Xi'an University of Posts and Telecommunications, Liu Rui, Xi'an Jiaotong University, Wang Xiaodong
1 Introduction
In recent years, with the rapid development of computer technology, large-scale integrated circuits and special components, the 256-level grayscale full-color LED large display has developed rapidly in China, but its display effect is not ideal at present: on the one hand, LED Luminous efficiency is inherently different by the manufacturing process, and this difference also changes with time, so that a large screen composed of a large number of LEDs will show some random dark spots or bright spots, which seriously affect the display requirements. Online point correction is used to eliminate this effect. On the other hand, the existing full-color large screen generally has insufficient brightness level. Even with the use of nonlinear gray control technology, the ability to express color at low brightness levels is still poor. The layering is not strong, and another problem caused by insufficient brightness level is that it is not easy to perform γ correction, so that the full-color LED large display screen produces a certain color distortion.
TI's latest TLC5941 driver chip features dot correction and high brightness levels. The large-screen driver solution composed of him solves the above problems to a certain extent and can constitute a high-performance display system.
2 TLC5941 chip introduction
2.1 chip features
TLC5941 has 28 pins and is a 16-channel LED constant current driver. It can drive 16 LEDs at the same time. The maximum driving capacity of each channel is 80mA. Each channel can be 4096-level brightness control according to the value of the internal brightness register.
The length of each internal brightness register is 12 bits. In addition, the driving circuit for each channel LED is controlled by the internal 6-bit point correction register for 64 levels, and the maximum value of the drive current can be passed through the off-chip resistor. set up.
64-level current control provides LED point brightness correction, and 4096-level brightness adjustment ensures that even at lower brightness levels, each point in the dot has up to 256 levels of gray representation, resulting in red, green, and blue The full color screen can have a color rendering capability of 16M color, which is extra important for high quality color large screen display.
Compared with the traditional color large-screen display system, the programmable logic chip (or high-speed CPU) is used to generate PWM for brightness control. After adopting TLC5941, the programmable chip (or high speed) is completed because the driver chip TLC5941 completes the PWM brightness control. CPU) only needs to handle the output of buffer management, brightness and point correction data, and the design complexity is reduced. Moreover, since the brightness control of PWM is independent of the serial shift of data, it is convenient to obtain a higher frame rate and achieve good dynamics. display effect.
2.2 pin function
All internal data registers, brightness registers, point correction registers and error status information of the TLC5941 are accessed through the serial interface with a maximum serial clock efficiency of 30MHz.
The serial interface of the TLC5941 is similar to the 74HC595, and the interface section consists of five signal lines.
Mode: Mode=0 is the brightness signal input mode, Mode=1 point correction signal input mode.
SCLK (serial clock), at the rising edge of each SCLK, when Mode=0 input data and output data is shifted in and out of the internal 192-bit (16 channels × 12) luminance serial shift register, when Mode=1 input data And the output data is shifted in and out of the internal 96 (16 channels × 6) bits of the point correction serial shift register.
SOUT: Serial data output.
SIN: Serial data input.
XLAT: Data latch, on the rising edge of XLAT, if Mode=0, the luminance serial shift register is latched into the brightness control register, and the brightness PWM output is randomly controlled. If Mode=1, the point correction serial shift register is latched. The point correction control register controls the output of the current.
In order to ensure the reliable operation of color large screen, TLC5941 provides the ability of LED open circuit and over temperature detection. The pin XERR is the open collector output, which is used for alarm when error occurs. No matter which channel has errors in 16 channels, XERR Will be pulled low, by querying the internal state information of the chip, you can know which way to fault, the XERR pin of all TLC5941 in the system can be connected together, through the pull-up resistor to the high level, through monitoring This signal allows the system to self-diagnose during operation.
In addition, TLC5941 also provides GCLK pin, inputting a clock signal to synchronize PWM generation.
3 Dynamic scanning drive circuit based on TLC5941
The design object is a 640×480 full-color display system. Only the driver part is introduced here. The whole screen is composed of 4 sub-screens. Each sub-screen manages a range of 640×120 pixels, and has a separate driving circuit. Because it is an indoor screen, the drive design uses a dynamic 1/8 scan drive. The control of the driver circuit is realized by the programmable logic device EPM1270 (Altera). In order to increase the frame rate, the serial data adopts 15 parallel output methods, each of which refreshes the range of 640×8 pixels, which is given in Figure 1. It is a single circuit block diagram of the sub-screen driver.
Here, TLC5941 is cascaded to form a column driver for the LED dot matrix, and the row driving portion is composed of 74HC138 and STM4953 (PMOS transistor, 4.5A).
The EPM1270 chip is responsible for managing the display buffer, processing the external bus interface portion and maintaining the dynamic scanning process of the LED dot matrix. The internal module structure of the EPM1270 is shown in Figure 2.
In order to prevent the access between the register during the dynamic scanning of the LED and the access of the external bus when updating the display data, a double buffer structure is also used. When the LED scanning process accesses a memory, it is exposed to the bus interface. It is another memory. When the specific scan control register of the external interface operates, it causes the exchange of two registers, and the display content is also updated. The memory uses two pieces of static RAM - IDT71V424 (512k × 8), interface of EPM1270 and memory. The lower 8 bits use address data multiplexing to save the I/O pins of the EPM1270.
The first 26k of memory stores the color information of each point. Each pixel has 2 bytes of 24 bits representing the color. Each byte corresponds to the color luminance value of 3 pixels of red, green and blue of one pixel. At the beginning of 256k, the point correction data corrected by the γ correction is stored.
The brightness of the whole screen is controlled by the brightness register extended by the EPM1270. When each TLC5941 is written, the EPM1270 controls to read the color element value of each display unit from the memory space corresponding to the current pixel, and then obtains 12 bits after the operation with the brightness register value. The brightness value of each channel TLC5941 (control the brightness and color of each pixel), output after parallel/serial conversion, while maintaining Mode=0; after outputting the brightness, read 6-bit correction data from the corresponding space of 256k later , and / string conversion output, while maintaining Mode = 1, thus completing the output of a channel data, after a row corresponds to all channel data output, pause the serial clock, set Mode = 0, generate a positive in the XLAT foot Pulse, then set Mode=1, at XLAT foot
A positive pulse is generated, and the data is respectively latched into the corresponding control register in the TLC5941, and one line of data is output.
The specific process is given in Figure 3.
4 Conclusion
The ERM1270 is logically designed in Verilog language, which takes up 79% of the chip resources. The color large-screen system with the above design has a refresh rate of 60Hz. Through gamma correction and dot correction, the brightness of each pixel in the whole screen is uniform, and the layering is very Strong, to meet the design requirements, this TLC5941-based full-color large-screen drive solution online screen and offline screen can be used, and has proved to have a good display effect.
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