LED large screen display because of its eye-catching, flexible content and other characteristics, has been more and more used in advertising, information dissemination, traffic instructions and other public places, and achieved good results. LED display is divided into digital display and dot matrix display two large numbers, this article only discusses the dot matrix display. The current LED display is basically the first request by the user, the production plant according to the need to build, each time to repeat the design of circuit and mechanical structure, resulting in waste of resources, and if the user's needs change, changes will be very difficult.
In fact, regardless of the size of the display, the principle is the same, so you can design a standardized, modular LED display, will be different needs, only need to combine the appropriate module. This article describes a modular LED display, you can flexibly change the size according to need, and can be run independently from the computer, you can also achieve such as flashing, rolling display and other effects. A problem with the flashing of the overall display refresh rate is not a problem in this design because it is split into small modules.
Basic principles
The basic design idea is to divide the entire display into several display modules of the same size and the same function, and additionally control with a central control module to synchronize the operation of all display modules to achieve the overall coordinated display.
So the system consists of two parts: display module and control module.
The display module is actually the same size of the small LED driver board, can drive LED array, with the basic display function, and can receive data and commands from the control module, in addition to some commonly used display such as flashing, rolling, etc. are also included in the display Module. The existing national standard dot matrix Chinese character library has 16 × 16 and 24 × 24 two, taking into account the convenience of software and hardware capabilities, we show the module as 16 × 16 dot matrix size.
The control module is responsible for controlling the display modules and sending data and commands to each display module. The control module stores the lattice data information to be displayed and transmits the data to each display module at regular intervals. Control module also comes with a small keyboard and universal asynchronous serial port, which can operate with the keyboard, you can also use the computer control, you can also use the computer to update the control module data.
Display module and control module between the bus connection, we use the I2C bus.
The design of the whole system has a lot of freedom: the number of display modules controlled by the control module can be changed, the arrangement of the display module can also be changed (such as 16 display modules, either 2 × 8 or 4 × 4) , The same piece of the screen can save a number of conditions with the information were displayed, each information lattice size can be different, more than the size of the screen information can be scrolled out. This basically can meet a variety of needs.
2 circuit structure
Using PIC16C73 as the central controller. The microcontroller has a built-in I2C interface, serial port, and has a strong mouth line drive capability (25mA per port), use it can simplify a lot of peripheral circuits.
Data storage using two EEPROM: X24256 provides a total of 64K bytes of data space, you can repeatedly erase the reset, power-down data is not lost. If all used to display 16 × 16 dot matrix Chinese characters, you can display about 2000. The data memory shares the I2C bus with each display module, depending on the address of the memory or display module.
Small keyboard external 4 current limiting resistor directly connected to the CPU port line, the keyboard with 4 × 4 scan mode.
Control module and the computer connection using RS232 serial port, with a MAX202 do level conversion.
In addition the control module also has a light intensity sensor, its role is to adjust the brightness of the display. As the day and night the ambient light intensity is very different, during the day feel the appropriate bright Fang to the evening may be very sleepy, so according to the outside world, the use of the sensor automatically adjust the degree of the display. Here is the use of TI's TLS235 optical conversion components.
Each display module to drive a 16 × 16 LED array, due to the small number of single-chip mouth line, the use of static display is not possible, where the use of dynamic scanning display. Taking into account the refresh rate factor, the use of 16 lines of scan refresh may be flickering, so use 8 lines of scanning, 32-bit data.
Each data line at each time only corresponds to a light-emitting unit, requiring a smaller current; and when the display is fully open, 8-way common anode scan line will correspond to 32 light-emitting unit, the current requirements, so the two ends of the drive circuit Are designed separately. Data terminals use TI's 4 TPIC6B595. The scanning side uses Allegro's two pieces of 2944.
The display module is connected to the control module using the I2C bus. Since the task of the display module is only displayed, no additional memory is added, and only the internal RAM unit holds the dot matrix data to be displayed and scans the display to the LED array. Such as scrolling, blinking and other display is only on the display data to do additional processing, such as rolling the actual corresponding data shift operation. These functions are also programmed in the display module, which are activated by the control module to activate these functions.
In addition, it should be noted that the display module of the drive circuit and LED array can be seen as two parts of the separation, as long as the drive circuit drive capacity range, you can use the LED unit LED array, such as a light-emitting unit is only 1 LED small display card or a light-emitting unit, including 5 to 7 LED large display card.
3 software design
The system software includes a part: display module, control module and PC.
Display module software, including scanning display, special effects processing and I2C communication in three parts. Three display module CPU RAM to open up 32 bytes as a display buffer, scan display program is only from the display buffer to send data to the mobile register, and set the corresponding scan line to achieve a scan. The scan shows that the data in the buffer is not recognized. Scanning is the use of the human eye image retention effect, if the scanning frequency of 60Hz or more, the scanning frequency fluctuations are not sensitive, so here the scan display program does not use the timer interrupt, only by the cycle to complete.
Special effects processing refers to the operation of the scan buffer data shift, mainly corresponding to the rolling display. Scrolls one pixel row or column each time, most of the data already exists in the display module, the control module sends a scroll command when the United Nations adds two bytes (1 row or 1 column) of the new data. As the use of the scan mode is 32 × 8 way, so the data storage is not completely linear, and there are jump storage part, plus the characters scroll up and down about four directions, to move the new data into the buffer and remove the word Section of the old data, which requires more complicated RAM address calculation.
I2C communication is PIC16C73 internal hardware implementation of the function, relatively simple, only the interrupt can be achieved.
The adjustment function of the display brightness is achieved by adjusting the lighting time of the light emitting unit. Specifically, because it is scanning display, each light-emitting unit up to 1/8 of the time can be lit, if the 1/8 of all the time light, the effect is the maximum brightness, if in this 1/8 of the time There is still some time is not bright, the overall display brightness drops. The display module receives the ambient luminance data from the control module and adjusts the ratio of the lighting time to achieve the function of adjusting the brightness.
The control module program includes I2C communication with the display module, I2C communication with the EEPROM, serial communication with the computer, data address calculation, external light intensity detection several parts.
Two I2C communication sections and serial communication are supported by hardware, making it easier. External light intensity sensor directly into the frequency into the CPU into the frequency, so as long as the timer can be counted. The more complex part is the address calculation of the data. Due to the degree of freedom of the system, the data is stored in the EEPROM using the indexing method. The address calculation is logically simple but it is complicated to implement. It is necessary to send different contents to each display module. For the rolling display of the situation is more complex.
If it is static display, the control module only receives the command to send data once to enter the idle wait: If it is flashing, rolling and other display, you need to take the data from the EEPROM to take the display module.
PC part of the program is mainly lattice editing and RS232 communication functions. Dot matrix editing can not only write a variety of characters characters, but also can draw a simple graphics; RS232 communication not only includes all the functions can be achieved with the keyboard, you can also display the graphics directly on the display without having to save to the control module, Immediate release of information.
The system of the commonly used LED large screen display device made a standardized, modular and useful attempt, and has been used in the traffic situation display occasions, and achieved good results to prove that the design is reasonable and feasible. As the design for the use of a lot of free to retain the free, so that it can be flexible combination of various applications in a variety of occasions, without having to repeat the design, effective saving of resources. As the original system corresponds to a relatively small display, afraid to use the I2C bus, if you want to do a considerable display of zero can also be replaced by RS485 bus, to provide greater room for expansion. In addition, based on this design ideas, you can further develop color LED large screen display and other products.