This is the best flash popular science article I have ever seen!

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source ： Bread board community

One 、Flash Memory brief introduction

Flash Memory Is a nonvolatile memory . In embedded system, it is usually used to store the system 、 Applications and data . stay PC In the system , It is mainly used in SSD and motherboard BIOS in .

in addition , Most of them U disc 、SDCard And other mobile storage devices are also used Flash Memory As a storage medium .

Two 、Flash Memory The main characteristics of

Compared with traditional hard disk storage ,Flash Memory With light weight 、 Low energy consumption 、 Small volume 、 Strong anti-seismic ability , But there are also many limitations , Mainly as follows ：

1. You need to erase before writing
   Flash Memory There are certain restrictions on writing data . It can only set the current as 1 The bits of are rewritten as 0, It is not possible to have 0 The bits of are rewritten as 1, Only in the erase operation , To rewrite the whole block of bits into 1.

2. The number of block erasures is limited
   Flash Memory There is a limit on the number of erasures for each block of data （ 100000 to millions of times ）, After more than a certain number of erasures , This data block will not be able to reliably store data , Become a bad block .
   In order to maximize the extension Flash Memory Life span of , In software, you need to do erasure balance （Wear Leveling）, Write through dispersion 、 Dynamic mapping and other means to balance the use of various data blocks . meanwhile , The software also needs bad block management （Bad Block Management,BBM）, Identify bad block , Do not let bad blocks participate in data storage .（ notes ： In addition to the bad blocks caused by erasure ,Flash Memory Bad blocks will also be produced in the production process , That is, inherent bad block .）

3. Read and write interference
   Due to the physical characteristics of hardware implementation ,Flash Memory When reading and writing , It may cause bit flipping of other adjacent bits , Cause data exception . This exception can be recovered by re erasing .Flash Memory It is often used in applications ECC And other algorithms for error detection and data correction .

4. Charge leakage
   Stored in Flash Memory Charge of storage unit , If not used for a long time , There will be a charge leak , Causing data errors . But it takes a long time , Usually about ten years . This abnormality is not permanent , Re erasure can restore .
   

3、 ... and 、NOR Flash and NAND Flash

According to the different storage principles on the hardware ,Flash Memory Mainly can be divided into NOR Flash and NAND Flash Two types of . The main differences are as follows ：

• NAND Flash The reading speed is the same as NOR Flash Close , It varies according to the interface ;

• NAND Flash Write speed ratio of NOR Flash Much faster ;

• NAND Flash Erase speed ratio NOR Flash Much faster ;

• NAND Flash Maximum wipe times ratio NOR Flash many ;

• NOR Flash Support on-chip execution , You can run code directly on it ;

• NOR Flash Software driver ratio NAND Flash Simple ;

• NOR Flash Data can be read randomly by byte ,NAND Flash Need to read by block .

• Large capacity NAND Flash Than NOR Flash The cost is much lower , And smaller ;

（ notes ：NOR Flash and NAND Flash All erasures are performed in blocks , When performing an erase or write operation ,NOR Flash About need 5s, and NAND Flash Usually no more than 4ms.）

1、NOR Flash

NOR Flash According to CPU Different end interfaces , Can be divided into Parallel NOR Flash and Serial NOR Flash Two types of .

Parallel NOR Flash You can access Host Of SRAM/DRAM Controller On , The stored content can be directly mapped to CPU address space , No need to copy to RAM Can be CPU visit , Therefore, on-chip execution is supported .Serial NOR Flash Cost ratio Parallel NOR Flash low , Mainly through SPI Interface and Host Connect .

Whereas NOR Flash The erasure speed is slow , High cost characteristics ,NOR Flash Mainly used in small capacity 、 Scenarios with less content updates , for example PC a main board BIOS、 Router system storage, etc .

2、NAND Flash

NAND Flash It needs to be done through special NFI（NAND Flash Interface） And Host End to end communication , As shown in the figure below ：

NAND Flash According to the number of memory bits in each storage unit , Can be divided into SLC（Single-Level Cell）、MLC（Multi-Level Cell） and TLC（Triple-Level Cell） Three types of . among , In a storage unit ,SLC Can be stored 1 A bit ,MLC Can be stored 2 A bit ,TLC You can store 3 A bit .

NAND Flash Inside a storage unit of , Through different voltage levels , To represent the stored information . stay SLC in , The voltage of the storage unit is divided into two levels , respectively 0 and 1 Two states , namely 1 A bit . stay MLC in , The voltage of the memory cell is divided into 4 Level , respectively 00 01 10 11 Four states , namely 2 A bit . Empathy , stay TLC in , The voltage of the memory cell is divided into 8 Level , Storage 3 Bits of information .

NAND Flash The more bits are stored in a single storage unit of , The worse the read-write performance will be , The shorter the life , But the cost will be lower .Table 1 in , The cost and life data under the specific process and technical level are given .

Compared with NOR Flash,NAND Flash Good write performance , Low cost with large capacity . at present , The vast majority of mobile devices such as mobile phones and tablets eMMC Inside Flash Memory All belong to NAND Flash.PC It is also used in the solid state drive in NAND Flash.

Four 、ROW flash and managed flash

because Flash Memory There is erasure by block 、 Limit of erasure times 、 Read and write interference 、 Limitation of charge leakage , In order to give full play to Flash Memory The value of , Usually there needs to be a special software layer , Implement bad block management 、 Erasure balance 、ECC、 Garbage collection, etc , This software layer is called FTL（Flash Translation Layer）.

In the implementation , according to FTL Different locations , You can put Flash Memory It is divided into Raw Flash and Managed Flash Two types of .

1、Raw Flash

In such applications , stay Host The end usually has a special FTL perhaps Flash File system to achieve bad block management 、 Functions such as erasure balance .Host The software complexity of the terminal is high , But the cost of the overall scheme is low , It is often used in price sensitive embedded products .

What we usually say is NOR Flash and NAND Flash All belong to this type .

2、Managed Flash

Managed Flash In its internal integration Flash Controller, Used to complete erasure equalization 、 Bad block management 、ECC Check and other functions . Compare with directly Flash Access to Host End ,Managed Flash Shielded Flash The physical properties of , Yes Host Provide standardized interfaces , Can reduce the Host The complexity of end software , Give Way Host Focus on the upper business , Omit right Flash Take special care of .

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