ROM（Read Only Memory）
ROM can still hold data when the system stops powering, such as CD-ROM, which can save files and can only be read.
RAM (Random Access Memory)
RAM usually loses data after power down, such as a computer’s memory stick is RAM, and memory data is automatically cleared after power off.
2 PROM & EPROM && EEROM
ROM can be further divided into several categories
PROM
Programmable ROM, but can only be written to once (a bit like a CD-ROM, which can only be burned once), this is an early product that is no longer used.
EPROM
The original program is erased by ultraviolet light, and is no longer used.
EEPROM
Electric erasable, such as the AT24C02 chip used when learning microcontroller, size 2K, used to save a small amount of power-down data not lost, microcontroller through the IIC to read and write the contents of this EEPROM.
3 SRAM & DRAM
RAM can be divided into two categories
SRAM (StaTIc RAM)
SRAM that is static RAM, read and write speed is very fast, but the price is also more expensive, used for CPU’s first level buffer, second level buffer.
SRAM is also available in microcontrollers, such as STM32F103VET6 with 64K SRAM for program runtime variables, stack staging, etc. In addition, external SRAM can be extended through the FSMC interface, such as the 1M byte IS62WV51216 external SRAM chip, which is used for occasions requiring large amounts of data calculation (running algorithms, GUI).
DRAM (Dynamic RAM)
That is, dynamic RAM, which retains data for a very short time and is slower than SRAM (but faster than any ROM), computer memory is DRAM.
DRAM is divided into many kinds, the common ones are FPRAM/FastPage, EDORAM, SDRAM, DDR RAM, RDRAM, SGRAM, and WRAM, etc.
SDRAM (Synchronous Dynamic RAM)
Synchronous Dynamic Random Memory, mainly don’t mistake it as a combination of the above two! Synchronous means that the memory needs a synchronized clock to work, dynamic means that it is refreshed to ensure that data is not lost, and random means that data is not stored in a linear order, but is read and written to freely specified addresses.
DDR RAM
Computer memory sticks are often referred to as DDR3, DDR4.
4 NAND Flash & NOR Flash
FLASH memory, also known as flash memory, combines the strengths of ROM and RAM, which not only has the function of electrically erasable programmable (EEPROM), but also does not lose data in power failure, and can read data quickly (the advantage of NVRAM), and USB flash drives use this kind of memory.
Flash
Flash, such as in microcontrollers, is used to save programs, such as the STM32F103VET6 has 512K of Flash.
Microcontroller can also be external Flash, such as W25Q128, 16M size, can be used to save pictures and other larger data, the microcontroller through the SPI to read and write its contents.
NOR Flash
It can be read in the same way as our common SDRAM, and the user can run the code loaded in NOR FLASH directly, which reduces the capacity of SRAM and thus saves cost.
NOR is characterized by simple application, no special interface circuit, high transfer efficiency, and it belongs to on-chip execution (XIP, eXecute In Place), so that applications can run directly in the (NOR type) flash memory without having to read the code into the system RAM. It is very cost effective at small capacities of 1 to 4MB, but the very low write and erase speeds greatly affect its performance.
NAND Flash
NANDFlash has a larger storage capacity than NORFlash, e.g. K9F1208U08 is 64M. It does not take the random read technology of memory, but does so in the form of reading one piece at a time, usually 512 bytes at a time, and Flash with this technology is relatively inexpensive. Users cannot run the code on NAND Flash directly, so many development boards that use NAND Flash come with a small NOR Flash to run boot code in addition to NAND Flah.
The NAND structure provides a very high cell density, allowing for high storage density and fast write and erase speeds. The difficulty in applying NAND is the management of flash and the need for a special system interface.
Comparison of the two
NOR Flash has a slightly faster read speed than NAND.
The write speed of NAND Flash is much faster than NOR.
NAND Flash’s 4ms erase speed is much faster than NOR’s 5s.
NAND Flash has smaller erase cells and correspondingly less erase circuitry.
NOR Flash comes with an SRAM interface with enough address pins to address and can easily access every byte inside it.
NAND Flash uses complex I/O ports to access data serially, with 8 pins to transfer control, address and data information, and more complex timing.
NOR Flash generally has a smaller capacity and more complex process, so it is more costly.
NAND Flash is simpler to produce, so its capacity is larger and its price is lower.
5 eMMC
eMMC (Embedded MulTI Media Card) is a standard specification for embedded memory set by the MMC Association, mainly for cell phones or tablet PCs. eMMC consists of Nand flash, Nand flash controller and standard interface package.
Review Editor: Peng Jing
