Data Protection in AT28C Series EEPROM

1 Introduction
　　EEPROM is a memory with power-off memory function. Its content can be rewritten like ordinary RAM, and it can automatically erase and replace it with new content when rewriting. It does not need ultraviolet erasure like EPROM; it only needs electricity to erase and rewrite the content stored inside. EEPROM usually has a programming power supply inside. Since it only needs a single +5V power supply, it is very convenient to use. Compared with serial chips, the circuit interface and programming design of parallel EEPROM are much simpler, so it is very convenient to use EEPROM memory when the circuit board area requirements are not very strict.
2 Reasons for data loss in parallel EEPROM Parallel
　　EEPROM usually adopts the bus expansion interface method. Figure 1 shows the interface circuit based on AT28C256, in which the chip select signal generated by address decoding is the read and write signal of the microcontroller.

During programming, only one MOVX instruction is needed to complete the read and write operations of the EEPROM memory, which is very convenient to use. However, parallel EEPROM is often disturbed in industrial sites, causing the data content stored in it to change, thereby causing data damage or loss.
　　After analysis, data loss in the EEPROM memory mainly occurs when the system is powered on, powered off or reset. It mainly manifests in the following two situations:
　　(1) When the CPU in the entire data storage system is released after the reset signal, it will generally delay for hundreds of μs. Therefore, during this period of time, the read signal, write signal, address signal and data signal may change randomly, causing the memory to be written incorrectly and the data therein to change.
　　(2) During the slow rise and fall of the power supply, when the CPU is in a critical working state, its read and write timing may be chaotic, causing the data in the memory to change.
3 Introduction to AT28C Series EEPROM
　　Figure 2 shows the logic block diagram of ATMEL's AT28C series parallel EEPROM memory. The main products in this series include AT28C64 (8K×8), AT28C256 (32K×8), AT28C010 (128K×8), AT28C040 (512K×8) and other models, and their structures are basically the same. Compared with ordinary 28 series EEPROM, AT28C series EEPROM has the following characteristics:
　　● It has 64, 128 or 256 bytes of identification bytes (different models have different);
　　● It can be read quickly, and the reading time range is 120ns~150ns;
　　● It has two write modes, byte and page. In the page write mode, the internal timer control timing of the EEPROM memory can write a page of data in one internal programming cycle , and its page capacity is 64, 128 or 256 bytes (depending on the model);
　　● It can provide data protection function and has excellent anti-interference ability.

　Among the above features, the most attractive one is undoubtedly the perfect data protection function.
4 Data protection measures of AT28C series
　　AT28C series EEPROM generally adopts two types of measures: hardware data protection and software data protection (SDP).
　　4.1 Hardware data protection
　　The hardware data protection measures of AT28C series EEPROM are as follows:
　　The first measure is to monitor VCC. When Vcc is lower than 3.8V, writing to EEPROM is prohibited; the second is to use power-on delay, that is, when the system is powered on, after Vcc rises to 3.8V, it will delay for 5ms before allowing data to be written; the third is write prohibition, that is, when any of the three conditions of low level, high level or high level occurs, data writing is prohibited; the last one is to use noise filtering so that pulses narrower than 15ns on the signal line cannot trigger write operations.