ROM and RAM refer to semiconductor memory, ROM is an abbreviation of Read Only Memory, and RAM is an abbreviation of Random Access Memory. The ROM can still hold data when the system stops supplying power, and RAM usually loses data after power down. The typical RAM is the computer's memory.
There are two main categories of RAM:
One is called Static RAM (SRAM). SRAM is very fast. It is the fastest storage device for reading and writing, but it is also very expensive, so it is only used in demanding places, such as CPU level. Buffer, secondary buffer.
The other is called dynamic RAM (DRAM). DRAM keeps data for a short time and slower than SRAM, but it is faster than any ROM, but DRAM is cheaper than SRAM in terms of price. Many, computer memory is DRAM.
There are many types of DRAM. Common ones are FPRAM/FastPage, EDORAM, SDRAM, DDR RAM, RDRAM, SGRAM, and WRAM. One of them is described here.
DDR RAM (Date-Rate RAM) is also called DDR SDRAM. This improved RAM and SDRAM are basically the same, except that it can read and write data twice in one clock, which doubles the data transfer speed. . This is the most used memory in the computer, and it has a cost advantage, in fact defeating Intel's other memory standard - Rambus DRAM. On many high-end graphics cards, high-speed DDR RAM is also used to increase bandwidth, which can greatly improve the pixel rendering capability of 3D accelerator cards. 
How memory works:
Memory is used to store the data and programs that are currently in use (ie, in execution). The memory of the computer we usually refer to is dynamic memory (ie, DRAM). The so-called "dynamic" in dynamic memory refers to When we write data to DRAM, after a while, the data is lost, so an extra circuit is needed for the memory refresh operation.
The specific working process is as follows: whether a DRAM memory cell stores 0 or 1 depends on whether the capacitor has a charge, a charge representing 1 and no charge representing 0. But for a long time, the capacitor representing 1 will discharge, and the capacitor representing 0 will absorb the charge, which is the reason for data loss; the refresh operation periodically checks the capacitor. If the power is greater than 1/2 of the full charge, it is considered to be 1 And fully charge the capacitor; if the charge is less than 1/2, it is considered to represent 0, and the capacitor is discharged, thereby maintaining data continuity.
There are also many kinds of ROMs. The difference between PROM is programmable ROM, PROM and EPROM (erasable programmable ROM) is that PROM is one-time, that is, after the software is poured, it cannot be modified. This is early The product is now impossible to use, and EPROM is a general-purpose memory that wipes out the original program by ultraviolet light. Another type of EEPROM is electronically erased, which is expensive, has a long write time, and is very slow to write.
For example, the mobile phone software is usually placed in the EEPROM, we call, some of the last number dialed, temporarily stored in the SRAM, not immediately written through the record (the call record is saved in the EEPROM), because there is a very important job at the time (Call) To do, if written, the long wait is for the user to endure.
FLASH memory, also known as flash memory, combines the advantages of ROM and RAM, not only with electronically erasable programmable (EEPROM) performance, but also does not power off data and can quickly read data (NVRAM advantage), U disk This memory is used in MP3. In the past 20 years, embedded systems have used ROM (EPROM) as their storage device. However, in recent years, Flash has completely replaced the position of ROM (EPROM) in embedded systems, as a storage bootloader and operating system or program. The code is used directly by the hard disk (U disk).
Currently Flash has two main types of NOR Flash and NADN Flash.
The reading of NOR Flash is the same as the reading of our common SDRAM. Users can directly run the code loaded in NOR FLASH, which can reduce the capacity of SRAM and save costs.
NAND Flash does not take random reading technology of memory. Its reading is performed in the form of reading one block at a time, usually reading 512 bytes at a time. Flash using this technology is relatively cheap. Users can't run the code directly on NAND Flash, so many NAND Flash development boards use a small NOR Flash to run the boot code in addition to NAND Flah.
Generally, NOR Flash is used for small capacity. Because it is fast, it is used to store important information such as operating system. For large-capacity NAND FLASH, the most common NAND FLASH application is DOC (Disk On Chip) used in embedded systems. And the "flash disk" we usually use, can be erased online. Currently, FLASH is mainly from Intel, AMD, Fujitsu and Toshiba, while the main manufacturers of NAND Flash are Samsung and Toshiba.
Comparison of NAND Flash and NOR Flash
NOR and NAND are the two main non-volatile flash technologies on the market today. Intel first developed NOR flash technology in 1988, completely changing the situation that was originally dominated by EPROM and EEPROM. Then, in 1989, Toshiba announced a NAND flash architecture that emphasizes lower cost per bit, higher performance, and can be easily upgraded via the interface like a disk. But after more than a decade, there are still quite a few hardware engineers who can't distinguish between NOR and NAND flash.
Phase "flash memory" can often be used interchangeably with phase "NOR memory". Many in the industry are also confused about the advantages of NAND flash technology over NOR technology, because in most cases flash is only used to store a small amount of code, then NOR flash is more suitable. NAND is the ideal solution for high data storage density.
