Application of UPD6450 and uPD6453 in video character overlay system

    Abstract: This paper introduces the performance characteristics of the special character overlay chips uPD6450 and uPD6453 launched by NEC and their application in video character overlay systems, including control commands, serial port timing, hardware peripheral circuit design, and software design.

    Keywords: Video character overlay uPD6450 uPD6453

In recent years, with the widespread application of video surveillance systems, their system capacity has continued to expand. Character superposition circuit is an indispensable part of the video surveillance system. Therefore, its display and editing functions, reliability, circuit size and complex procedures have become a matter of great concern to designers.

In order to ensure low-cost, high-performance character overlay when the system capacity is large, we tried to use the dedicated character overlay chips uPD6450 and uPD6453 launched by NEC. Practice has proven that the character overlay circuit designed with these two chips as the core has the advantages of high reliability, strong display and editing functions, and simplified hardware circuits. It is a better choice for video character overlay. The performance characteristics of these two chips and their specific applications in character superposition circuits are briefly introduced below.

1 Performance characteristics

uPD6450 and uPD6453 are special character overlay chips launched by NEC. Their common feature is strong display and editing functions. They can display 12 rows and 24 columns of characters on the screen. Each character is a 12×18 dot matrix. The size of the characters, blinking The frequency can be adjusted as needed, and in order to achieve display diversity, the background color of the screen, the edge color of the characters, and the color of the characters themselves can also be modified. However, in some applications, there are still some differences between the two chips, which are mainly reflected in two aspects. First, the chip uPD6450 only solidifies 128 fonts of Japanese, Siwan letters and numbers, and does not provide RAM space for custom characters, so Chinese characters cannot be expanded. For most users in our country, there are some limitations in use. UPD6453 solves this problem better. It not only provides a 240-byte font internally, but also provides 16 characters of RAM space for users to fill in custom characters, which provides the possibility for Chinese character display. Second, there are some differences in color control between the two chips. The 5-color internal video signal of the chip uPD6450e can be used as the background color during display, but its character color can only be black or white, and the edge color of the character There is only one black color; the chip uPD6453 has 7 byte background colors, edge colors, and character scenery. Therefore, the chip uPD6453 provides users with more choices in color control.

2 Control commands and serial port timing

2.1 uPD6450 control command and control sequence

2.1.1 Control commands

Each control command of uPD6450 consists of two bytes. The first byte is "format", as shown below:

The format command is mainly used to release the test mode (FR=1 means releasing the test mode), and provides one bit (F0) to be combined with the next byte to determine the command type. When the first byte of several consecutive control commands, that is, the content of "format" is the same, one "format" command can be shared. The chip uPD6450 provides 10 control commands. Table 1 lists the main functional characteristics of these control commands.

Table 1 Main functions of uPD6450 control commands

2.1.2 Control sequence

The control sequence is shown in Figure 1. DATA, CLK, STB,BUSY respectively corresponds to the serial data input terminal, clock input terminal, strobe signal input terminal and busy signal output terminal of the chip uPD6450. Only when the BUSY signal is low level, the uPD6450 can read the serial data input by the CPU. 8-bit serial data is written to its internal register.

2.2 uPD6453 control commands and control timing

2.2.1 Control commands

The control commands of uPD6453 are divided into two types: single-byte commands and double-byte commands. These two different command formats have different timings. Table 2 lists the main functional features of these control commands.

Table 2 Main functions of uPD6453 control commands

2.2.2 Control sequence

Figure 2 shows the control timing of single-byte commands and double-byte commands.

3 Application of uPD6450 and uPD6453 in video character overlay system

3.1 Hardware circuit design

When the chip uPD6453 is used for character superposition, the hardware structure block diagram is shown in Figure 3.

In the design of this circuit, you need to pay attention to the following issues:

(1) The fonts inside the chip uPD6453 (including the 16-character empty RAM area) are all 12×18 dot matrix Chinese characters. It is difficult to find out of the library, so the 12×12 dot matrix font data is used in the design, but it is displayed like this The characters are smaller. After experiments, it was found that when the inductance is 15μH and the capacitance is 56pF, it is more appropriate to display the Chinese characters in a 12×12 dot matrix in double.

(2) 89C52 can only directly manage 64K storage space. When using a 12×12 dot matrix Chinese character library, 128K ROM is required to store all national standard first-level Chinese characters. This requires distribution management. The highest address line is I/O port leads out.

(3) The separation of horizontal and vertical synchronization signals can use special integrated circuits such as LM1881, or discrete analog components. Regarding its specific circuit, it has been discussed in detail in Reference 2. It should be noted that the output end of uPD6453 only outputs the superimposed character signal.

(4) In the design block diagram shown in Figure 3, an external clock chip, such as DS12887, can be connected to the P0 port so that the current time can be superimposed on the video image signal.

When using the chip uPD6450, there are some differences in the hardware circuit design and when using the uPD6453, mainly reflected in two aspects: First, because uPD6450 can only display the 128 characters solidified inside it and cannot display Chinese characters, so the circuit design is no longer Use font library ROM; secondly, the chip uPD6450 itself has input and output pins for video signals. The video signal after character superposition is directly output from this pin, unlike uPD6453 which only outputs character signals. Figure 4 and Figure 5 show the design of the uPD6450 video input and output circuit.

3.2 Software design

Generally speaking, it is necessary to superimpose information such as title and time on the video image signal so that the image signals in the video archive can be distinguished and searched in the future; at the same time, in order to prevent the superimposed character information from affecting the normal image display, the display position of the characters must also be Should be adjustable. Based on these two points, the software design of the entire system is divided into 6 states, and the state diagram is shown in Figure 6.

When programming the software, you can choose the background color on the 5 setting interfaces. When setting the title, if the chip uPD6453 is used, the Chinese character area code filled in by the user needs to be edited, the dot matrix data is extracted from the font ROM, and then the corresponding characters are displayed through the synthesis of uPD6453; but if the chip uPD6453 is used, Then some commonly used characters (such as elegant characters and numbers, etc.) solidified inside the chip can be directly displayed on the setting interface for users to choose. In addition, the title setting should have the functions of character replacement, character insertion, and character deletion. The software programming for time setting is relatively simple. You only need to display the current time set by the user and write it into the clock chip. In terms of programming time movement and title movement, the display position adjustment function provided by chips uPD6450 and uPD6453 brings great convenience to software design.

The above introduces the application of chips uPD6450 and uPD6453 in video character overlay systems, including their performance characteristics, control commands, serial port timing, hardware peripheral circuit design, and software design. When designing a video character overlay system using these two chips, it has the advantages of strong display and editing functions, simple design, high reliability, and easy practicality. It is a better choice for video character overlay.