Phison PS2251-21 40Gbps USB4 Portable SSD Controller Launched

Phison U21
Phison U21

At CES 2024, Phison launched a few new controllers. One is particularly interesting and is something we hope to see make it to products later in 2024. This is the Phison PS2251-21. It is a new USB4 controller that should offer more speed for portable SSDs.

Phison PS2251-21 40Gbps USB4 Portable SSD Controller

The Phison PS2251-21 supports USB4 but is backward compatible with USB 2.0. The new controller is a 40Gbps controller supporting up to 4GB/s reads and writes. Some of the other key controller specs are:

  • 500GB up to 8TB support
  • Triple-level cell (TLC) and Quad-level cell (QLC) flash memory support
  • Compliant with Toggle 1.0/2.0/3.0/4.0 NAND flash interface
  • Compliant with ONFI 2.0/3.0/4.1
  • Transfer rate: up to 1600 MT/s
  • Support up to 4 channels/ 16 Chip Enable (CE) within Single Design
  • Flash IO Operating voltage supply 1.2v
  • TSMC 12nm Process

These may not be the fastest controller specs for SSDs in general in 2024, but they are very high-end specs for USB drives today. Having a controller like this means that manufacturers do not need to add a bridge chip to create these external SSDs. As a result, the SSD can usually be made smaller and with less complexity. That often leads to more drives in the market driving competition and even lower prices for consumers. All good things for the market.

Final Words

Phison expects USB IF Compliance to be complete in Q2 of 2024. Once that is complete,it should help pave the way for more portable SSDs with the controller. For our readers, a 4GB/s read/ write USB4 drive will be awesome both for transferring data via sneakernet and as potential extra SSDs in a system that has run out of slots. Hopefully, the single-chip solution will help more vendors offer drives in this class, and at lower prices. We have been waiting for a controller like this to help push the market for USB4 SSDs, and it appears to be close to fruition.


  1. Unfortunately don’t expect cheap drives to result from this as is usual with external SSDs… Having a ASM2464 case with Nvme ssds probably gonna run cheaper

  2. The asm2464 at its current state is causing some troubles if you want to save a little money. The most outstanding issue is that the controller runs extremely hot(hotter than the ssd controller itself usually) and caused a lot of instability due to that. Other than that, it is picky about the cables that you are using. Don’t expect a random Thunderbolt cable would work with it well. From my own testing, you will need a Thunderbolt4 cable or USB4 cable for a high success rate, and the older Thunderbolt 3 cables would very likely to fail to recognize the disk.

  3. What about USB 3 (g1 / g2 / g2x2) compatibility?
    USB 2.0 is nice but not having at least USB3 gen1 compatibility would be quite bad…

  4. More 40Gb devices like SSD’s are great and should put even more pressure to build PC’s and servers with 40Gb host controllers. All the OEMs today avoid high speed interconnects like the plague, except for Apple. Super happy with Mac Mini M2 Pro with 10Gb Ethernet plus four 40Gb Thunderbolt 4 ports. OEMs need to focus on AQC-113 10Gbe and a couple ASM4242 USB4 controllers by default on all motherboards, laptops, and desktops.


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