NOR is the main non-volatile flash technology on the market today. NOR is generally only used to store a small amount of code; NOR is mainly used in code storage media. NOR is characterized by simple application, no need for special interface circuit, high transmission efficiency, it belongs to in-chip execution (XIP, eXecute In Place), so that the application can run directly in the (NOR) flash memory, no need to put the code Read into system RAM. It is cost-effective at small capacities of 1 to 4 MB, but the low write and erase speeds greatly affect its performance. The NOR flash has an SRAM interface with enough address pins to address and easy access to every byte in its internals. NOR flash occupies most of the 1 to 16MB flash memory market.
The NAND structure provides extremely high cell density, high memory density, and fast write and erase speeds. The difficulty in applying NAND lies in the management of flash and the need for special system interfaces.
1, performance comparison:
Flash flash memory is a non-volatile memory that can be erased and reprogrammed for blocks of memory cells called blocks. Any flash device write operation can only be done in empty or erased cells, so in most cases, the erase must be performed before the write operation. It is quite simple for a NAND device to perform an erase operation, while NOR requires that all bits in the target block be written as 1 before erasing.
Since the erasing of the NOR device is performed in blocks of 64 to 128 KB, the time for performing a write/erase operation is 5 s. In contrast, the erase NAND device is performed in blocks of 8 to 32 KB, and the same is performed. The operation only takes up to 4ms.
The difference in block size when performing an erase further widens the performance gap between NOR and NADN. Statistics show that for a given set of write operations (especially when updating small files), more erase operations must be performed. Performed in a NOR-based unit. Thus, when choosing a storage solution, the designer must weigh the following factors:
â— NOR reads faster than NAND.
â— NAND write speed is much faster than NOR.
â— NAND's 4ms erase speed is much faster than NOR's 5s.
â— Most write operations require an erase operation first.
â— The NAND erase unit is smaller and the corresponding erase circuit is less.
(Note: NOR FLASH SECTOR erasure time varies by brand and size. For example, 4M FLASH, some SECTOR erasure time is 60ms, and some need up to 6s.)
2, interface differences:
The NOR flash has an SRAM interface with enough address pins to address and easy access to every byte in its internals.
NAND devices use complex I/O ports to serially access data, and the methods of individual products or vendors may vary. Eight pins are used to transfer control, address and data information.
NAND read and write operations use 512-byte blocks, which is a bit like hard disk management. Naturally, NAND-based memory can replace hard drives or other block devices.
3. Capacity and cost:
The cell size of NAND flash is almost half that of NOR devices. Due to the simpler production process, the NAND structure can provide higher capacity within a given die size, which in turn reduces the price.
NOR flash occupies most of the 1 to 16MB flash memory market, while NAND flash is only used in 8-128MB products. This also shows that NOR is mainly used in code storage media, NAND is suitable for data storage, and NAND is in CompactFlash. Secure Digital, PC Cards and MMC memory cards account for the largest share of the market.
4. Reliability and durability:
One important consideration when using flahs media is reliability. For systems that need to extend MTBF, Flash is a very suitable storage solution. The reliability of NOR and NAND can be compared in terms of lifetime (durability), bit swapping, and bad block processing.
A) Life (durability)
The maximum number of erasures per block in NAND flash is one million, while the number of NOR erases is 100,000. In addition to having a 10 to 1 block erase cycle advantage, a typical NAND block size is eight times smaller than a NOR device, and each NAND memory block has fewer deletes in a given amount of time.
B) Bit swapping
All flash devices suffer from bit swapping. In some cases (rarely, NAND occurs more often than NOR), a bit bit is inverted or reported to be inverted.
A single change may not be obvious, but if it occurs on a critical file, this small failure can cause the system to shut down. If you only have a problem with the report, you may solve it by reading it a few more times.
Of course, if this bit really changes, you must use the Error Detection/Error Correction (EDC/ECC) algorithm. The problem of bit reversal is more common in NAND flash, where NAND vendors recommend using EDC/ECC algorithms when using NAND flash.
This problem is not fatal when storing multimedia information with NAND. Of course, if you use a local storage device to store operating systems, configuration files, or other sensitive information, you must use an EDC/ECC system to ensure reliability.
C) bad block processing
Bad blocks in NAND devices are randomly distributed. In the past, there have been efforts to eliminate bad blocks, but found that the yield is too low, the cost is too high, it is not cost-effective.
NAND devices require an initial scan of the media to find bad blocks and mark bad blocks as unavailable. In a fabricated device, if this process cannot be performed by a reliable method, it will result in a high failure rate.
5, easy to use:
You can use NOR-based flash memory very directly, you can connect like other memories, and you can run code directly on it.
NAND is much more complicated due to the need for an I/O interface. Access methods for various NAND devices vary from manufacturer to manufacturer.
