Do you know how the control scheme of LED display screen works

The basic working principle of LED display is dynamic scanning.The process of display control is to read the font data from the data memory first, and then write the data to the LED dot matrix through the serial port or parallel port of the single chip computer, and then scan the data again.

The dynamic scanning scheme and the static display scheme save driver elements, but the refresh rate is higher than 50 Hz to avoid flashing the image or text displayed.Due to the limitation of refresh rate, the number of chips that a single chip can control display elements is less.

Now the large-screen LED display is more and more widely used.In order to realize orderly and rapid display control for hundreds or thousands of LED dot matrix chips, people have made a lot of efforts. Dual-cpu, dual-ram schemes and FPGA schemes have been successfully applied.However, the display control process of these schemes is still read before writing.

In this scheme, a reading instruction is used to complete the reading and writing in one step, which can greatly improve the efficiency of display control, and the circuit is simple.

1. Working principle of LED display

The basic working principle of LED display is dynamic scanning.Dynamic scanning is divided into line scan and column scan, the common way is line scan.Line scan is divided into 8 - line scan and 16 - line scan.

LED display screen manufacturer said that in line scan mode, each LED lattice has a set of column drive circuit, and there must be a latch or shift register in the column drive circuit to lock the font data to be displayed.In line scanning mode, the eponymous line control pins of the same row LED lattice are connected to a line, a total of 8 lines, and finally connected to a line driving circuit.There must also be a latch or shift register in the line drive circuit to latch the line scan signal.

The column drive circuit and line drive circuit of LED display screen are generally controlled by single chip microcomputer. The common single chip microcomputer is MCS51 series.Generally, the content displayed by the LED display is stored in the external data storage of the single chip computer in the form of font, which is 8-bit binary number.

Single - chip computer to LED display control process is first read and then write.According to the order of LED array chips on the screen, the single-chip computer first drives the latch on the column of the first LED array chip in the first row, writes the font data read from the external data storage, and then on the second, thirdUntil the last piece of the row is finished, the MCU writes the line scan signal to the row drive latch of the row, so the row 1 and the row 1 light emitting diode related to the font data will light up.Then row 2, row 3, row 1Until the first row of the last row lights up.After each row of row 1 is lit up, it will be delayed for a period of time, and then the screen will be black. In this way, the single-chip computer can complete the one-line scan control of LED display.

Single chip microcomputer on LED display screen line 2 scan control, line 3 scan control..Until the scanning control of line 8, the process is the same as that of line 1.After the control process of all 8 lines is completed, the LED screen will complete the full display of 1 frame image.

Although the LED display is lit up line by line in this way, only one line at a time, but as long as each line can be lit up more than 50 times per second, that is, the refresh frequency is higher than 50 Hz, then due to human visual inertia, the LED display is still a full-screen stable image.

2 traditional control method of LED display

References [1] summarized and compared the control circuit of LED display screen.Among them, the display control circuit works in line scan mode, and the column control circuit is divided into two categories.In the column control circuit, a kind of chip such as 74LS377 is used as the latch of the column drive circuit. The CPU writes font data to the latch of the column drive circuit through the parallel bus.The other is to use a chip such as shift register 74LS595 as the latch of the column drive circuit. The CPU writes the font data to the latch of the column drive circuit through the serial bus.

Whether the control mode of the parallel bus or the control mode of the serial bus, its working process is to first assign A value to the data pointer DPTR, and then accumulator A reads the font data from the external data memory RAM according to the direction of the data pointer DPTR.Then, when the parallel bus is running, the data pointer DPTR is assigned. Then, the CPU sets the font data in the accumulator A, and writes the latch of LED array drive circuit according to the direction of the data pointer DPTR.In the serial bus, the CPU writes the font data in accumulator A to the latch that drives the circuit through the serial port.

In general display control, the more used microcontroller is the MCS51 series.Assuming that the frequency of the single chip microcomputer system is 12 MHz and the machine cycle is 1 wheel s, the display control of one LED lattice in the above two control modes should be more than ten wheel s.

