Differences between nor flash and NAND Flash, and between RAM and Rom

Rom and RAM are both semiconductor memory, Rom is short for read only memory, and Ram is short for random access memory. ROM can still maintain data when the system stops power supply, while Ram usually loses data after power loss. A typical Ram is the computer memory.
Ram has two categories: static RAM (static RAM/SRAM). The speed of 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 the first-level buffer of the CPU and second-level slow flushing. The other is dynamic RAM (Dynamic RAM/Dram). DRAM retains data for a short period of time and is slower than SRAM, but it is faster than any Rom, but in terms of price, DRAM is much cheaper than SRAM, and computer memory is dram.

DRAM is divided into many types. Common types include fpram/fastpage, edoram, SDRAM, ddr ram, RDRAM, sgram, and wram. Here we will introduce a ddr ram.

Ddr ram (date-rate RAM) is also called ddr sdram. The improved Ram is basically the same as that of SDRAM. The difference is that it can read and write data twice at a clock, this doubles the data transmission speed. This is the most widely used memory in computers, and it has a cost advantage. In fact, it beat Intel's other memory standard-Rambus DRAM. In many high-end graphics cards, high-speed ddr ram is also provided to increase the bandwidth, which can greatly improve the pixel rendering capability of the 3D accelerator card.

Memory Working principle:

Memory is used to store the data and programs currently in use (that is, execution). The memory of the computer we usually refer to dynamic memory (that is, Dram ), in dynamic memory, the so-called "dynamic" means that after we write data into dram, data will be lost after a period of time, so we need a memory refresh operation by using a peripheral circuit.

The specific working process is as follows: whether a dram storage unit stores 0 or 1 depends on whether the capacitor has a charge. If there is a charge, it indicates 1, and if there is no charge, it indicates 0. However, after a long period of time, the capacitance of 1 will be discharged, indicating that the capacitance of 0 will absorb the charge, which is the cause of data loss. The refresh operation regularly checks the capacitance, if the power is greater than 1/2 of the full power, it indicates 1 and the capacitor is fully charged. If the power is less than 1/2, it indicates 0 and the capacitor is discharged, to maintain data continuity.

There are also many types of Rom. The difference between the prom and the EPROM is that the prom is one-time, that is, after the software is filled in, it cannot be modified. This is an early product and cannot be used now. The EPROM is used to erase the original program through ultraviolet radiation and is a general memory. Another type of EEPROM is an electronic interface. It has a high price, a long write time, and a low write speed.

For example, the mobile phone software is usually placed in the EEPROM. When we call, some of the last called numbers are temporarily stored in the SRAM, rather than writing the call records immediately (the call records are stored in the EEPROM ), at that time, there was a lot of important work (CALLS) to do. If you write data, a long wait would be unbearable for users.
Flash Memory, also known as flash memory, combines the strengths of ROM and Ram, not only has the electronic Erasable Programmable (EEPROM) performance, data will not be lost after power failure, and data can be quickly read (NVRAM advantage), which is used in USB flash drives and MP3. In the past 20 years, embedded systems have been using ROM (EPROM) as their storage devices. However, in recent years, Flash has completely replaced the position of RoM (EPROM) in embedded systems, it is used to store bootloader, operating system or program code, or directly used as a hard disk (USB flash disk ).

Currently, there are two types of FLASH: nor flash and nadn flash.

The reading of nor flash is the same as that 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 adopt the random read Technology of memory. It reads a block at a time, usually 512 bytes at a time. Flash with this technology is cheaper. Users cannot directly run the code on NAND Flash. Therefore, many development boards that use NAND Flash use a small nor flash to run the startup code in addition to NAND flah.

Nor flash is usually used for small capacity. Because of its fast reading speed, it is used to store important information such as the operating system, while NAND Flash is used for large capacity, the most common nand flash application is the DOC (Disk On Chip) used in the embedded system and the "flash disk" we usually use, which can be erased online. Flash on the market is mainly from Intel, AMD, Fujitsu and toshba, while the main manufacturers of NAND Flash are Samsung and toshba.

