UFS is like SSD in mobile phones. What are the challenges of master control?
With the development of technology, mobile phone has become an indispensable mobile device at present, whose configuration has evolved from single-core to eight-core or even ten-core, making its functions more and more powerful. The high-end mobile phone upgraded the standard of storage chips from eMMC to UFS, and began to popularize in thousand-yuan smartphone models.
Some people say that UFS is like an SSD in a mobile phone. But after all, compared with a PC, the components in the mobile phone are relatively opaque. The size requirements and the integration of the mobile phone are very high so that UFS is not common to its true face. In 2018, low-end mobile phones used 16GB, compared with some high-end mobile phones more than 64GB or 128GB NAND Flash. Except that Apple has its own specifications, as long as it is an Android phone, the specifications and interfaces of embedded NAND Flash are basically either eMMC or UFS.
Today almost 90% of Android's cellphones are eMMC interfaces. However, there is no significant increase in the interface speed defined from 200MB/s for eMMC 4.5 to 400MB/s for eMMC 5.0 in 2013. While eMMC 5.1 in 2015 only supported Command Queue to speed up random access, the JEDEC Association's related vendors have already started to research the next generation of flash interface standards, releasing UFS1.0 in 2011, UFS2.0 in 2013 and the lastest UFS3.0 in 2016.
The battle between eMMC and UFS on speed and power consumption
Speed and power consumption are the most important requirements for storage in mobile phones. In addition to speed, the size of the mobile phones limits the capacity of the batteries. Low power consumption is an important issue for memory chips. Unlike eMMC's 8-bit parallel I/F, UFS data transmission is a Serial mode and CMD set is SCSI. Therefore, UFS can combine the speed of SSDs with the low power consumption of eMMC. UFS has adopted the 2-lane transmission mode since version 2.0, and the transmission rate is up to 1200MB/s, which is more than three times that of eMMC 5.1 version 400MB/s. The version 3.0, using MIPI m-PHY Gear4, can achieve 2400MB/s in 2-Lane mode, six times that of eMMC 5.1. UFS defines the card standard as well as be used as an embedded memory chip. This is the part discarded in eMMC specifications. The UFS card specification is currently version 1.1, using UFS 2.1 1-lane to reach 600MB/s, which is also higher than the built-in eMMC 5.1.
The challenge of NAND Flash control chip factory on UFS
From eMMC to UFS, the threshold will only get higher and higher. After all, in order to design an IC that can meet the specifications of the flagship mobile phone, the resources invested are different. When UFS is upgraded to 3.0 specification, the transmission speed of 2400MB/s,low latency and other requirements mean that the control chip should be designed with SSD architecture. There must be a powerful 32-bit processor. Many SRAMs may not be as simple as eMMC because of the cost considerations of 1-CH (channel), but 2-CH or 4-CH.
For low latency, a lot of processes are too slow to use FW and may need to be solved from the hardware, which is quite complex and costly for the master IC. Secondly, in order to solve the problem of battery life, power consumption or overheating, the master IC must have a variety of processing methods to save power for modules that do not need to work, and to have multiple temperature management. For example, when detected that the system temperature is overheated, temperature management need to adopt various mechanisms such as speed reduction. Third, due to the increases in unit storage density and the frequency of errors, NAND Flash technology has changed so rapidly that it has been updated almost in less than a year, and has entered the 3D era, especially in TLC or QLC with above 64/96 storeys. The master ECC is to determine the durability of NAND Flash, which is related to the reliability and competitiveness of the product.
UFS's challenges such as durability, data integrity, platform compatibility, and so on
In support of new 3D NAND, the ECC hardware capabilities required by UFS, such as LDPC + RAID, are already comparable to PCIe SSD. In addition, it is important for NAND Flash to management the FW’s know-how, requiring good cooperations with NAND Flash’s original plant and technical supports. As the main component of the memory chip in the mobile phone, the main control chip must demand the highest quality requirements of the international manufacturers. For example, how to ensure that the SRAM in the control chip does not cause FW collisions or data errors due to cosmic rays. Even if the probability is fairly low, a hardware mechanism should be designed to ensure data integrity. In terms of testing and validation, the specifications of the mobile phone are relatively less open than those of PCs. Thus the compatibility with the hardware of the mobile phone chip manufacturers and the testing of the OS/driver of the mobile phone system are all very important.
The largest application market for embedded NAND Flash lies in mobile phones,but the saturation of high-end smart phones has led to almost single-digit growth in shipments in recent years. With the rapid development of smart phones, not only UFS is used in high-end machines, but also low-end and middle-end models are being transformed from eMMC to UFS, thus increasing the volume. In addition, applications such as tablets, OTT boxes, and intelligent hardware continue to increase, which has a great incentive for the growth of embedded NAND Flash.