1 Introduction
With the continuous development of computers, computer alone can not meet people's needs, scalability computers become an important performance index of computer knowledge, common interface has a mouse, keyboard interface, printer interface, scanner interface.
Microcomputer common interface external view shown below, which is a computer motherboard, mainly by a CPU slot, PCI expansion slots, memory slots, BIOS, CMOS battery, the CMOS jumper, ATX power socket, an audio interface, AGP expansion slots, USB interface, mouse interface, keyboard interface and other components.
Computer interface (interface) is a microprocessor CPU and the "outside world" connection circuit, a relay station CPU and the outside world information exchange. For example to send the original data source, or from an input device through the interface into the operation result via the interface to send it out to the input apparatus; a control command sent out via the interface, the interface live status taking in, and from which the information is to be transformed through the interface transit. The term "outside world" means all devices other than the CPU itself or current including a memory, I / O equipment, control equipment, measuring apparatus, communication device, a multimedia device, A / D and D / A converter, etc. .
As can be seen from the figure, various types of external devices (the peripherals) and a memory, via respective interface circuits are connected to the bus microcomputer system up, the user according to their own requirements, use of different types of peripherals, a respective interface circuit is provided, to hang them on the system bus, configured for different purposes, different size applications.
2. Interface features
From the viewpoint of solving the contradiction present in the CPU and peripheral connection point of view, generally have the following functions
(1) addressing the external apparatus
In the computer system generally has a variety of peripherals, in the same peripherals may be more than one, and a CPU at the same time, with only a peripheral exchange of information, it is necessary to set the I / O interface in peripheral port address decoding circuitry for addressing. The CPU port address code for I / O devices to the interface address decode circuit, and by the decoding circuit, the code into translated address selection signal I / O devices. Typically the upper address is used to select the interface chip, the internal chip select low address registers to select the switching device needs its own port information, only the selected devices to exchange data with the CPU or communication. The device is not selected, you can not exchange data with the CPU.
(2) signal conversion function
The signal level of the external apparatus may not be TTL level or the CMOS level, it is necessary to complete the signal level conversion by the interface circuit.
(3) a data buffer function
The work rate of the external device is far below the rate of the CPU, in order to improve the efficiency of the CPU and avoid loss of data, the interface must have a data buffer. Data provided in the interface buffer register as an intermediary between the two, temporary data sent by the sender, waits for the receiver to get data for a sufficient time. By means of the data buffer, the data exchange between the CPU and high-speed operation to low speed operation to the external device may be coordinated. Since the data buffer directly connected to the data bus system, it should have a three-state characteristics.
(4) contact function
It includes a handshake interfaces. Control signal sent by the CPU, operating conditions of the external device and a transponder signal must interact with "handshake" approach through an interface with the CPU.
(5) interrupt management functions
Using interrupt communication between the CPU and peripherals, help to improve CPU utilization.
(6) Programmable Function
In order to make the interface with strong flexibility, scalability to adapt to a variety of operational state or mode of operation, the interface should have programmable features.
Requirements for each of the functional interfaces is not provided, the computer system for different uses, different interface function which large complexity of the interface circuit is not the same. But the first three functions is the core of the interface circuit, the interface is generally in demand.
3. Composition Interface
To achieve the above various functions, the interface requires a physical basis - the hardware be supported; there is also a corresponding program - the software, to be driven. So, an interface can actually run, should the hardware and software of two parts.
3.1 hardware circuit
The basic structure of the interface circuit shown in FIG. Three buses: an address bus AB, a data bus DB and a control bus CB connected to the CPU. The address bus AB for providing access to the interface circuit address information. Data bus DB interface for circuit switched data information, command information and status information. Control bus CB includes I / O, ..., INTR signal lines and the like.
3.2 Software Programming
To enhance the interface practical range, semiconductor manufacturers generally in accordance with general-purpose, programmable interface chip programmable pattern design. Therefore, in order to use interface, you need to write a special interface program to the CPU. Multi interface program implemented by assembly language programming, the initialization interface chip generally includes determining the data transmission mode, the control operation of interface hardware and other major features.