Does Usb 3.1 use PCIE Lanes / why there is no fully 3.1 motherboards ?

rakanbst · August 18, 2017

First of all,

I Didn't know which category should I put this post "Motherboards, Storage, or General". if someone from the admin forgive me for that and if he/she transfer it to proper category.

second of all,

I'm one of Linus Tech Fan which I always watch there videos and always take there recommendations. Thanks a lot for everything. Most of my knowledge of the Tech is because of you guys.

Now lets go to the subject,

I have 4770K cpu and etc. and thinking to upgrade but when I feel there is something worth it (SATA 4, or full USB 3.1, or when CPU can get to above 5GHz (in boost mode) without overclocking :P)

I was wondering why there is no Motherboards yet support full 3.1 Gen 2 usbs where is should be back compatible for older versions.

I read somewhere that Usb 3.1 Gen 2 build-in motherboard used 2 PCIE Lanes ??? is that true ?

I tried to search online and didn't find any article or video explaining this.

Can someone clarify it ? or someone with Build-in USB 3.1 to see if it really consuming PCIE lanes or no.

Best Regards,

Rakan Tawabini

Petroleum Engineer

typographie · August 18, 2017

Actually, yes, you're right. An ASMedia USB 3.1 controller does indeed eat up two potential PCIe lanes, as explained here. Keep in mind these are chipset lanes, though, not CPU lanes. The USB 3.1 controller does not compete with your graphics card for its 16 lanes. It just means you're giving up two of your Z170/Z270 chipset's 20 or 24 lanes, which is inconsequential for most users.

The reason you haven't seen any motherboards with integrated USB 3.1 yet is because no Intel or AMD chipsets currently offer it. It has to be included as a third-party controller. This is what always happens, I think… my old i5-750 system had a separate USB 3.0 controller because Intel P55 didn't have integrated 3.0 support yet.

mariushm · August 18, 2017

There are ICs (chips) which create one or two USB 3.1 ports and use one or more pci-e lanes.

For example, AsMedia ASM1142 creates two USB 3.1 gen 2 (10gbps) ports, but connects to the rest of the system through either a single pci-e v3.0 x1 lane (~970 MB/s in both directions) or two PCI-E v2.0 lanes (maximum 2 x 500 MB/s in both directions).

So while a single port is designed to handle up to 10gbps (~ 1250 MB/s) in real world very few devices actually get close to that speed, so being limited to maximum 970-1000 MB/s to the system is not a big deal.

Also, rarely both ports are used anywhere close to maximum speed so again, it's not a big deal that there is a "choke point" at the exit from the chip.

Some usb 3.1 10 gbps ports are created by the chipset. Most modern chipsets talk to the processor through a connection that's very similar to or is actually just a bunch of pci-express lanes. Ryzen chipsets actually use a pci-e v3.0 x4 link, while intel processors use DMI or something like that, which behaves pretty much like pci express.

The chipsets take this big data link and split it into multiple internal smaller "highways" to which various devices can connect. Think of the chipset as a network switch that has 16-48 1gbps ethernet jacks and a single 10gbps ethernet jack which goes somewhere else (the processor). Those 16-48 ethernet ports have cables which go to various parts of the chipset : the usb 2.0 controller, the usb 3.0 controller, the usb 3.1 controller, m.2 connectors (if any) , pci-e slots (usually x4 and x1 slots on the motherboards ), the sata controller, pci-e links to the onboard sound, the onboard network card, additional usb controllers...

The idea is that you don't use all the usb ports, all the sata connectors, all the pci-e slots on the motherboard at the maximum throughput all the time, so it's acceptable to just take the "data traffic" from all these and mix it together into the pci-e 3.0 x4 link or DMI and send it to the cpu, and rarely all the devices connected to the chipset will actually reach the limitations of that connection (4 GB/s in both direction)

In some cases, data doesn't even have to go to the processor, for example you have an external hard drive connected to usb 3.0 which comes from chipset, and you copy data on a sata drive connected to the sata controller in the chipset, so the flow of data may not have to reach the processor, it may simply pass through between controllers in the chipset.