In the high-speed control scheme proposed in this paper, the display control of 1 LED lattice is only about 4 wheel s.By this calculation, one single chip MCS51 series can almost control the display of more than 600 LED dot matrix chips.Compared with the traditional control method, the efficiency of the control is improved several times.

3 high-speed control scheme of LED display

Figure 1 is the principle of LED display circuit of high-speed control scheme.The LED display is controlled by MCS51 series microcontroller.Ram 62512 is used as the data storage of LED display, to store the font data of the content to be displayed;The eight-line scan mode is adopted, and the multiple LED arrays share one set of row driving circuits.Each LED lattice has a set of column drive circuits. 74LS377 is used as the column drive latch. The CPU writes font data to the latch of the column drive circuit through the parallel bus.Address decoding circuit used to generate the chip selection address of LED array line drive circuit and column drive circuit.

This scheme has two characteristics: first, although the CPU still writes font data to the latch of the column drive circuit through the parallel bus, the latch signal of the latch switch to the control signal RD of the CPU instead of the conventional WR;Second, the address decoding circuit ensures that the selected address of the LED array drive circuit and the logical address of a segment of data storage are overlapped, rather than in regular usage, and that the two groups of addresses must be separated.

LED display manufacturers said that due to some simple changes in the circuit described above, the display control efficiency of single-chip microcomputer on LED display will change significantly.The process is as follows: it is assumed that the address of data storage has been loaded in the data pointer DPTR, and the instruction "MOVXA, @dptr" is executed.The function of this instruction is that the CPU reads the font data from the external data memory according to the direction of DPTR to the accumulator A;However, in this circuit, because the chip selection address of the LED array drive circuit and the logical address of a segment of data storage are overlapped, that is, when executing the instruction "MOVXA, @dptr", DPTR not only points to an address of the external data storage, but also selects a latch of an LED array drive circuit.If the latch pin of the selected latch comes in at the right moment, the latch locks the font data sent from the external data store.This impulse is RD.RD is the read control signal that the CPU sends to the external data storage while executing the instruction "MOVXA, @dptr".Since the reading control signal RD and writing control signal WR of MCS51 series MCU have the same timing sequence [2], RD can replace WR to realize latch function, of course, there is no suspense.This instruction, when executed, completes the reading of the data store while also completing the writing of the LED dot matrix, thus speeding up the display control process.

As mentioned above, when the parallel bus is used, the CPU completes the programming process of writing the font data to the latch of the column drive circuit of LED lattice sheet once, which requires about a dozen qualifications.Now, as long as 4 mu s faster, because now complete 1 to LED lattice column driver circuit of latch letter data application process as long as the two steps, first of all, give it an effective address DPTR data pointer, then the CPU according to DPTR point to read font data from external data memory, at the same time will also be passed on to the LED dot matrix font data column driver circuit latch.Two instructions, four machine cycles, minus s.It should be added here that when compiling the program for writing font data for all LED lattice-driven circuits, do not use circular instructions, because then the process would have to add 2 again each time.To adopt the method of chip - by - chip LED lattice programming, so that the programmed procedures although space, but save time.The design method of changing space for time is sometimes also a method worth trying.

This circuit‘s row-driven latch control, or the CPU write control signal WR, do not change.The line - driven latch selection signal is also from the address decoding circuit.To avoid interference between the data storage and the LED lattice, this portion of the data storage corresponding to this set of addresses is not used.

