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Oct 8 2018

A Comparison of the NorFlash, NandFlash and eMMC

Introduction

Flash memory is a non-volatile EEPROM used for storage and transfering data between a computer (PC) and digital devices, which can be electrically erased and reprogrammed. It has two main types: NOR-type flash and NAND-type flash. eMMC is an internal storage card, its architecture puts the MMC (MultiMediaCard) into a small ball grid array (BGA) IC package for use in circuit boards, which is widely used in portable devices. Since most of the time flash is used to store only a small amount of code, which is more suitable, while NAND is an ideal solution for high data storage density. This tech note talks about differences between the three.

What Is Flash Memory?

Catalog

Introduction

Ⅰ Types of Flash Memory

1.1 NOR Flash

1.2 NAND Flash

Ⅱ Embedded MMC (eMMC)

2.1 eMMC Structure

2.2 eMMC Applications

2.3 eMMC Advantages

Ⅲ Comparison of the NOR Flash, NAND Flash, and eMMC

3.1 Performance Comparison

3.2 Capacity and Cost

3.3 Storage Physical Form

3.4 Reliable Durability


Ⅰ Types of Flash Memory

1.1 NOR Flash

NOR flash is one of the two main technologies on the nowadays market. Intel was the first person who exploited the NOR flash technology in 1988, which demoted the place of EPROM (Electrically Erasable Programmable Read - Only Memory) and EEPROM (Electrically Erasable Programmable Read - Only Memory) in the flash memory lose-hardness field. And then, in 1989, Toshiba published the NAND Flash structure, which emphasized cost reduction per bit, and has a higher performance, and can easy upgrade over the interface like a disk. The character of the NOR flash is XIP (execute in place). So the application program can be operated inside the Flash instead of reading the code into the RAM system. NOR’s transmission efficiency is high, with high cost-effectiveness at 1-4mb of small capacity but low write and erase speed greatly affects its performance. NAND's structures provide extremely high unit density, and can achieve high storage density, and can be written and erased quickly. The difficult of applying NAND is that flash's management requires a special system interface. Reading NOR is generally faster than NAND, and the written of the NAND is much faster than NOR, which should be considered in design.

It provides the complete address and data bus, and allows any area of random access memory, making it a good fit to replace the old ROM chip. At the time, ROM chips were mainly used to store code that required little update, such as the computer's BIOS or the set-top Box's firmware. NOR flash can tolerate 10, 000 to 1, 000, 000 wipe cycles and was the basis of earlier removable flash storage media. Compact flash was originally based on NOR flash, although it later moved to the lower-cost NAND flash.

 

1.2 NAND Flash

NAND flash memory is a kind of flash memory, which adopts nonlinear macro mode internally, and provides a cheap and effective solution for the realization of solid large memory. It was developed by Toshiba a year after NOR was produced. Nand flash memory has the advantages of large capacity and fast rewriting speed, and is suitable for the storage of large amounts of data. NAND flash has faster wipe time and smaller area per storage unit, which allows NAND flash to have higher storage density and lower cost per bit than NOR. It can also be erased 10 times more than NOR flash. While NAND flash's I/O interface does not randomly access an external address bus, it must be read in a block-based manner, and NAND flash typically has a block size of hundreds to thousands of bits.
Because most microprocessors and microcontrollers require byte level random access, NAND flash is not a good replacement for the ROM used to load programs. From this perspective, NAND flash is more like a secondary storage device like a CD or a hard disk. NAND flash is great for large storage devices such as storage CARDS. The first removable storage media created based on NAND flash was smart media.

NAND flash is widely used in the industry, such as embedded products including digital camera, MP3 player memory card, compact U disk and so on.

flash memory

Ⅱ Embedded MMC (eMMC)

eMMC (Embedded MultiMediaCard) is the standard specification of embedded memory developed by MMC association, which is mainly for products such as mobile phones or tablet computers. The eMMC integrates a controller into the package, provides a standard interface and manages flash memory, allowing the phone maker to focus on other parts of the product development and reduce the time it takes to market the product. It is equivalent to Nand flash+ main control IC. The interface speed of eMMC is up to 52Mb per second, and has fast and scalable performance. The interface voltage can be 1.8V or 3.3V. Their features are equally important to NAND vendors looking to reduce lithographic size and cost.

2.1 eMMC Structure

It consists of an embedded storage solution with MMC interface, flash memory device and main controller. All of these are in a small BGA package. The interface speed is up to 52Mb per second, and eMMC has fast and scalable performance. The interface voltage can be 1.8v or 3.3v.

2.2 eMMC Applications

eMMC is used in consumer electronics products with high storage capacity. eMMC has been used in some of the hottest products that were mass-produced in 2011, such as the Palm Pre, Amazon Kindle II and Flip MinoHD. To find out exactly what kind of memory these products use, iSuppli used the teardown analysis business to disassemble them and found that eMMC was in it.

