Common bus and interface in embedded system design

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Common bus and interface in embedded system design


Any microprocessor is connected to a certain number of parts and peripherals, but if each part and each peripheral device is connected directly to the CPU with a set of lines, the connection will be intricate and even difficult to achieve. In order to simplify the hardware circuit design, simplify the system structure, commonly used a set of lines, configured with the appropriate interface circuit, with the components and peripherals connected, this group of common connection lines known as the bus.

The use of bus structure facilitates the expansion of components and equipment, especially the establishment of a unified bus standard is easy to achieve interconnection between different devices.

The bus in microcomputer generally has internal bus, system bus and external bus.

(1), the internal bus is the microcomputer inside the peripheral chip and processor between the bus, used for chip level interconnection;

(2), the system bus is the microcomputer between the plug-in board and the system board between the bus, for the plug-in board level interconnection;

(3), the external bus is the bus between the microcomputer and the external equipment, the microcomputer as a device, through the bus and other equipment for information and data exchange, it is used for equipment level interconnection. In addition, in a broad sense, the computer communication mode can be divided into parallel communication and serial communication, the corresponding communication bus is called parallel bus and serial bus. Parallel communication speed is fast, real-time good, but because of the large number of mouth lines, not suitable for miniaturized products, and the serial communication rate is low, but in the data communication throughput is not very large micro-processing circuit is more simple, convenient and flexible.
Serial communication can generally be divided into asynchronous mode and synchronous mode. With the development of microelectronics technology and computer technology, bus technology is also constantly developing and perfecting, and makes computer bus technology a wide variety, unique.

The following is only an introduction to the current popular bus technology in all kinds of microcomputer buses.

First, the internal bus

1.I2C Bus I2C (Inter-IC) bus was launched by Philips more than 10 years ago and is a new bus standard widely used in the field of microelectronics communication control in recent years. It is a special form of synchronous communication, which has the advantages of less interface line, simplified control mode, small package form of device and high communication rate.

In the master and slave communication, there can be multiple I2C bus devices connected to the I2C bus at the same time, through the address to identify the communication object.

2.SPI Bus Serial Peripheral Interface SPI (Serial Peripheral Interface) bus technology is a synchronous serial interface introduced by Motorola. The vast majority of MCUS (microcontrollers) produced by Motorola are equipped with SPI hardware interfaces, such as the 68 series MCU.

SPI Bus is a third-line initializes, because of its strong hardware features, so the SPI related software is quite simple, so that the CPU has more time to deal with other transactions.

3.SCI Bus The Serial Communication Interface SCI (serial communication Interface) was also launched by Motorola.

It is a universal asynchronous communication interface UART, which is basically the same as the asynchronous communication function of MCS-51.
Second, the system bus

1.ISA Bus The ISA (Industrial Standard architecture) bus standard is the system bus standard established by IBM in 1984 to launch the PC/AT machine, so it is also called the at bus. It is an extension of the XT bus to accommodate the 8/16-bit data bus requirements. It is so widely used in the 80286 to 80486 era that ISA bus slots are now retained in the Pentium machine.

ISA Bus has 98 pins.

2.EISA Bus The EISA bus is a bus standard jointly launched in 1988 by 9 companies such as Compaq. It uses a double-decker socket on the basis of the ISA bus, adding 98 signal lines to the 98 signal lines of the original ISA bus, that is, adding a EISA signal line between the two ISA signal lines.

In practical, the EISA bus is fully compatible with ISA bus signals.

3.VESA Bus The Video Electronics Standard Association bus is a local bus jointly launched in 1992 by 60 accessory card manufacturers, referred to as the VL (VESA local bus) bus. Its introduction lays a foundation for the innovation of bus architecture of microcomputer system. The bus system takes into account the direct connection of the CPU to the main memory and Cache, which is usually referred to as the CPU bus or the main bus, and the other devices are connected to the CPU bus via the VL bus, so the VL bus is called the local bus. It defines a 32-bit data cable and can be extended to 64 bits through the expansion slot, using a 33MHz clock frequency with a maximum transfer rate of 132MB/s, which can work synchronously with the CPU.

Is a high-speed, efficient local bus, can support 386SX, 386DX, 486SX, 486DX and Pentium microprocessor.

