O Introduction
To accommodate the need for CCTV (closed-circuit television) security industry to generate text and images and overlay them onto video images, MAXIM introduced the MAX7456 video captioning module. The device is a single-channel, monochrome on-screen display (OSD) generator with built-in non-volatile memory (EEPROM). The device generates all the functions required for a user-defined OSD and inserts it into a video signal, while being compatible with NTSC and PAL video standards. This paper discusses the implementation process of the monochrome OSD system based on MAX7456 with examples.
1 MAX7456 Internal Structure The MAX7456 integrates all the functions and EEPROM required to generate a user-defined OSD and insert it into a composite video signal. The device includes an input clamp, sync generator, video timing generator, OSD plug-in MUX, non-volatile (EEPROM) character memory, display memory, OSD generator, crystal oscillator, SPI-compatible interface for reading/writing OSD data. And a video drive. The internal structure of the MAX7456 chip is shown in Figure 1.
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The MAX7456 provides flicker, inversion, and background control. At the same time, the MAX7456 accepts signals common to security industry video standards such as standard NTSC or PALCVBS. Therefore, the device can be adapted to the needs of most security markets.
2 OSD module system structure The MAX7456 single-channel OSD generator will greatly reduce the complexity and cost of the OSD system. The system requires only a common microcontroller chip and the corresponding software to achieve flexible and variable subtitle display requirements. Figure 2 shows the application system block diagram of the MAX7456. In the figure, the PC can download the software generated font information to the microcontroller (MCU) through RS232 (serial port), and the font information is stored by the microcontroller (MCU) through the SPI (Serial Peripheral Interface Bus) interface to the MAX7456. In the EEPROM. The analog video signal output by the camera is processed by the OSD module to obtain the desired image with subtitles. The software running in the microcontroller can control the display content of the subtitles in real time through the SPI interface.
3 OSD module hardware structure MAX7456 as the core component of the OSD system, the corresponding peripheral devices need to cooperate to play their own role. Figure 3 shows the hardware block diagram of the OSD module. Among them, MAX74 56 is responsible for subtitle processing of the input analog video signal; MAXQ2000 as a low-voltage micro-power single-chip microcomputer, can provide corresponding control signals for MAX7456 through GPIO analog SPI interface timing, and is responsible for communication with PC through serial port. Get user-defined character set and configuration information; MAX3002 is a bidirectional level conversion chip that converts MXXQ2000's 2.5V system signal into 5V system signal that MAX7456 can use; MAX3311 is a serial port chip that is compatible with MAX02000. Responsible for providing serial communication signals with the PC; the MAX8881 converts the 5V supply to the 2.5 V supply required by the microcontroller and the buffer chip.
4 OSD module software structure The software that works with the OSD module consists of two parts: one is the firmware running in the microcontroller, which includes the SPI interface driver, the memory configuration program, the register configuration program, the serial port transaction communication, etc. The other part is The user interface running on the PC provides users with easy OSD settings and memory upgrades.
4.1 MAX7456 Control Routine The MAX7456's SPI interface supports up to 10 MHz interface clock (SCLK). Pull low when writing registers 
Enable serial interface to read data from SDIN on the rising edge of SCLK; When going high, the data is latched into the input register. If during transmission When it goes high, the program terminates (that is, the data is not written to the register). After going low, the device will wait to read the first byte from SD-IN to determine the type of data transfer being performed. Figure 4 shows the write data timing.
Pull low when reading registers . The address will be latched into SDIN on the rising edge of SCLK. The data can then be output from SDOUT on the falling edge of SCLK. Figure 5 shows the timing of reading data from the device.
The following is the specific operation method code: 
4.2 PC-side user interface The software on the PC side can provide users with common OSD configuration information, such as video system selection, subtitle brightness, flip mode, flashing, and selection of subtitle files. Figure 6 shows its user interface diagram.
In this way, through the software, the user can also edit the subtitle display content, customize the character set, and set the OSD background color.
4.3 Compiling the Development Environment The IAR EmbeddedWorkbench 2.10A running under the Windows platform is one of the integrated development tools for the MAXO series of microcontrollers. It has been used by most MAXQ series chips. However, after creating a new project under IAR, you also need to set the compile option: General Option/Target/Device and select MAX200X.
5 Conclusion The MAX7546 is a single-channel, monochrome on-screen display (OSD) generator that allows you to develop an OSD system that not only saves board space and design time, but also delivers higher performance at a lower cost. This paper introduces the basic structure of the OSD system, and expounds the technical points of using the MAX7456 to build the analog TV subtitle display system from both hardware and software, and hopes to play a guiding role for the OSD system developers.
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