2.3 eMMC Advantages

1) lean design

eMMC is currently the most popular mobile device local storage solutions, and it aims to simplify the design of mobile phone memory due to different brand including Samsung NAND Flash chips, KingMax, Toshiba or Hynix, Micron, etc., so it needs to according to each company's products and technical characteristics to redesign, and there was no technology to general label of NAND Flash chips in the past.

2) fast update speed

Every time the NAND Flash process technology changes, including the evolution from 70 nm to 50nm, and from 40nm to 30nm process technology, the mobile phone customers are also needed to redesign, but every year the technology of semiconductor product will come out of the old , and memory problems also dragged down the speed of new mobile phone model, so all the storage and management such as eMMC NAND Flash control chip package in 1 MCP concept will become popular in the market as the society developed continually.

3) running speed improvement

The design concept of the eMMC is to simplify the use of mobile phones’ internal memory. It designs NAND Flash chips and control chips into one MCP chip. Mobile customers only need to purchase eMMC chips and put them into new phones without dealing with other complicated NAND Flash compatibility and management issues.

memory

Ⅲ Comparison of the NOR Flash, NAND Flash, and eMMC

3.1 Performance Comparison

Flash memory is non-volatile memory and can be erased and reprogrammed into a memory cell block which called a block. Writing to any flash device can only be done in an empty or erased cell, so in most cases erasure must be done before writing to the device. While NAND devices are easy to erase, NOR requires all the bits in the target block to be written to 0 before erasure.

Since the erasure of NOR devices is performed in blocks ranging from 64 to 128KB, a write/erase operation takes 5s; in contrast, the erasure of NAND devices is performed in blocks ranging from 8 to 32KB, with a maximum of 4ms to perform the same operation.

The difference in block size during erasure further widens the performance gap between NOR and NAND, and statistics show that for a given set of writes (especially when updating small files), more erasure operations must be performed in nor-based units. Thus, when choosing storage solution, the designer must weigh the following factors.

1) NOR's reading speed is slightly faster than NAND's.

2) NAND's writing speed is much faster than NOR's.

3) NAND's 4ms erasing speed is far faster than NOR's 5s.

4) Most write operations require an erase operation first.

5) NAND has smaller erasure units and fewer erasure circuits.

In addition, the practical application of NAND is much more complex than NOR. NOR can be used directly and can also run the code above it directly. However NAND requires an I/O interface, so drivers are required for using. In fact, today's popular operating systems support Flash for NAND structures. In addition, the Linux kernel provides Flash support for NAND structures.

eMMC is widely used in high performance applications such as portable consumer electronic products for e.g. smartphones, digital tablets, multi-media players, PDAs, navigational systems and digital cameras.

 

3.2 Capacity and Cost

NAND flash units are almost half the size of NOR devices, and because the production process is simpler, NAND structures can provide higher capacity within a given mold size, which reduces the price accordingly.

NOR flash occupies the majority of the flash memory market with capacity of 1 ~ 16MB, while NAND flash is only used in 8 ~ 128M B products, which means that NOR is mainly used in code storage and storage media. NAND is suitable for data storage, and NAND has the largest share in CompactFlash, Secure Digital, PC Cards and MMC memory card market.

eMMC memory hides the complexities of flash memory technology in a convenient embedded package. So this reduces time and cost for chip developers. Also it has no need to develop interface software for all types of NAND memory by integrating the embedded controller into the memory chip.

 

3.3 Storage Physical Form

NAND Flash data is stored in memory cells in bit mode. Generally, only one bit can be stored in a cell. These cells are connected to bit lines in 8 or 16 units, forming what is called byte (x8) /word (x16), which is the bit width of NAND Device. These lines will form page again (NAND Flash has a variety of structures. NAND flash's reads and writes data are in pages and erases data are in blocks. In this way, three types of addresses can be formed:

1) Column address: starting address of the Register

2) Page address

3) Block address

 eMMC refers to a package consisting of both flash memory and a flash memory controller integrated on the same silicon board. Its secure erase and trim and high-priority interrupt are highly satisfied with the demand for high performance and security.

 

3.4 Reliable Durability

When adopt the flash media, we should take the reliability into consideration. Flash is a great storage solution for systems that need to extend MTBF. The maximum number of touches per block in NAND flash is a million, and NOR is 100,000.In addition to the 10 to 1 block erasure cycle advantage, a typical NAND block is eight times smaller than NOR devices, and each NAND block is deleted a little less in a given amount of time.

Flash also has better dynamic shock resistance than hard drives. These features are exactly the reasons that flash is so widely used on mobile devices. Flash memory also has one feature: it is very reliable when made into a memory card, and even submerged in water can withstand high pressure and extreme temperatures. Flash writing is often significantly slower than reading.

The eMMC v4.41 standard provides better performance, security and reliability features such as high-priority interrupt and secure erase. The eMMC applications market has expanded year by year.

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