4.PCI Bus The PCI (Peripheral Component interconnect) bus is one of the most popular buses available today, and it is a local bus launched by Intel Corporation. It defines a 32-bit data bus and can be expanded to 64 bits. PCI Bus motherboard slot is smaller than the original ISA bus slot, its function is greatly improved than VESA, ISA, support burst read and write operation, the maximum transmission rate of up to 132MB/s, can support multiple sets of peripherals at the same time.

The PCI local bus is not compatible with the existing ISA, EISA, MCA (Micro Channel architecture) bus, but it is not subject to the processor and is a bus developed based on a new generation of microprocessors such as Pentium. 5.
Compact PCI Several of the system buses listed above are generally used in commercial PCS, in the computer system bus, there is another large class to adapt to the industrial field environment and design of the system bus, such as STD bus, VME bus, PC/104 bus and so on. Only one of the popular buses of the current industrial computer--Compact PCI is described here. Compact PCI means "solid PCI", is the first PCI system using passive bus backplane structure, is the PCI bus electrical and software standard plus European Card Industrial assembly standard, is the latest industrial computer standards. Compact PCI is a retrofit based on the original PCI bus, which leverages the advantages of PCI to provide high-performance core systems that meet the requirements of industrial environmental applications, while also considering taking full advantage of traditional bus products such as ISA, STD, VME, or PC/104 to extend the I/O and other functions of the system. Third, the external bus

1.RS-232-C Bus RS-232-C is a serial physical interface standard developed by the Electronic Industry Association of the American Electronics Society. RS is the abbreviation for the English "recommended standard", 232 is the identification number, and C indicates the number of modifications. The RS-232-C bus standard has 25 signal lines, including a main channel and an auxiliary channel, in most cases mainly using the main channel, for the general duplex communication, only a few signal lines can be achieved, such as a send line, a receiving line and a ground line. The RS-232-C standard provides for data transfer rates of 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 Porter per second. The RS-232-C standard states that the drive allows for a capacitive load of 2500pF and that the communication distance will be limited by this capacitance, for example, when using a 150pF/m communication cable, the maximum communication distance is 15m, and if the capacitance per meter cable decreases, the communication distance can be increased.

Another reason for the short transmission distance is that the RS-232 is a single-ended signal transmission, there are common noise and can not suppress common-mode interference and so on, so it is generally used for communication within 20m.

2.RS-485 Bus The RS-485 serial bus standard is widely used when the communication distance is required to be dozens of meters to thousands of meters. The RS-485 adopts balanced sending and differential reception, so it has the ability to suppress common-mode interference. Coupled with the high sensitivity of the bus transceiver, the voltage can be detected as low as 200mV, so the transmission signal can be recovered outside the kilometer. RS-485 uses a half-duplex mode of operation, at any time only a point is in the sending State, therefore, the transmission circuit must be controlled by the enabling signal. RS-485 is very convenient for multi-point interconnection and can save many signal lines.

The application RS-485 can be networked to form a distributed system that allows up to 32 drives and 32 receivers in parallel.

3.IEEE-488 Bus The above two external buses are serial bus, while IEEE-488 bus is the parallel bus interface standard. IEEE-488 bus is used to connect the system, such as micro-computer, digital voltmeter, digital display and other equipment and other instrumentation can be assembled with IEEE-488 bus. It transmits the signal according to bit parallel, byte serial bidirectional asynchronous mode, the connection mode is bus mode, the instrument equipment is directly parallel to the bus and does not need the intermediary unit, but the bus can connect up to 15 devices.

The maximum transmission distance is 20 meters, the signal transmission speed is generally 500KB/s, the maximum transmission speed is 1MB/s.

4. USB Bus Universal Serial Bus Serial Bus is made by Intel, Compaq, Digital, IBM, Microsoft, NEC, Northern A new interface standard jointly launched by 7 world-famous computer and communications companies, such as Telecom. It is based on universal connection technology, to achieve a simple and fast connection of peripherals, to facilitate users, reduce costs, expand the range of PC connection peripherals. It provides power to peripherals, unlike ordinary devices that use strings and ports that require a separate supply system. In addition, fast is one of the outstanding features of USB technology, USB Maximum transmission rate of up to 12Mbps than serial port 100 times times faster, than and burnt nearly 10 times times, and USB can also support multimedia. However, you cannot interconnect your computer via USB.

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