Four common memory concepts in 51 single-chip microcomputers

PROM is called programmable memory. It is like our exercise book, which is blank when we buy it and can be written on it, but once written, it cannot be erased, so it can only be written once. If you write something wrong, it will be scrapped.

EPROM is called UV-erasable programmable read-only memory. After writing the contents, if you are not satisfied with them, you can use a special method to erase them and rewrite them. That is, use ultraviolet light to irradiate them. Ultraviolet light is like an "eraser" that can erase the words and then rewrite them. Of course, if you erase them too many times, they will not work, so the number of times this chip can be erased is limited - a few hundred times.

EEPROM, also known as  E2PROM, is called electrically erasable programmable read-only memory. It is similar to EEPROM in that what is written can also be erased and rewritten, but it is more convenient and does not require light. Data can be erased or rewritten using electricity, so it is much more convenient and has a long lifespan (ranging from tens of thousands to hundreds of thousands of times).

FLASH, also known as flash memory, is an improved product of EEPROM. Its biggest feature is that it must be erased in blocks (the size of each block is not fixed, and products from different manufacturers have different specifications), while EEPROM can only erase one byte at a time. FLASH is now commonly used for large-capacity storage, such as USB flash drives.

Once again, all the writing here does not refer to normal working conditions. Whether it is PROM or EPROM, their writing requires special conditions. Generally, we use a device called "programmer" to do this work. Once it is installed in its working position, it cannot be rewritten casually.

Classification of semiconductor memory

According to the function, it can be divided into two categories: read-only memory and random access memory. The so-called read-only, literally means that you can only read from it, not write into it. It is similar to our books. After it is sent to us, we can only read the contents inside and cannot change the contents of the book at will. The abbreviation of read-only memory is ROM (READ ONLY MEMORY)

The so-called random access memory can be rewritten at any time and the data inside can be read out. It is similar to our blackboard. I can write things on it at any time and can also erase it with an eraser and rewrite it. The abbreviation of random access memory is RAM (READ RANDOM MEMORY). The abbreviations of these two memories must be remembered.

Note: The so-called read-only and random access refer to normal working conditions, that is, when the memory is used, not when the chip is manufactured. Otherwise, how does the data in the read-only memory come from? In fact, this principle is also easy to understand. Books cannot be changed after they are in our hands. When they are still raw materials - white paper, of course they can be printed by the printing factory.