TLC5941-based full-color large screen LED driver design
In recent years, with computer technology, large-scale integrated circuits and the rapid development of specialized components, the 256 gray-scale full-color LED large screen at home has developed rapidly, but its display is not ideal: on the one hand, LED's light-emitting efficiency of manufacturing processes by showing the impact of inherent differences, but this difference also changes over time, so that a large number of LED components from the large screen display and there will be some random dark spots orseriously affect the display request, require the use of point-line calibration to eliminate these influences, on the other hand, the existing large-screen full-color general lack of brightness levels, even if the use of non-linear gray-scale control technology, in the low brightness level on the ability of the performance of color are still less shows that the level of flu is not strong, by the lack of brightness levels is another problem caused by correction to γ is not easy, so that large full-color LED display a certain color distortion.
TI's latest chip TLC5941 driver with dot correction and high brightness levels of the characteristics of the composition of his big-screen-driven program to solve a certain extent, the above-mentioned problems, can constitute a high-performance display systems.
Introduction 2 TLC5941 chip
2.1 Chip characteristics
A total of 28 pin TLC5941 is a 16-channel constant current LED driver, able to drive 16 LED, the maximum drive capability per channel 80mA, each channel can be the basis of internal PWM mode register value of brightness 4096 Brightness control brightness of each channel within the length of register 12, In addition, the flow of each channel LED driver circuit 6 from the point of the internal calibration registers 64 control values, and the maximum drive current through the resistor chip set .
64 provides a current control LED brightness calibration point the ability to adjust the brightness of 4096 to ensure that even at lower brightness levels of small, dot-matrix, each point as many as 256 gray said that in order to red-green-blue 16M full color display can be color-color expression, these two points for high-quality large-screen color display is extra important.
Compared with traditional large-screen color display system, designed to make use of programmable logic chips (or high-speed CPU) have focused on PWM control for brightness, the TLC5941, as the driver completed the TLC5941 chip PWM brightness control, programmable logic chip ( or high-speed CPU) only need to deal with cache management, brightness and dot correction data output, to reduce design complexity, and as a result of PWM brightness control and serial data out of nothing to do, it is easy to obtain a higher frame rate, and achieved very good dynamic display effect.
2.2 Pin function
All the TLC5941 internal data registers, register brightness, dot correction registers and error status information is accessible through the serial interface, maximum efficiency of the serial clock 30MHz.
TLC5941 a manner similar to the serial interface 74HC595, Interface in part by the five signal lines.
Mode (mode signal): Mode = 0 is the luminance signal input mode, Mode = 1 dot correction signal input mode.
SCLK (serial clock), in each of the rising edge of SCLK, when Mode = 0 input data and output data into and out of the internal 192 (16 channel × 12) the brightness of the serial shift register, when Mode = 1 input data and output data into and out of internal 96 (16 channels × 6)-bit serial shift register point correction.
SOUT: Serial Data Output.
SIN: Serial data input.
XLAT: data latches, the rising edge of the XLAT If Mode = 0, brightness serial shift register to the brightness control register latch, randomized controlled PWM output brightness, if Mode = 1, point correction serial shift register latch calibration point control register, control the output current.
Color in order to safeguard the reliable operation of large-screen, TLC5941 provides a way for each road and over-temperature LED's ability to detect, XERR pin is open collector output for alarm goes wrong, 16 channels, no matter which channel an error occurred, XERR will be pulled low through an internal inquiry chip status information, you can know which path fails, the system of all the TLC5941 can be received with XERR pin through a pull-up resistor from high, through the monitoring the signal, the system can run the self-diagnosis process.
Another TLC5941 provides GCLK pin, a clock input signal can be synchronized PWM generation.
3 Based on the dynamic scanning TLC5941 driver circuit
This design is the 640 × 480 full-color display system, only the driver on his part, the entire screen by the four sub-screens, each managing a sub-screen 640 × 120 pixel size range, have a separate drive circuit, as is the indoor screen, drive the design of dynamic 1 / 8 scan drive. Driving circuit control programmable logic device EPM1270 (Altera) to achieve, in order to increase the frame rate, serial data output 15-way parallel way, every road on the size of 640 × 8-pixel refresh scope, figure 1 to is the son of a single-screen drive circuit diagram of the road.
Here the composition of the use of cascading TLC5941 column dot-matrix LED driver, the driver in part by the 74HC138 and STM4953 (PMOS tube, 4.5A) pose.
EPM1270 chip responsible for managing the display buffer, to deal with the external bus interface LED dot matrix parts and the maintenance of the dynamic scanning process, EPM1270 internal module structure as shown in Figure 2.
In order to prevent the LED process of dynamic scanning register access and update the display in the external bus data register access conflict between, here is a dual buffer structure, when the LED scanning process is a memory access, exposure to the total line interface is another chip memory, external interface of a particular control register scan operation, caused by the exchange of two registers, but also to update the display, memory using two static RAM - IDT71V424 (512k × 8), EPM1270 and low memory interface address 8-bit data used for the rehabilitation of the EPM1270-saving I / O pin.
The beginning of the first 26k memory storage is the color information for each point, three bytes per pixel, said 24 colors, each byte of a pixel corresponding to the three RGB color pixel brightness value, after 256k after the beginning of the storage is amended γ correction point calibration data.
The brightness of the entire screen brightness by EPM1270 extended control registers, each TLC5941 write, EPM1270 control the corresponding pixel from the current memory space for each display unit to read out the value of the pigment, and then register with the brightness value of 12 after operation each channel of the TLC5941 brightness value (brightness control for each pixel and color), through and / string conversion output, while maintaining Mode = 0; output brightness from the corresponding space for 256k after reading six point calibration data and / string conversion output, while maintaining Mode = 1, so the completion of a channel data output will be his party all the channel data corresponding to the output after the suspension of the serial clock, home Mode = 0, in a foot is XLAT pulse, and then home Mode = 1, at the foot XLAT pulse is generated, the data were locked into the corresponding TLC5941 internal control register, the completion of data output line.
Figure 3 gives the specific process.
EPM1270 using Verilog language for logic design, integrated chip resources after the occupation of 79 percent, using the above-mentioned construction of the color design of large-screen system refresh frequency of 60Hz, through the point γ correction and correction, full-screen uniform brightness of the pixel point, the level of influenza strong to achieve the design requirements, the TLC5941-based large-screen full-color screen-driven program of online and offline screen can be used in practice proved to have good effects.
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