Basic knowledge of UFS that must be understood for mobile phone storage

There are three major components in a mobile phone: CPU, memory DDR, and storage device (EMMC/UFS, etc.). The memory is used to temporarily store the data required for program execution (data is lost when power is off), while the storage device is used to store data permanently (data is not lost when power is off).

On mobile phones, from SD cards to EMMC, and then to UFS cards, the speed of memory cards is getting faster and faster. Now ordinary mobile phones are configured with EMMC, while flagship mobile phones are configured with UFS. It is obvious that UFS will be the mainstream mobile phone storage for some time to come, and speed is the only reason.

The so-called UFS stands for Universal Flash storage. This concept usually represents two meanings:

(1) Mobile phone storage interface protocol, similar to SATA, PCIE/NVME and other concepts;

(2) Storage devices using the protocol;

Compared with EMMC, UFS is much faster, and the most important thing is that its signal is differential transmission, which is the basis of UFS data transmission speed. All high-speed transmission lines, such as SATA, PCie, and SAS, are serial differential signals.

Serial signals can use faster clocks (clock information can be embedded in the data stream) and differential signals, that is, the level difference between the two signal lines is 0 or 1.

Compared with single-ended signal transmission, differential signal has stronger anti-interference ability and can provide wider bandwidth. Secondly, UFS, like PCIE, supports multi-channel data transmission, currently supporting up to two channels. Multi-channel allows UFS to make trade-offs between cost, power consumption and performance.

UFS supports command queues, which means that the host can send many commands at once, and the UFS device supports parallel and out-of-order execution. The one that completes first returns the status first. This command processing method is called asynchronous command processing. Its predecessor, eMMC, does not support command queues. Commands are executed one by one, or in packets (each packet contains several commands). If the previous command is not completed, the subsequent command cannot be sent. This command processing method is called synchronous command processing.

Let's compare the command processing methods and command execution efficiency of "full-duplex + asynchronous command processing" and "half-duplex + synchronous command processing".

Half-duplex + synchronous

The host sends a write command W1 to the device, and then the host writes the data to the device; because it is a synchronous transmission mode, the commands are processed one by one, so before sending the read command R2, the previous write command W1 must be completed; similarly, before sending the write command W3, the R2 command must be completed.

Full-duplex + asynchronous

Because it supports command queues, the host can send several commands to the device at once. As shown in the figure above, the host sends a write command W1 and a read command R2 to the device at once. The device can process these two commands in parallel. Since the protocol supports full-duplex operation, the host transmits the data of the write command W1 to the device at the same time.

The device can also return the data of the read command R2 to the host. The processing of the subsequent commands R3, R4, W5... is similar.

Let's use the example of moving goods to compare the eMMC and UFS command execution methods:

Today's mobile phones have a wide variety of applications. You can play Landlord, listen to music, chat on WeChat, and perform multi-threaded operations. Due to the existence of full-duplex and command queues, the efficiency of UFS in processing commands is greatly improved, giving users an excellent experience.

Earlier we made some comparisons between UFS and eMMC, but I seem to have forgotten what eMMC is. Some people may be confused, what is eMMC?

eMMC, Embedded Multi Media Card, like UFS, is also a mobile storage protocol developed by JEDEC. It is the previous generation protocol standard of UFS.

The latest eMMC standard is eMMC5.1 released in 2015, with a maximum speed of 400MB/s. JEDEC already has UFS, and it is uncertain whether it will release a new eMMC standard. After all, parallel transmission of eMMC is limited by physical signals, so it is unrealistic to expect a qualitative leap in speed.