Any microprocessor must be connected to a certain number of components and peripherals, but if each component and each peripheral are directly connected to the CPU by a set of lines, the connection will be complicated or even difficult to implement. In order to simplify the hardware circuit design and simplify the system structure, a set of lines is commonly used, and the appropriate interface circuit is configured to be connected with various components and peripheral devices. This set of shared connection lines is called a bus. The use of a bus structure facilitates the expansion of components and equipment, and in particular, the development of a unified bus standard makes it easy to interconnect different devices.
The bus in a microcomputer generally has an internal bus, a system bus, and an external bus.
(1) The internal bus is a bus between each peripheral chip and the processor inside the microcomputer, and is used for interconnection at the chip level;
(2) The system bus is a bus between each plug-in board and the system board in the microcomputer, and is used for interconnecting at the level of the plug-in board;
(3) The external bus is a bus between the microcomputer and the external device. The microcomputer acts as a device through which information and data are exchanged with other devices, and is used for interconnection at the device level.
In addition, in a broad sense, the computer communication method can be divided into parallel communication and serial communication, and the corresponding communication bus is called a parallel bus and a serial bus. Parallel communication speed is fast and real-time is good, but it is not suitable for miniaturized products due to the large number of occupied lines; while the serial communication rate is low, it is simpler in the micro-processing circuit where the data communication throughput is not very large. 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 constantly developing and perfecting, and the computer bus technology has a wide variety and unique features. In the following, only the bus technologies that are currently popular in various types of microcomputers are introduced.
First, the internal bus1. I2C bus
The I2C (Inter-IC) bus was introduced by Philips Corporation more than 10 years ago and is a new bus standard widely used in the field of microelectronic communication control in recent years. It is a special form of synchronous communication, which has the advantages of less interface lines, simplified control mode, small device package form and high communication rate. In master-slave communication, multiple I2C bus devices can be simultaneously connected to the I2C bus to identify the communication object by address.
2. SPI bus
The Serial Peripheral Interface (SPI) bus technology is a synchronous serial interface introduced by Motorola. Most MCUs (microcontrollers) manufactured by Motorola are equipped with SPI hardware interfaces, such as the 68 series MCUs. The SPI bus is a three-wire synchronous bus. Because of its strong hardware capabilities, the SPI-related software is quite simple, giving the CPU more time to process other transactions.
3. SCI bus
The serial communication interface SCI (serial communication interface) was also introduced by Motorola. It is a universal asynchronous communication interface UART, which has basically the same asynchronous communication function as the MCS-51.
1. ISA bus
The ISA (industrial standard architecture) bus standard is a system bus standard established by IBM in 1984 for the introduction of PC/AT machines, so it is also called the AT bus. It is an extension of the XT bus to accommodate 8/16-bit data bus requirements. It has been widely used in the 80286 to 80486 era, so that the ISA bus slot is still reserved in the Pentium. The ISA bus has 98 pins.
2. EISA bus
The EISA bus is a bus standard jointly introduced by nine companies such as Compaq in 1988. It uses a double-layer socket on the basis of the ISA bus. In addition, 98 signal lines are added to the 98 signal lines of the original ISA bus, that is, an EISA signal line is added between the two ISA signal lines. In practice, the EISA bus is fully compatible with ISA bus signals.
3. VESA bus
The VESA (video electronics standard association) bus is a local bus jointly introduced by 60 accessory card manufacturers in 1992, referred to as VL (VESA local bus) bus. Its introduction laid the foundation for the innovation of the microcomputer system bus architecture. The bus system considers the direct connection between the CPU and the main memory and the Cache. Usually, this part of the bus is called a CPU bus or a main bus, and other devices are connected to the CPU bus through the VL bus, so the VL bus is called a local bus. It defines a 32-bit data line and can be extended to 64 bits through an expansion slot, using a 33MHz clock frequency with a maximum transfer rate of 132MB/s, which works in sync with the CPU. It is a high-speed, high-efficiency local bus that supports 386SX, 386DX, 486SX, 486DX and Pentium microprocessors.