The design of address decoding circuit should ensure that the selected address of LED array drive circuit and the logical address of a segment of data storage are overlapped.Examples of the specific design are as follows:

It is assumed that a certain LED display screen is equipped with 240 LED dot arrays, which can display 60 Chinese characters of 16 x 16, and a MCS51 series MCU is used for high-speed control.The selected address of the 240 LED array drive circuits should be 240, and the address decoding circuit must ensure that the effective address after decoding is greater than this number.In the address decoding circuit in figure 1, the input address signals are A0 ~ A7 and A11 ~ A15, without access to A8, A9 and A10.With 74LS138 decoder, 256 effective address lines can be obtained after three-level decoding. The first effective address line corresponds to 8 addresses of external data storage: 0000H, 0100H, 0200H, 0300H, 0400H, 0500H, 0600H and 0700H.The second valid address line corresponds to 8 addresses of external data storage: 0001H, 0101H, 0201H, 0301H, 0401H, 0501H, 0601H and 0701H.......The 256th effective address line corresponds to 8 addresses of external data storage: 00FFH, 01FFH, 02FFH, 03FFH, 04FFH, 05FFH, 06FFH and 07FFH.These 256 valid address lines, 240 for the column drive circuit, and the remaining for the row drive circuit;If not enough, the line drive circuit can be considered to change to serial bus mode for control.The above analysis results show that the I/O interface address of 1 LED lattice is overlapped with the address of 8 bytes of data storage.This is because each LED dot matrix has eight rows, and each row corresponds to one byte of font data.

The above analysis results also show that the mapping relationship is established between the I/O interface address of all LED arrays and the address segment of 0000H ~ 07FFH of data storage.The data storage in 0000H ~ 07FFH contains all the font data of exactly one frame of image.

Application of high speed control scheme in LED display

Now commercially available large LED displays use hundreds, thousands, even thousands of leds.The control of LED display by SCM includes communication between SCM and PC, data processing of font data and display control.A single chip to communicate with PC, data processing, display control, is certainly busy.

In order to solve the control problem of large screen LED display, many literatures have successfully designed the control scheme.The basic idea of many schemes [36] is that the data processing is completed by one single chip, and the display control is completed by another single chip or a specially designed circuit.Although the control efficiency of these schemes is high, the circuit is complex.

The basic idea of this scheme is that communication, data processing and display control between MCU and PC are completed by 1 MCU.The display control adopts the high-speed control scheme proposed in this paper. The circuit is simple and the display control is efficient.For example, when the LED grid plate adopts the commonly used LED grid plate with an external size of 6 cm x 6 cm, and the screen area is less than 2 m2, one single chip MCS51 series can be completed.However, the high-speed control scheme for large LED displays still has some problems to be solved:

The communication problem between computer and PC.When the large LED screen is connected to the PC, the PC is used to edit the content to be displayed and send the content to the single chip computer in the large LED screen.PC and MCU communication, will not interfere with the work of the display screen.Because when the display screen works, it is displayed one by one, and there is black screen time between fields, so there is no problem in communicating with the black screen time.

A problem with increasing the number of shows.The way most screens work is by displaying one thing, one thing, another, and so on.The previous design only considered the problem of establishing a mapping relationship between the I/O interface address of the LED lattice and a section of data memory when displaying one frame of image, so only one frame of image can be displayed.The addition of figure 2 on the basis of figure 1 enables the I/O interface address of the LED lattice and the multi-segment of data storage to establish a mapping relationship.At work, p1-port controls multiplexed switches, switching between different segments of data storage and I/O interface mapping of LED dot matrix chips, so that the display screen can be displayed in a field loop.If the number of external data memory chips is expanded and one of them can be effective by P1 port, then more segments and I/O interface addresses of LED lattice chips can be expanded to establish a mapping relationship. In this way, some display effects such as drawing screen and flowing water can be achieved.

Data processing problem for create type data.There are many ways to display, such as stop grid, draw screen, water flow, and water flow mode to the left, right water, and so on.In the conversion display mode, the data processing of font data must be carried out once, using a single chip microcomputer, which will not become a problem.Because when the display mode was changed, the screen was supposed to be black from 1 s to several s, and this time was just used for data processing.

conclusion

The high speed control scheme of LED display is proved to be stable, reliable, and simple in circuit.In addition, the flexible use of reading instructions proposed in this paper can also provide people as a reference.