Comparison between NAND Flash and nor flash

Nor and NAND are two major non-loss flash technologies on the market. Intel first developed nor Flash technology in 1988, which completely changed the previous situation where EPROM and EEPROM were integrated. Next, in 1989, Toshiba published the NAND Flash structure, emphasizing reducing the cost per bit and higher performance, and easily upgrading it through interfaces like disks. However, after more than a decade, a considerable number of hardware engineers are still confused about nor and NAND Flash.

Phase "Flash Memory" is often used with phase "nor memory. Many insiders do not know the advantages of NAND Flash compared with nor, because in most cases, Flash is only used to store a small amount of code, so nor flash is more suitable for some. Nand is an ideal solution for high data storage density.

Nor is the primary non-easy-to-lose Flash technology on the market. Nor is generally used to store a small amount of code. Nor is mainly used in the code storage media. Nor is characterized by simple application, no special interface circuit, and high transmission efficiency. It is an internal chip execution (xip, execute in place ), in this way, the application can run directly in the (NOR) flash memory without having to read the code into the system Ram. In 1 ~ The small size of 4 MB has high cost efficiency, but the low write and erase speeds greatly affect its performance. Nor Flash has an SRAM interface and enough address pins to address it, so it can easily access every byte in it. Nor flash occupies 1 ~ Most of the 16 MB flash memory market.

The NAND structure provides a very high unit density, achieves a high storage density, and writes and erases quickly. The difficulty of using NAND lies in Flash management and special system interfaces.

1. Performance Comparison:

Flash flash memory is a non-loss-prone memory. It can be used to erase and re-program memory unit blocks called blocks. Write operations on any flash device can only be performed in an empty or erased unit. In most cases, the flash device must be erased before writing. It is very easy for the NAND device to perform the erasure operation, and nor requires that all the bits in the target block be written to 1 before the erasure.

Since the nor device is erased with 64 ~ For blocks of KB, the time for performing a write/erase operation is 5 S. In contrast, the erased NAND device is 8 ~ For 32 KB blocks, it takes up to 4 ms to perform the same operation.

When the block size is erased, the performance gap between nor and nadn is further extended. Statistics show that for a given set of write operations (especially when updating small files ), more erasure operations must be performed in the nor-based unit. In this way, when selecting a storage solution, the designer must weigh the following factors:

● Nor reads faster than NAND.

● NAND writes much faster than nor.

● NAND's 4 Ms erasure speed is far faster than nor's 5s.

● Most write operations require erasure first.

● NAND has fewer erased units and fewer erased circuits.

(Note: The erasure time of nor flash sector varies depending on the brand and size. For example, for 4 M Flash, some sector erasure times are 60 ms, while some require a maximum of 6 s .)

2. Interface differences:

Nor Flash has an SRAM interface and enough address pins to address it, so it can easily access every byte in it.

Nand devices use complex I/O ports to access data in a serial manner. Different products or vendors may use different methods. Eight pins are used to transmit control, address, and data information.

Nand read and write operations use 512 bytes of blocks, which is a bit like hard disk management. Naturally, NAND-based memory can replace hard disks or other Block devices.

3. Capacity and cost:

The unit size of NAND Flash is almost half the size of the nor device. Because the production process is simpler, the NAND structure can provide a higher capacity within the given mold size, thus reducing the price accordingly.

Nor flash occupies 1 ~ Most of the 16 MB flash memory market, while NAND Flash is only used in 8 ~ Among the MB products, this also shows that nor is mainly used in code storage media. NAND is suitable for data storage. NAND shares the largest share in the compactflash, secure digital, PC cards, and MMC memory cards markets.

4. Reliability and Durability:

Reliability is an important issue to consider when using the flahs media. Flash is a suitable storage solution for systems that need to expand MTBF. The reliability of nor and NAND can be compared in terms of life (durability), bit switching and bad block processing.

A) Life (durability)

In nand flash memory, the maximum number of writes to each block is 1 million, while that of nor is 100,000. In addition to having a block erasure cycle of 10 to 1, the typical NAND block size is eight times smaller than that of the nor device, each NAND memory block is deleted less frequently within a given period of time.

B) Bit Switching

All flash devices are plagued by bit switching. In some cases (rarely, Nand occurs more often than nor), a bit may be reversed or reported to be reversed.

One-bit changes may not be obvious, but if it occurs on a critical file, this small fault may cause system downtime. If there is only a report problem, it may be solved by reading multiple times.

