniaturization and specialization of EOS,EOS begins to develop from a single weak function to a highly specialized strong function. Embedded operating system has outstanding characteristics in system real-time efficiency, hardware dependence, software solidification, application specificity and so on.

(2)file system

Embedded file system is relatively simple, mainly provides file storage, retrieval, update and other functions, generally does not provide protection, encryption and other security mechanisms. It provides various file operations in the form of system calls and commands, mainly setting and modifying access rights to files and directories, providing services such as creating, modifying, changing and deleting directories, and providing services such as creating, opening, reading and writing, closing and revoking files.

(3)graphical user interface

The widespread application of graphical user interface (GUI) is one of the great achievements of computer development. It greatly facilitates the use of non-professional users. People no longer need to memorize a large number of commands by rote, but instead operate conveniently through windows, menus, buttons and so on. Embedded GUI has the following basic requirements: light weight, less resources, high performance, high reliability, easy to transplant, configurable and so on.

4. functional layer

The functional layer is also known as the application software layer. The application software is composed of applications developed based on real-time systems, running on embedded operating systems, and generally separated from the operating system. Application software is used to realize the control function of the controlled object. Function layer is to face the controlled object and users, in order to facilitate user operation, often need to provide a friendly man-machine interface.

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