4. PCI bus
The PCI (peripheral component interconnect) bus is one of the most popular buses at present, and it is a local bus introduced 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 operations, maximum transfer rate of up to 132MB / s, can support multiple sets of peripherals equipment. The PCI local bus is not compatible with the existing ISA, EISA, and MCA (micro channel architecture) buses, but it is not subject to the processor and is a bus developed based on a new generation of microprocessors such as the Pentium.
5. Compact PCI
The system buses listed above are generally used in commercial PCs. In the computer system bus, there is another system bus designed to adapt to the industrial field environment, such as STD bus, VME bus, PC/104. Bus, etc. Here is only one of the popular buses for industrial computers today - Compact PCI. Compact PCI means "solid PCI". It is the first PCI system adopting passive bus backplane structure. It is the industrial assembly standard of PCI bus electrical and software standard plus European card. It is the latest industrial computer. standard. Compact PCI is based on the original PCI bus. It takes advantage of PCI to provide a high-performance core system that meets the requirements of industrial environments. It also considers leveraging traditional bus products such as ISA, STD, VME or PC/ 104 to expand the system's I / O and other features.
Third, the external bus1. RS-232-C bus
RS-232-C is a serial physical interface standard developed by the Electronic Industry Association (EIA). RS is an abbreviation of the English "recommendation standard", 232 is the identification number, and C is the number of revisions. The RS-232-C bus standard has 25 signal lines, including one main channel and one auxiliary channel. In most cases, the main channel is mainly used. For general duplex communication, only a few signal lines are needed, such as a strip. Send line, one receive line, and one ground line. The data transmission rate specified by the RS-232-C standard is 50, 75, 100, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud per second. According to the RS-232-C standard, the driver is allowed to have a capacitive load of 2500pF, and the communication distance will be limited by this capacitor. For example, when using a 150pF/m communication cable, the maximum communication distance is 15m; if the capacitance per meter is reduced The communication distance can be increased. Another reason for the short transmission distance is that RS-232 is a single-ended signal transmission, which has common ground noise and cannot suppress common mode interference. Therefore, 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 several tens of meters to several kilometers. RS-485 uses balanced transmit and differential receive, so it has the ability to reject common-mode interference. In addition to the high sensitivity of the bus transceiver, it can detect voltages as low as 200mV, so the transmitted signal can be recovered beyond the kilometer. RS-485 uses a half-duplex mode of operation, and only one point can be sent at any time. Therefore, the transmitting circuit must be controlled by an enable signal. RS-485 is very convenient for multi-point interconnection and can save many signal lines. Applications RS-485 can be networked to form a distributed system that allows up to 32 drives and 32 receivers to be connected in parallel.
3. IEEE-488 bus
The above two external buses are serial buses, and the IEEE-488 bus is a parallel bus interface standard. The IEEE-488 bus is used to connect systems. Devices such as microcomputers, digital voltmeters, digital displays, and other instruments can be assembled using the IEEE-488 bus. It transmits signals according to bit parallel and byte serial bidirectional asynchronous mode. The connection mode is bus mode. The instrument equipment is directly connected to the bus without intermediation unit, but up to 15 devices can be connected on the bus. The maximum transmission distance is 20 meters, the signal transmission speed is generally 500KB/s, and the maximum transmission speed is 1MB/s.
4. USB bus
Universal Serial Bus (USB) is a new interface standard jointly developed by seven world-renowned computer and communication companies including Intel, Compaq, Digital, IBM, Microsoft, NEC and Northern Telecom. It is based on universal connection technology, which enables simple and fast connection of peripherals, which is convenient for users, reduces costs, and expands the range of PC connected peripherals. It can provide power to peripherals, unlike conventional strings and parallel devices that require a separate power supply system. In addition, fast is one of the outstanding features of USB technology. The maximum transfer rate of USB is up to 12Mbps faster than the serial port by 100 times, nearly 10 times faster than the parallel port, and USB can also support multimedia. However, it is not possible to interconnect computers via USB.
5.IEEE1394
Apple's FireWire is based on standards set by the IEEE. There is a great similarity with USB. A tree or daisy-chain structure can be used to connect up to 63 different types of devices on a single interface in a cascaded manner. The transmission rate is high, up to 3.2Gb/s; the real-time performance is good, the bus provides power, the relationship between the devices in the system is equal, the connection is convenient, and the hot plug and plug and play are allowed.
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