LED display control technology

LED display screens need to be controlled by a control system based on an application-specific integrated circuit (ASIC). Companies such as Maxin, Agilent and Toshiba are the world's major manufacturers of LED display screen control ASICs. In addition to the development of LED display technology itself, due to the continuous advancement of network technology and practical application needs, network-based and intelligent LED control technology has also seen a new development momentum, which has posed new challenges to the traditional control of one or more LED displays by a microcomputer. Since LED display technology and network technology were originally independent of each other, in order to realize network control of LED displays, it is necessary to develop relevant clauses that comply with network system protocols and specifications.

1. Agilent LED display control solution

Agilent is not only an important manufacturer of LED, but also one of the world leaders in LED applications. The company has launched three series of outdoor LED full-color screens and two series of indoor LED full-color screens, which are the most complete product series of LED display screens in the world market. The LED display screen technology solutions provided by Agilent are as follows:

(1) Data transmission of LED display system

In terms of transmission, large-screen systems generally use serial mode to transmit video data, and optical fiber mode is usually used for reliable transmission over long distances. |

Agilent's high-speed serial/deserializer chipset can meet customers' requirements for speed and synchronization. Agilent's HDMP-1032/1034 and HDMP-1022/1024 GLINK chipsets can achieve serial transmission rates of 1200Mbit/s and 1500Mbit/s respectively. The parallel port of the chipset is a parallel 16-22-bit TTL level with a maximum speed of 70MHz and 75MHz respectively, while the serial port uses a high-speed PECL level. The chipset has a built-in CDR to ensure clock recovery and synchronization under long-distance conditions, which is very suitable for long-line transmission. Users can send 24-bit RGB signals and control signals to the GLINK chipset within two clock cycles. This reduces the demand for bus bits and the number of chipsets, which can effectively reduce costs. The chipset supports command transmission and can use the same serial channel to transmit control commands. The receiving end can easily interpret the command. The chipset also supports additional parity bit transmission. In the short distance, users can use a coaxial cable to connect; in the long distance, users can use a pair of optical modules to transmit video signals. For high-bandwidth applications, such as customers need higher color depth, a pair of serial/deserializer chipsets may not be enough. The GLINK chipset supports two groups in parallel, which can be easily upgraded to 2.5GHz bandwidth to meet customers' higher bandwidth requirements. The two chipsets are strictly synchronized, which can simplify the transceiver circuit and logic design, as shown in Figure 1.

Figure 1 Transceiver circuit and logic design

In long-distance applications, optical modules are required to ensure reliable transmission, usually megahertz sampling optical modules. Agilent can provide users with a full range of optical module products, including single-mode and multi-mode optical module products from 100THz, 1PHz to 10PHz. Currently, the HFBR-53D5 1.25GHz optical module is widely used in the large-screen field.

In general, a pair of GLINK chipsets can meet user requirements, so only one coaxial cable or optical fiber is needed, greatly simplifying on-site wiring and maintenance.

(2) Ethernet interface

In order to support multimedia applications, some large screens are equipped with Ethernet interfaces to support the playback of MPEG2 and other video streams. As a leading Ethernet chip supplier, Micrel's Ethernet products include Ethernet switching chips, Ethernet controllers, Ethernet physical layer products, etc., which can meet the Ethernet requirements of the Ethernet screen system. These products support l0THz, 100THz Ethernet, with IVDI/MDI¨x JllffE, can automatically adjust the transmit and receive line pairs, and can be used with straight or crossover cables.

Micrel's Ethernet products support cable and fiber modes. The fiber mode supports placing the Ethernet screen in a relatively remote location. The Ethernet controller supports 1 or 2 Ethernet channels, and the CPU supports 8/16/32-bit ISA interface or PCI interface. The Ethernet physical layer chip can provide MII, RMII and other interfaces. The flexible and diverse product series allows customers to choose a more general CPU to implement the design.