When using a NAND device, the driver must be written before proceeding with other operations. Writing information to a NAND device requires considerable skill, because the designer must never write to bad blocks, which means that virtual mapping must be done from start to finish on the NAND device.
6, software support:
When discussing software support, you should distinguish between basic read/write/erase operations and a higher level of software for disk emulation and flash management algorithms, including performance optimization.
No software support is required to run code on a NOR device. When doing the same on a NAND device, a driver, usually a memory technology driver (MTD), is used for NAND and NOR devices during write and erase operations. Both need MTD.
There are relatively few MTDs required to use NOR devices, and many vendors offer more advanced software for NOR devices, including M-System's TrueFFS driver, which is powered by Wind River System, Microsoft, QNX Software System, Symbian. Used by vendors such as Intel.
The driver is also used for emulation of DiskOnChip products and management of NAND flash, including error correction, bad block processing and wear leveling.
NOR FLASH's main suppliers are INTEL, MICRO and other manufacturers. It used to be the mainstream product of FLASH, but now it is more uncomfortable to be squeezed by NAND FLASH. Its advantage is that it can run programs directly from FLASH, but the process is complicated and the price is more expensive.
The main suppliers of NAND FLASH are SAMSUNG and Toshiba. These FLASHs are in U disk, various memory cards and MP3 players. Due to different processes, it has more storage capacity than NOR FLASH and is cheaper. But there are also disadvantages, that is, the program cannot be directly run, and only data can be stored. In addition, NAND FLASH is very prone to bad areas, so a verification algorithm is needed.
Use NAND FLASH to store data and programs on your handheld, but you must have NOR FLASH to boot. In addition to the SAMSUNG processor, other mainstream processors used in handheld computers do not support direct startup from NAND FLASH. Therefore, you must start the machine with a small NOR FLASH, and run the software such as OS into the SDRAM from NAND FLASH. It is very troublesome.
DRAM uses the charge on the gate capacitance of the MOS transistor to store information. Once the power-down information is lost, the gate will leak, so a refreshing mechanism is needed to recharge the gate capacitors at regular intervals, and each read. After the data is once again, it needs to be replenished. This is called dynamic refresh, so it is called dynamic random access memory. Since it uses only one MOS tube to store information, the integration can be high and the capacity can be made very large. SDRAM has one more synchronization with the CPU clock.
SRAM uses registers to store information, so once power is lost, the data will be lost. As long as the power is supplied, its data will always exist. It does not need dynamic refresh, so it is called static random access memory.
The above is mainly used for the internal memory of the system, and the capacity is large, and the data is not saved after the power is turned off.
The Flash ROM uses the capacitance on the floating gate to store the charge to save the information. Because the floating gate does not leak, the information can still be saved after the power is turned off. Also because of its simple structure, the integration can be done very high and the capacity can be large. Before the Flash rom is written, it needs to be erased by power, and the erase and the EEPROM can be performed in units of bytes. The flash rom can only be performed in units of sectors. However, it can be written in bytes. The flash rom is mainly used for devices such as bios, U disk, Mp3, etc. that require large capacity and no power loss.
PSRAM, pseudo static random access memory.
background:
PSRAM has a single-transistor DRAM cell that is quite different from a traditional SRAM cell with six transistors or four transistors and a two-load resistor SRAM cell, but it has a stable SRAM-like interface and an internal DRAM architecture. Give PSRAM some advantages over low-power 6T SRAM, such as being lighter and more competitive. Currently, 90% of manufacturers in the overall SRAM market are producing PSRAM components. In the past two years, important SRAM/PSRAM suppliers on the market include Samsung, Cypress, Renesas, Micron and Toshiba.
Fundamental:
PSRAM is a pseudo SRAM. The internal memory particles are similar to the SDRAM particles, but the external interface is similar to SRAM. It does not require a complicated controller and refresh mechanism like SDRAM. The interface of PSRAM is the same as the interface of SRAM.
PSRAM capacity is 8Mbit, 16Mbit, 32Mbit, etc. The capacity is not as high as SDRAM, but it is definitely much higher than the capacity of SRAM. The speed supports burst mode, not very slow, Hynix, Coremagic, WINBOND.MICRON. CY And other manufacturers have to supply, the price is only slightly more expensive than the same capacity SDRAM, much cheaper than SRAM.
PSRAM is mainly used in mobile phones, electronic dictionaries, PDAs, PDAs, PMP.MP3/4, GPS receivers and other consumer electronics products and SRAM (using 6T technology), PSRAM uses 1T+1C technology, so in The volume is smaller, and the I/O interface of the PSRAM is the same as the SRAM. In terms of capacity, there are currently 4MB, 8MB, 16MB, 32MB, 64MB and 128MB. Compared to SDRAM, PSRAM consumes much less power. So for many portable products that require a certain amount of cache capacity, this is an ideal choice.
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