Of course, if this bit changes, you must use the error detection/error correction (EDC/ECC) algorithm. The problem of bit inversion is more common in nand flash memory. The NAND supplier recommends using the EDC/ECC algorithm when using nand flash memory.

This problem is not fatal when using NAND to store multimedia information. Of course, if you use a local storage device to store operating systems, configuration files, or other sensitive information, you must use the EDC/ECC system to ensure reliability.

C) Bad Block Processing

Bad blocks in NAND devices are randomly distributed. I have tried to eliminate bad blocks before, but I found that the yield rate is too low, the cost is too high, and it is not cost-effective at all.

The NAND device needs to initialize the media to detect Bad blocks and mark them as unavailable. In an existing device, a high failure rate may occur if this processing is not performed in a reliable way.

5. ease of use:

The nor-based flash memory can be used very directly, and can be connected like other memory, and code can be directly run on it.

Due to the need for I/O interfaces, Nand is much more complicated. The access methods for various NAND devices vary from manufacturer to manufacturer.

When using the NAND device, you must first write the driver to continue other operations. Writing information to a NAND device requires considerable skill, because designers must not write information to Bad blocks, which means virtual ing must be performed from beginning to end on the NAND device.

6. Software support:

When discussing software support, we should distinguish basic read/write/erase operations from high-level software for disk simulation and flash management algorithms, including performance optimization.

Running code on the nor device does not require any software support. When performing the same operation on the NAND device, the driver is usually needed, that is, the memory technology driver (MTD ), the NAND and nor devices require MTD for write and erase operations.

Fewer MTDS are required to use the nor device. Many vendors provide more advanced software for the nor device, including the trueffs driver of the M-system, this driver is used by Wind River system, Microsoft, QNX software system, Symbian, Intel, and other vendors.

The driver is also used to simulate diskonchip products and manage nand flash memory, including error correction, Bad Block Processing, and loss balancing.

Nor flash is mainly supplied by Intel, micro, and other vendors. It was once a mainstream flash product, but it is hard to squeeze it by NAND Flash. Its advantage is that it can run programs directly from flash, but the process is complicated and the price is relatively high.

The main vendors of NAND Flash are Samsung and Toshiba. flash is used in USB flash drives, various memory cards, and MP3 players. due to technological differences, NAND Flash has a larger storage capacity than nor flash, and cheap. However, there are also disadvantages: you cannot directly run programs with addressing, and you can only store data. In addition, NAND Flash is prone to bad areas, so verification algorithms are required.

In the handheld computer, you must use NAND Flash to store data and programs, but nor flash must be enabled. In addition to the Samsung processor, other mainstream processors used in handheld computers do not support programs directly started by NAND Flash. Therefore, it is very troublesome to start the machine with a small nor flash, and load OS and other software from the NAND flash into the SDRAM.

DRAM uses the charge on the gate capacitor of the mos tube to store information. Once power loss occurs, all information will be lost, because the gate will leak, therefore, a refresh mechanism is required to charge these gate capacitors at a certain time, and each time the data is read, the charge is also required. This is called Dynamic Refresh, so it is called dynamic random memory. Because it only uses one MOs to store information, the integration level can be very high and the capacity can be very large. SDRAM has one more synchronization with the CPU clock than it.

SRAM uses registers to store information. Once the power is down, all the data will be lost. As long as the power is supplied, its data will always exist, and no Dynamic Refresh is required. Therefore, it is called static random memory.

The above is mainly used for memory in the system, with a large capacity and data will still be stored after power failure.

Flash ROM uses the capacitor on the floating gate to store the information. Because the floating gate does not leak electricity, the information can still be saved after power failure. Because of its simple structure, the integration level can be very high and the capacity can be very large. Flash ROM needs to be erased by electricity before writing, and it can be erased in bytes (bytes). Flash ROM can only be in the unit of Sector (sector. However, it can be written in bytes. Flash ROM is mainly used for bios, USB flash drives, MP3, and other devices that require large capacity and no data loss during power failure.

