VGA (Video Graphics Array) is the video graphics array, which is the meaning of IBM with the PS / 2 machine in 1987 (PS / 2 was originally "Personal System 2", "Personal System 2" is a product launched by IBM in 1987 A kind of personal computer. The keyboard and mouse interface used on the PS / 2 computer is the current PS / 2 interface. Because the standard is not open, the PS / 2 computer has failed in the market. Only the PS / 2 interface has been used until today.) A video transmission standard using analog signals at that time had the advantages of high resolution, fast display rate, and rich colors. It was widely used in the field of color displays. This standard is very outdated for today's personal computer market. Even so, VGA is still a standard commonly supported by most manufacturers, and personal computers must support the VGA standard before loading their own unique drivers. VGA first refers to the display mode of the display 640X480.
The application of VGA technology is also mainly based on VGA display cards such as computers, notebooks and other devices. However, on some devices that require color high-resolution images and do not require the use of computers, the application of VGA technology is rarely seen. This paper studies the implementation method of embedded VGA display.
The embedded VGA display system based on this design method can realize the display and control of VGA images without using a VGA display card and a computer. The system has the advantages of low cost, simple structure, and flexible application. It can be widely used in advertising and display of prompt information in public places such as supermarkets, stations, airports, etc., and can also be used in operation information display in the production process of the factory floor. Used in daily life in multimedia form.
VGA principle
VGA display and VGA timing realization
The general VGA display card system is mainly composed of three parts: control circuit, display buffer area and video BIOS (Basic Input Output System) program. The control circuit is shown in Figure 1. The control circuit mainly completes functions such as timing generation, data operation of the display buffer, master clock selection, and D / A (Digital to Analog) conversion of digital signals to analog signals; the display buffer provides display data buffer space; and the video BIOS as a control program It is solidified in the ROM (Read-Only Memory) of the display card.
1 VGA timing analysis
Through the analysis of the basic working principle of the VGA display card, we can know that to realize the VGA display, we must solve the problems of data source, data storage, and timing implementation. The key is how to realize the VGA timing. VGA's standard reference display timing is shown in Figure 2. Both line timing and frame timing need to generate four parts: sync pulse (Sync a), display back edge (Back porch b), display timing segment (Display interval c) and display front edge (Front porch d). The timing parameters of several common modes are shown in Table 1.
2 VGA timing realization
First, determine the main clock frequency according to the refresh frequency, and then calculate the total number of line cycles from the main clock frequency and image resolution, and then according to the main count pulse source for the timings of the a, b, c, and d timing sections given in Table 1 The frequency is converted to the number of clock cycles. Use counters and RS flip-flops in CPLD to generate pulse signals of different widths and cycles based on the calculated clock cycles of each timing segment, and then use their logical combination to form a, b, c, d in Figure 2 The blank signal BLANK and the synchronization signal SYNC of each timing segment and the D / A converter.
VGA reference timing 3 Generation method of reading SRAM address
The main clock is used as the pixel count pulse signal, and provides the read signal of the memory SRAM and the D / A conversion clock. The output of the counter driven by it is used as the low address of the read SRAM. The line synchronization signal is used as the line count pulse signal, and the output terminal of the counter driven by it is used to read the high-order address of the SRAM. Since two SRAMs are used, the most significant address is used as a chip select for SRAM. Because there is a certain time delay when the signal passes through the CPLD internal logic device, when the CPLD generates the address and read signal to read the data, the read signal, address signal and data signal cannot meet the timing requirements of the SRAM read data. You can use the hardware circuit to perform certain timing adjustments on the read signal, so that each signal can meet the timing requirements of reading SRAM and inputting data for the DAC.
4 Data width and format
If the VGA displays a true-color BMP image, then each of the three components of R, G, and B needs 8 bits, that is, 24 bits represent a pixel value, and in many cases 32 bits are used to represent a pixel value. In order to save the storage space of the video memory, a high-color image can be used, that is, each pixel value is represented by 16 bits, and the three components of R, G, and B use 5, 6, and 5, respectively, which is half of the data amount of the true color image , And at the same time can meet the display effect.
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