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Types of Flash Memory Comparison: NAND vs NOR

Author: Apogeeweb
Date: 19 Jul 2021
what is flash memory


FLASH is a kind of non-volatile memory, even the power is off, the data will not be lost. However, flash memory does not rewrite data in bytes like RAM to replace it. A flash memory chip is composed of a set of data storage cell arrays, including many blocks, and each block contains many pages (usually 32 pages), a page is usually 512 bytes, because it was developed to replace the disk initially.

What Is Flash Memory?



Ⅰ Types of Flash Memory

Ⅱ NAND Flash Memory

Ⅲ NOR Flash Memory

Ⅳ NAND Flash vs NOR Flash

Ⅴ Universal Flash Storage Versions Comparison

Ⅰ Types of Flash Memory

Flash is generally used as a storage medium in digital cameras, handheld computers, MP3 and other small digital products. So it looks small like a card, sometimes it is called a flash memory card. According to different manufacturers and different applications, flash memory cards may include U disk, SmartMedia (SM card), Compact Flash (CF card), MultiMediaCard (MMC card), Secure Digital (SD card), Memory Stick, XD-Picture Card (XD card) and Microdrive Card. Although these flash cards have different appearances and specifications, their technical principles are the same.
Flash memory usually includes two main types, NOR and NAND Flash. NOR Flash is a random access device, with dedicated address and data lines (similar to SRAM), read and write in bytes, allowing access to any location in the memory, which is a good alternative to ROM, like the BIOS chip of a computer. While NAND flash memory does not have a dedicated address line and cannot be directly addressed. It is controlled by sending commands and addresses through an I/O-like interface, which means that NAND flash memory can access in pages only.

Types of Flash Memory

Ⅱ NAND Flash Memory

NAND Flash is widely used in various large-capacity devices such as memory cards, U disks, SSD, eMMC, etc. Its particles can be divided into three types: SLC (Single-Level Cell), MLC (Multi-Level Cell) and TLC (Triple-Level Cell) according to the number of bits stored in each storage unit. Among them, in a memory cell, SLC can store 1 bit, MLC can store 2 bits, and TLC can store 3 bits.
The more bits stored in a single storage unit of NAND Flash, the worse the read and write performance and the shorter the life, but the cost will be lower.





Bits Per Cell




P/E Cycles




Read Time




Program Time




Erase Time




SLC is mainly aimed at military industry and enterprise-level applications, with the advantages of high-speed writing, low error rate, and long durability.
MLC is mainly for consumer-level applications. It has twice the capacity of SLC with low-cost. It is suitable for USB flash drives, mobile phones, digital cameras and other storage cards. Now it is also used in civil solid state drives. Nowadays high-end SSDs will choose MLC or even SLC, low-end SSDs will choose TLC, and SD card generally selects TLC.
As a practical solid-state hard disk storage medium, NAND Flash has its own unique physical characteristics and requires basic management technology to use it. For designers, there are mainly the following challenges:
1) Erase before write
2) Loss mechanism, with durability limitation
3) Interference caused by reading and writing will cause data errors
4) Data retention period
5) Management of bad blocks at initial and runtime

The five major factors affecting these are:
1) SLC and MLC
2) Balanced wear algorithm
3) Ensure data integrity through bad block management.
4) Use error detection and correction technology.
5) Write amplification
Data is stored in the form of electrical charge in the Flash memory unit. The amount of stored charge depends on the voltage applied to the Control gate in the figure, which controls whether charge is charged or discharged.
1) For the writing (program) of NAND Flash, it is to control the Control Gate to charge (pressurize the Control Gate), so that the floating gate stores enough charge. If it exceeds the threshold Vth, it means 0.
2) For NAND Flash erasing (Erase), it is to discharge the floating gate. If it is lower than the threshold value Vth, it means 1.


Ⅲ NOR Flash Memory

NOR Flash is similar to ordinary memory in that they can support random access, which makes it also support XIP (eXecute In Place), and can execute programs like ordinary ROM. This makes it an excellent carrier for the code to be executed when the BIOS is turned on.
NOR Flash can be divided into two types: Parallel NOR Flash and Serial NOR Flash according to the difference of the interface with the Host side.

Parallel NOR Flash & Serial NOR Flash

Parallel NOR Flash can be connected to the Host controller, and its stored content can be directly mapped to the CPU address without copying to RAM. NOR Flash was the first such interface in the BIOS, called FWH (Firmware HUB). Because it was a parallel interface, the speed was slow, and it has basically been eliminated now. The cost of Serial NOR Flash is lower than that of Parallel NOR Flash, and it is mainly connected to the Host or PCH through the SPI interface.
Now almost all BIOS and some Set Top Boxes use NOR Flash. Their size is generally between 1MB and 32MB, which is expensive.
The biggest problem of NOR Flash is slow erasing and fewer erasing times, but it rarely causes BIOS speed reduction and damage due to this reason.