Psram, false static random memory

Background:

Psram has a single-transistor DRAM bucket, which is very different from the conventional SRAM bucket with six transistors or four transistors and the two-load resistor SRAM bucket, however, it has a stable interface similar to SRAM. The internal DRAM Architecture gives psram some advantages over low-power 6 t sram. For example, it is more lightweight and the price is more competitive. Currently, 90% of the manufacturers in the overall SRAM market are producing psram components. In the past two years, key suppliers of SRAM/psram in the market include Samsung, cypress, Renesas, micron and toshba.

Basic Principles:

Psram is a pseudo-SRAM. The internal memory particles are similar to those of SDRAM, but the external interfaces are similar to those of SRAM. It does not need the complicated controller and refresh mechanism as the SDRAM does, the interfaces of psram are the same as those of SRAM.

Psram has a capacity of 8 Mbit, 16 Mbit, 32 Mbit, etc. The capacity is not as high as the density of SDRAM, but it must be much higher than the capacity of SRAM. The speed supports the burst mode and is not very slow, hynix, coremagic, WINBOND. micron. cy and other manufacturers have supply, the price is only a little more expensive than the same capacity of the SDRAM, much cheaper than the SRAM.

Psram is mainly used in mobile phones, electronic dictionaries, handheld computers, PDAs, and PMP. in the comparison of MP3/4, GPS receivers and other consumer electronic products with SRAM (using 6 T Technology), psram uses 1 t + 1C technology, so it is smaller in size. At the same time, the I/O interface of psram is the same as that of SRAM. currently, 4 MB, 8 Mb, 16 MB, 32 MB, 64 MB, and 128 MB are supported. Compared with SDRAM, psram consumes much less power. Therefore, it is an ideal choice for many portable products that require a certain cache capacity.

Flash cards:
Digital flash cards: mainstream digital storage media
Digital cameras, MP3 players, handheld computers, mobile phones, and other digital devices are the primary markets for flash. As mentioned above, the mobile phone industry is dominated by nor flash memory, and Flash memory chips are directly deployed on internal circuit boards, however, digital cameras, MP3 players, handheld computers, and other devices require that the storage media be replaceable. Therefore, interface standards must be developed to achieve connection. The flash card technology came into being. A flash card uses flash memory as its core storage device. In addition, it also has an interface control circuit and external encapsulation. Logically, it can be classified as a flash disk, flash cards only have more dedicated colors, while flash disks use a universal USB interface. Due to historical reasons, the flash card technology has not formed a unified industrial standard, and many manufacturers have developed their own flash card solutions. Currently, CF cards, SD cards, smcards, mmccards, and Sony memory stick are common.

CF card (compactflash)
CF card is a flash card introduced by SanDisk in 1994. It is the earliest portable storage device with large capacity. Its size is only 43mm × 36mm × 3.3mm, equivalent to 1/4 of the size of the PCMCIA card of the laptop. The cfcard has an independent controller chip and has a full PCMCIA-ATA function. It is connected to the device in a similar way as the PCMCIA card, but the cfcard has up to 50 pins. This connection method is stable and reliable, and will not affect its stability due to frequent plugging.
The CF card does not have any active components, does not have problems such as physical bad sectors, and has excellent seismic performance. The CF card is safer and more reliable than a floppy disk or hard disk. The power consumption of the CF card is very low. It can adapt to the voltage of 3.3 V and 5 V, and the power consumption is about 5% of that of the desktop hard disk. This feature is out-of-class, and the CF card becomes the preferred storage device for digital cameras. After years of development, the CF card technology has been very mature, and the capacity has soared from the initial 4 MB to the current 3 GB, the price is becoming more and more affordable, and is widely favored by various digital camera manufacturers, currently, the CF card ranks second in the digital camera memory card market.

Mmccard (multimediacard)
Mmcka is a new type of memory card jointly launched by SanDisk and Siemens in Germany in 1997. Its size is only 32mm × 24mm × 1.4mm, and the same stamp size is almost the same; its weight is less than 2 grams, and it has the characteristics of impact resistance, can read and write more than 0.3 million times repeatedly. In essence, MMC and CF are actually in the same technical system. Both of them have the same structure as the fast flash memory chip and controller chip. The functions are the same, but the size of the mmccard is super small, the connector must also be made in a small card, resulting in high production difficulty and manufacturing costs and high prices. MMC is mainly used for mobile phones, MP3 players, and other small-sized devices.