Video subtitles based MAX7456 module design and implementation (1)

O introduction

In order to adapt the CCTV (closed circuit television) security industry produces text and images and is superimposed over the video image on demand, MAXIM company launched MAX7456 video text module.

The device is a single channel, monochrome with screen display (OSD) generator, built-in nonvolatile memory (EEPROM). The device can generate user-defined OSD and insert the video signal of all the required functionality, while compatible with NTSC and PAL video standards. This article discusses the binding instance MAX7456 monochrome based OSD system implementation process.

The internal structure of 1 MAX7456

MAX7456 internal integration is generating a user-defined OSD and plug it into the composite video signal of all the features and EEPROM.

The device's built-in input clamp, synchronous generator, video timing generator, OSD insert MUX, non-volatile memory (EEPROM) characters, the display memory, OSD generators, crystal oscillator, can read/write OSD data SPI compatible interface and the video driver. MAX7456 chip internal structure as shown in Figure 1.

MAX7456 provides flicker, invert the colors and background control functions.

At the same time, MAX7456 acceptable standards NTSC or PALCVBS, etc in the security industry video standard signaling in common. Therefore, the device can be applied to most of the security needs of the market.

2 OSD module system structure

Single channel with MAX7456 OSD generators will greatly reduce the OSD system complexity and cost.

The system only needs a common microcontroller chip with the appropriate software, you can realize flexible captioning requirements. Figure 2 shows the application system block diagram MAX7456. Figure, the PC through the RS232 (serial interface) can be generated by the software information is downloaded to the character set the microcontroller (MCU), the character set information from the microcontroller (MCU), SPI (Serial Peripheral Interface Bus) interface is stored in the EEPROM of the MAX7456. Camera output of analog video signal through OSD module handle to get the needed images with captions. In the micro-controller software running in through the SPI interface you can control the display of subtitles.

3 OSD module hardware structure

MAX7456 as OSD system core components, you need the appropriate perimeter devices meet to play its role.

Figure 3 shows the OSD module hardware structure diagram. Of these, 56 MAX74 on input analog video signal for captions processing; a low voltage MAXQ2000 as micro-power SCM through GPIO analog SPI interface timing for MAX7456 provides appropriate control signal, and is responsible for communication with the PC through the serial port to get the user-defined character set and configuration information; MAX3002 is a bi-directional level translation chip, it may be a signal of MXXQ2000 2.5V system into MAX7456 can use 5V system signal; MAX3311 is supporting serial chip MAX02000, responsible for providing and PC communication serial signal; MAX8881 where you can put the power supply 5V into microcontroller and buffer chip needs 2.5 V power supply.

4 OSD module software structure

Meet the OSD module software consists of two parts: one part is running on the microcontroller in the firmware, it includes SPI interface driver, memory Configurator, registers configuration program, serial communication and other matters; the other part is running on the PC end user interfaces to provide the user with a simple memory upgrade OSD settings and features.

4.1 MAX7456 control routines

MAX7456 SPI interface can support up to 10 MHz interface clocks (SCLK).

Write registers, pulled down to the serial interface to the SCLK rising to read data from SDIN; and when changes to a high level, data is latched to input registers. If the transfer process becomes high, the program terminates (i.e., the data is not written to the register). Low, the device will wait from the first byte read SD-IN to determine ongoing data transfer type. Figure 4 illustrates how to write data time series.

Reading registers, pulled down.

Addresses are on the rise locks into the SCLK SDIN. Then, the data in the falling edge from the SCLK SDOUT output. Figure 5 shows the data from the device to read the time.

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