Ⅳ NAND Flash vs NOR Flash

Compared with NOR flash memory, NAND flash memory only needs fewer logic gates to store the same number of bits. Therefore, NAND flash memory is smaller and has a greater storage density than NOR flash memory. In terms of read speed, NOR flash memory is slightly faster than NAND flash memory. In terms of write speed, NAND flash memory is much faster than NOR flash memory. NAND flash memory is relatively simple to perform the erase operation, the entire block needs to be erased.
When erasing the NOR flash memory, all bits need to be written as 1. Although NOR flash memory has a faster and simpler access process, its storage capacity is relatively low, so it is more suitable for program storage. NAND flash memory can provide extremely high cell storage density (the current single chip has a storage capacity of 32GB), which is more suitable for storing a large amount of data, and the speed of writing and erasing is also very fast. In addition, the read and write operation unit of NAND flash memory is usually the size of a sector (512KB), which makes the NAND flash and disks very similar.
NAND Flash is more likely to have bit flips than NOR Flash, so error detection/error correction (EDC/ECC) algorithms must be used. At the same time, NAND Flash will gradually produce bad blocks as it is used. We often use FAT on SD cards that use NAND Flash. The file allocation table will be frequently rewritten, and the number of erasing and writing of each block is the decisive factor in the life of NAND Flash. How can we balance the erasing of each block and finding replacements for possible bad blocks? Usually need to have a special software level to realize the functions of bad block management, erasing and writing equalization, ECC, garbage collection, etc. This is called FTL (Flash Translation Layer). According to the location of FTL, Flash Memory can be divided into Raw Flash and Managed Flash.

Raw Flash & Managed Flash

Ⅴ Universal Flash Storage Versions Comparison

As we all know, CPU and GPU will affect the performance of mobile phones, or even more the flash memory will also have obvious effects. When buying smart phones, people start to pay attention to whether the flash memory is UFS2.1, UFS2.0 or eMMC5.1. Because the performance of the mobile phone is also greatly affected by it. With the release of UFS3.0 and UFS3.1 standards, what is the biggest difference among them we can see? Universal Flash Storage (UFS) provides a flash standard specification for digital cameras, mobile phones and other consumer devices. The numbers behind represent the version. The higher the version, the faster the speed. The number behind represents the version, the higher the version, the faster the speed. Here we will compare their differences.
✔️Version Comparison (Main Parameters)

UFS Version














Bandwidth per lane

300 MB/s

600 MB/s

1450 MB/s

Max. number of lanes



Max. total bandwidth

300 MB/s

1200 MB/s

2900 MB/s

M-PHY version




UniPro version




✔️UFS 2.1 vs UFS 2.2
The main difference between UFS 2.2 and UFS 2.1 is the Write Booster function, which improves the writing speed based on UFS 2.1. It will bring faster application startup speed, better browsing behavior of cache loading, faster coding time and many other good features. 
After eMMC gradually withdraws from the mainstream consumer market, and NAND flash memory price decreases continuously, which has accelerated the speed of popularization to some extent. That is, UFS 2.2 may become the main force.

✔️UFS 3.0 vs UFS 3.1
Compared with UFS 3.0, UFS 3.1 adds other three functions:
1) Write Turbo
Write Turbo is to increase the writing speed of the device. With this feature, the write rate of flash memory will be greatly increased. The write speed of UFS3.1 is increased to 700MB/s, while the sequential write of UFS3.0 is up to 500MB/s.
The Write Turbo is similar to the dynamic compression of some space in TLC/QLC solid state drives to simulate SLC cache. Data is first written from the part with stronger read and write performance, and the capacity is exchanged for performance, and then the files are automatically organized in free time and stored in a standard form. This frees up the extra space previously occupied by compression.
2) DEEP Sleep
DEEP Sleep can make the flash memory enter a low-power sleep state during idle time, ensuring that the mobile phone is more power-saving when it is idle. This helps to control the power consumption of the mobile phone during idle time and indirectly improves the battery service life of the mobile phone.
3) Host Performance Booster (HPB)
The main function of HPB is to improve the reading performance of mobile phones. The mobile phone response time becomes longer, one of the reasons is that the more the read, the slower, which is caused by the fragmentation of the file system and the degradation of random read performance of the device. The cache capacity of the device is limited, and frequent reloading of the L2P Map table causes excessive performance overhead. HPB uses the mobile phone's RAM to cache the L2P Map table to improve read performance, especially random read capability after long-term use. In short, HPB intends to solve the problem of slow running time.


Frequently Asked Questions about Types of Flash Memory

1. What are the different types of flash memory?
Flash memory comes in two basic types: NOR and NAND.


2. What are the characteristics of flash memory?
Flash memory has many features. It is a lot less expensive than EEPROM and does not require batteries for solid-state storage such as static RAM (SRAM). It is non-volatile, has a very fast access time and has a higher resistance to kinetic shock compared to a hard disc drive.


3. What is the purpose of flash memory?
Flash memory is a long-life and non-volatile storage chip that is widely used in embedded systems. It can keep stored data and information even when the power is off. It can be electrically erased and reprogrammed. Flash memory was developed from EEPROM (electronically erasable programmable read-only memory).


4. What is the difference between NAND flash and NOR flash?
NAND flash. NOR flash is faster to read than NAND flash, but it's also more expensive and it takes longer to erase and write new data. NAND has a higher memory capacity than NOR. NAND memory devices are accessed serially, using the same eight pins to transmit control, address and data information.


5. What is the difference between NOR and NAND gates?
NOR circuit basics. A NAND gate is conceptually implemented as an AND gate -- output 1 when both inputs are 1 -- followed by a NOT gate, which is a logical inversion. ... Likewise, a NOR gate is conceptually an OR gate -- output 1 when either input is 1 followed by the NOT gate, which is a logical inversion.

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