(3) Out-of-band serial communication

In order to meet the requirements of large screens to detect power supply voltage, temperature and other conditions, some large screen systems use out-of-band serial port communication. If the FPGA needs to download and update functions, data download can be achieved through the microcontroller. Silicon Labs' microcontrollers meet these requirements. Its microcontrollers have multiple analog input channels of more than 8 bits, small packages, fast speeds, and are compatible with traditional 8051s; internally integrated A/D conversion layer, D/A conversion layer, Flash and RAM; it is a complete soc microcontroller that supports industrial temperature ranges; at the same time, the microcontroller supports online upgrades in a variety of ways, and is currently used by large screen manufacturers.

FPGA plays a vital role in the screen. It is used to complete the core functions such as video stream storage and refresh, control LED, etc. Actel's third-generation FPGA based on Flash technology has the characteristics of low price, high speed, built-in boost programming circuit, online upgrade, etc., and it works immediately after power-on without loading configuration data. It can support the application of DDR memory. FPGA has built-in 18~500kbit RAM and supports LVDS transmission mode. It can be said to be a cost-effective FPGA. Actel's FPGA adopts Flash technology and has excellent radiation resistance and other properties, which is very suitable for outdoor environments with drastic climate changes.

Figure 2 is a solution block diagram of the LED display screen.

Shenzhen Shiqiang Guixun Co., Ltd. is an authorized distributor of Arilent, Micrel, Silicon Labs, and Actel. It can design suitable electronic display system solutions for customers and provide integrated circuit applications for displays, such as synchronous electronic displays, synchronous and asynchronous Ethernet electronic displays, and other applications.

Figure 2 LED display solution block diagram

2. Wireless communication LED display circuit

Although LED display technology is quite mature, its application scope is limited because it often needs to communicate with a microcomputer to display and update information. Figure 3 is a block diagram of an LED display system that uses wireless communication to update information.

(1) Drive control circuit

The LED driving circuit on the electronic display screen adopts data serial transmission mode, as shown in Figure 4.

A row address is decoded by a 4/16 decoder and input into an emitter follower. When the output is low, the LED of the row is in a valid state, and an array transfer instruction is used to transfer the data to complete the display of a row. In other words, when the base of a transistor in the 16 rows is low (normally high), the transistor is in a state of being turned on. In the 128 columns, whichever column is low, the LED of which column is turned on and emits light, thus completing the display of a column. In order to prevent all 128 points in a column from emitting light and causing the driving transistor to burn out due to excessive current, a Darlington device TIP 127 with a current of 5A is selected, so that even if the circuit is static, the transistor will not burn out.

The display brightness is controlled by changing the ratio of the number of LEDs that are illuminated and those that are not. PWM can be used to control the brightness level of the LED display.

(2) Single chip microcomputer control circuit

The single-chip microcomputer is the central processing unit for the control, driving, receiving/sending data and coordination of the entire electronic display board, and is responsible for important tasks. The single-chip microcomputer part of the system is mainly composed of a reset circuit (a combination of power-on reset and key reset), a 1-2MHz transistor oscillator, and a storage expansion circuit (expanded to 32KB). The communication mode of the single-chip microcomputer is serial asynchronous communication mode, receiving signals sent from the computer, with a bit rate of 2400bit/s.

Figure 4 shows the row and column driving circuit of the display panel

(3) Transmitter/receiver module

The DF data transmission module circuit is shown in Figure 5. The frequency of the transmission module is 315MHz. When the ambient temperature is -25～+85°C, the frequency drift is only 3 x 10-6/°C. The operating voltage of the transmission circuit is 3～12V. When the transmission voltage is 3V, the transmission distance in an open area is 20～50m; when the voltage is 5V, the transmission distance is 100～200m; when the voltage is 9v, the transmission distance is 300～500m; when the voltage is 12V, the transmission distance is 700～800m, and the transmission current is 60mA and the transmission power is 500mW.

The DF data transmission module adopts ASk modulation to reduce power consumption. When the data signal stops, the transmission current drops to zero. To ensure the normal operation of the transmission module, a resistor rather than a capacitor is used to couple the data signal to the module input terminal.

Figure 5 Transmitter module circuit

The receiving module circuit is shown in Figure 6. This superheterodyne receiving module uses the RX3310A wireless remote control and data transmission ASIC and a 316.8MHz PfJ F surface wave resonator. The superheterodyne receiver has high requirements for antenna impedance matching.

Figure 6 Receiver module circuit