Mainly divided into two categories: memory and flash memory, the data in the memory will be lost when the power is turned off, and the data in the flash memory will not be lost when the power is turned off.
Memory: SRAM, RAM, SDRAM, DDR
Flash memory: ROM, FLASH, EMMC
RAM: Random Access Memory random access memory, divided into: SRAM (static random access memory) and DRAM (dynamic random access memory). Random access memory, also called main memory, is an internal memory that directly exchanges data with the CPU. It can be read and written at any time, and the speed is very fast, and it is usually used as a temporary data storage medium for operating systems or other running programs.
The so-called "static" means that as long as the memory is powered on, the data stored in it can be kept constantly. However, when the power supply stops, the data stored in SRAM will still disappear (called volatile memory). SRAM is on-chip, directly in contact with the CPU, and the speed is very fast. It can be said that static RAM is currently the fastest storage The equipment is so good, the performance is so good, the price is expensive, so generally SRAM is only used in places that have strict requirements on storage performance, such as the first-level buffer and second-level buffer of the CPU;
The data stored in dynamic random access memory (DRAM) needs to be updated periodically. Dynamic random access memory (DRAM) is the most common type of storage in the embedded field. DRAM has been developed to this day and has gone through 5 generations. Each generation will There is a great improvement in performance, mainly in terms of operating frequency and energy saving.
SDRAM: Synchronous dynamic random access memory, the memory work requires a synchronous clock, and the sending of internal instructions and the transmission of data are based on this clock; the storage sequence needs to be continuously refreshed to ensure that the stored data is not lost; the data is not stored linearly at one time, Instead, freely specify the address to read and write data. This is the first generation of DRAM. SDRAM uses a single-ended clock signal.
DDR: Accurately, it should be called DDR SDRAM, double-rate synchronous dynamic random access memory. DDR uses a differential clock signal that can reduce interference as a synchronous clock. DDR can transmit data on the rising and falling edges of the clock, so the memory The speed is twice that of SDRAM, the running speed is faster in the same time, and the user experience has risen to a higher level. Since the emergence of DDR, DDR2, DDR3, and DDR4 have appeared later, and the performance of the latter is much stronger than that of the former. 2. It is more energy-saving. The working voltage of DDR is 2.5V, DDR2 is 1.8V, DDR3 is 1.5V, and DDR4 is 1.2V.
Main memory is the most important memory inside the computer, which is used to load various programs and data for the CPU to run and use directly. Due to its high cost performance and good scalability, DRAM is the most important part of the main memory of general computers today. In 2014, the main memory used in the production of computers was mainly DDR3 SDRAM. Since 2016, DDR4 SDRAM has gradually become popular. Notebook manufacturers such as ASUS and Acer have begun to replace DDR3L with DDR4 memory in notebooks.
The above is the memory, and then I will talk about flash memory. The concept of flash memory is similar to that of a hard disk. It is to store data, and the stored things will not be lost after the machine is powered off. ROM, FLASH and EMMC are such things.
ROM: ROM can be divided into PROM (programmable ROM), EPROM (programmable and erasable ROM), EEPROM (electrically programmable and erasable ROM). Because the overall writing time of ROM is long, slow, and the integration level is not high, it was replaced by FLASH later.
Therefore, FLASH is a combination of ROM and RAM, not only has the performance of EEPROM, will not lose data, and can read data faster. Here FLASH can be divided into NOR FLASH and NAND FLASH.
eMMC Embedded Multimedia Card (Embedded Multimedia Card) is a new storage technology established by the MMC Association. The architecture standard puts MMC components (flash memory plus controller) into a small ball grid array package (BGA), which is an embedded non-volatile memory system mainly used on printed circuit boards, with small size, high power With the advantages of low power consumption and large capacity, it is very suitable as a storage medium for consumer electronic devices such as smartphones, tablet computers, and mobile Internet devices. It is now widely used in embedded systems. eMMC is to put multimedia storage components into BGA-packaged chips. This multimedia component is Flash. At present, the Flash Memory inside eMMC used in most mobile devices such as mobile phones and tablets belongs to NAND Flash.
eMMC integrates the Flash Controller inside, which is used to complete functions such as erasing and writing balance, bad block processing, ECC verification, etc., so that the Host side can focus on the upper layer business, eliminating the need for special processing of NAND Flash.
In fact, the emergence of EMMC is to reduce the cost of research and development and speed up the time to market of the machine. For mobile phones, set-top boxes and other products, the NAND FLASH must be re-selected for each product development, and in order to adapt to the development trend, the NAND FLASH selected each time must have changed more than the previous version. Consider various compatibility and management issues, which will greatly increase the workload and time of research and development, and may delay the time to market of the product. Therefore, EMMC, which is similar to flash memory, appears. EMMC puts all the memory and the control chip that manages NAND FLASH on an MCP, and EMMC has the advantages of being faster and upgradeable. Manufacturers of mobile phones and set-top boxes You can directly buy an EMMC and use it on your own products, and it will be OK, and there will be no complicated compatibility issues.