Design and application of EEPROM based on I2C serial communication in TV

1 Introduction

In recent years, digital TV technology has developed rapidly, especially the emergence of remote control systems. Various manufacturers have added microprocessors (MCUs) to TV sets, and various artificial intelligence color TVs have completely replaced the manual (mechanical) control and memory methods of old TV sets. As the functions of TV-specific MCUs become increasingly powerful and the functions used by users become more and more abundant, the data required to be stored and the refresh rate continue to increase, and various models and specifications of memory have emerged one after another. At present, the most widely used in TV sets is E2PROM.

Because E2PROM stores a large amount of data closely related to the working status of the TV, its reliability is crucial for remote control of the TV and is considered the second core device. E2PROM maintains communication with the first core device (MCU) and stores the working information of the TV. Sometimes the loss of stored data causes significant losses, and even makes the data irreparable. Therefore, in the TV system, the electrically erasable memory E2PROM is generally used for the storage of system data and user data. E2PROM has the advantages of easy operation, long data storage time, wide storage temperature range, and many erasable times. The serial communication E2PROM represented by the AT24CXX series launched by ATMEL has gradually become the preferred device of E2PROM in TV product design with its reliable quality and simple interface.

2 AT24CXX Series Introduction

Compared with parallel communication memory, serial communication memory is widely used because of its advantages of occupying few resources and I/O bus, small size, etc. The typical serial communication E2PROM AT24CXX is an I2C bus serial device, with wide working power supply (1.8 ~ 5.5 V), strong anti-interference ability (built-in Schmitt trigger filter on input pin to suppress noise), low power consumption (maximum working current 3 mA in write state), high reliability (write times 1 million times, data storage for 100 years), support for online programming, etc.

Figure 1 shows the internal structure of the AT24CXX. The SDA pin can realize bidirectional serial data transmission. The data is sent to the E2PROM at the rising edge of the clock signal input at the SCL pin, and the data is read out at the falling edge of the clock.

According to the size of the memory space address, AT24CXX is divided into C02/2K, C04/4K, C08(A)/8K, C16/16K and other versions. Except for the different storage space, the internal performance of each version is basically the same. Users can reasonably choose the corresponding device according to the actual conditions of use. The A2, A1 and A0 pins are the device address input pins of the hardware connection of AT24CXX. A maximum of 8 AT24C02s, 4 AT24C04s, 2 AT24C08s(A) or 1 AT24C16 can be connected to one bus.

In addition, the AT24CXX series devices have a hardware data write protection pin. When this pin is connected to a low level, the device allows normal read/write operations; when this pin is connected to a high level, the device starts the write protection function. The description of the write protection function and storage structure of each version of the device are shown in Table 1.

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3 Design and Application of AT24CXX in Color TV

Although the AT24CXX has few peripheral pins and components and a simple interface circuit, in actual CRT color TV applications, due to interference signals such as high-voltage sparking inside the TV, the E2PROM will have some "strange" phenomena, causing the TV to work abnormally. The following are the main phenomena that cause abnormal TV operation.

3.1 Non-human factors change some E2PROM data

This phenomenon manifests itself as the TV working state being abnormal after a certain startup. If no treatment is done after the abnormality is found, the abnormal state will remain for a period of time. The most common phenomenon is the sudden change of key system data (row and field data). This phenomenon is generally caused by the abnormal operation of the I2C bus data line due to the sparking of the kinescope or other reasons when the TV is turned on. When this phenomenon occurs, if the mutated data is adjusted according to the standard data in the factory menu or user menu, it can be corrected and solved; if the mutated data cannot be adjusted, it must be solved by software reset, E2PROM initialization, etc.

The method to prevent this phenomenon from hardware is mostly to add a voltage regulator diode between the SDA and SCL data lines of the I2C bus near the E2PROM pins and the ground line, so as to protect the bus port, such as VD1 and VD2 in Figure 2. According to the results of long-term product tracking, the E2PROM data error rate using this method is less than 3‰. However, this method can only reduce the register data error rate, but cannot fundamentally eliminate data errors, but generally will not cause physical damage to the corresponding data area of ​​the E2PROM. The advantage of this method is that it is simple and easy to implement and almost does not occupy device resources.

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The most scientific method at present is to use the write protection function of E2PROM. The 7th pin of AT24C08A is the data communication write protection pin WP (active at low level), which is connected to the MCU processor I/O interface, as shown in Figure 3. During the E2PROM write operation, the MCU processor I/O interface outputs a low level, and the E2PROM allows the I2C bus to write data. During other operations, the I/O interface outputs a high level, and the E2PROM does not allow the I2C bus to write data. This method has low cost (saving two voltage regulator diodes) and a lower error rate, but requires software support. According to statistics, using the E2FROM write protection function, the data error rate is less than 1‰. If the above two methods are used at the same time, the E2PROM data error rate will be lower and the effect will be better.

3.2 Physical damage to some areas of E2PROM

At room temperature, physical damage to some areas of the E2PROM will not occur in the short term. Since the E2PROM can be erased and written 1 million times at room temperature. However, at high temperatures, the number of erasable times of the E2PROM is greatly reduced. If the E2PROM is erased and written frequently at high temperatures, it is easy to cause physical damage to some areas of the E2PROM in the short term. This is manifested as the inability to store specific data normally, and it cannot be restored even if it is forced to be burned using a burner. The method for determining physical damage to some areas of the E2PROM: Use a burner to set each address in the E2PROM to 0 and 1 respectively, and check whether there is physical damage to the area. If it is confirmed that the E2PROM is damaged, a new E2PROM must be replaced to eliminate the fault of the TV. This phenomenon is caused by improper software processing. To prevent this from happening, you must start with the software to avoid frequent operations on specific areas. A typical example is the software design of a high-definition CRT color TV AFT frequency automatic tracking system and some other discrimination systems. Taking the AFT frequency automatic tracking system as an example, when the ambient operating temperature and power supply voltage change, the high-frequency tuner will have a certain frequency point drift. When designing the software, after the frequency point shifts, the software program should immediately record the new frequency point offset voltage value and store its value in the E2PROM specified address. Under high temperature, due to the change in the parameters of related components, the frequency point will continue to drift with the increase in temperature, causing the E2PROM data to be refreshed abnormally frequently, resulting in physical damage to the E2PROM of the TV during the high temperature load test. After finding out the cause, the software should be modified to change the real-time storage of frequency point data to the frequency tracking power-off storage mode, that is, the data is only recorded when the TV is turned off and the power is off. This can reduce the number of E2PROM data refreshes, thereby solving the above problem.

4 Conclusion

As a general-purpose storage device, E2PROM is often easy to overlook some details when designing. Some problems are discovered and solved in the design and trial production stage, while some problems are exposed only after mass production or even after a period of after-sales service. Therefore, a complete modern electronic product system needs to rely more on the coordinated development of software and hardware to achieve the best reliability of the product.