This paper introduces a scanning controller implementation scheme that uses FPGA programmable logic device as the design platform and adopts large-screen full-color led display for full-color grayscale image display. After analyzing the principle of "19 field scanning" theoretical gray scale, a new type of high-order gray scale display is proposed for the shortcoming of the full-color LED display refresh frequency limited by serial shift clock. The bit-by-bit lighting control method uses a separate counter to control the refresh rate of the screen in the FPGA circuit design, so that the design of the full-color LED display is more flexible in adjusting the luminous efficiency and refresh rate of the L ED. Finally, according to the design requirements of the large-screen full-color LED display, combined with the gray-scale control method discussed in this paper, the internal circuit implementation structure framework of the FPGA screen scanning controller is given.
1 Introduction
As a kind of large-scale flat panel display device, LED display screen occupies an important position in the display field due to its long service life, low maintenance cost and low power consumption. Especially in recent years, the full-color LED display with red, green and blue primary colors and grayscale display effect has attracted much attention from the industry with its colorful display effect, and has become a product with relatively large growth in the LED display market in recent years. . The performance of life, brightness per unit area, degree of deviation of three primary colors, dot pitch, contrast, gray level (including gray level and linearity), scanning frequency, etc. are the criteria for measuring or comparing horizontally large display devices. The performance of these indicators is largely determined by the performance of the scan controller. Therefore, it is of great significance to study the scanning control method of large-screen full-color L ED display.
Since the luminance of the LED is approximately proportional to the illumination time in the scan period, the implementation of the gray scale is usually achieved by controlling the ratio of the illumination time of the LED to the scan period, that is, by adjusting the duty ratio. The full-color LED display generally adopts a scanning method of bit-by-bit illumination to realize grayscale image display. For an LED display that displays an 8-bit grayscale, the "19-field scan" principle is generally used to achieve 256-level grayscale display. The display data update of the L ED display generally adopts the serial output mode. For example, the static LED full color display designed by the 595 is based on the principle of “19 fields of scanningâ€. When the bit clock is determined, the refresh rate of the display and the luminous efficiency of the LED (the ratio of the longest LED illumination time in one scan period) are also determined. This paper proposes a new bit-by-bit illumination scanning method, which improves the typical "19-field scanning" mode, and can adjust the refresh rate and illumination within a certain range under the condition of serial shift clock determination. Efficiency is adjusted to increase the flexibility of the product to be designed according to the actual application environment and customer requirements.
2, the implementation of grayscale implementation algorithm for bit-by-bit lighting
Taking the 8-bit "19-field scan" theory as an example, the so-called bit-by-bit lighting, that is, one bit of data is sequentially extracted from one byte of data from the lower bit to the upper bit or from the upper bit to the lower bit, and the corresponding pixel is lit in eight times. The corresponding lighting time of each bit is different from the duty cycle of the off time. If the lighting time is multiplied from low to high, there will be 256 combinations of combined lighting times. Define the lighting time corresponding to the D0 bit plus the turn-off time as one time unit. Set to T to get the lighting and turn-off time of each bit shown in Table 1.