USB 3.0 data transaction path

[ChongHan]

Hi all,

In USB 3.0, there is an addition part in the USB 3.0 port design which SSTX and SSRX is added into the wire. The USB 2.0's D+ and D- are remained in the port.

So here,
if PC is USB 3.0 while device is USB 2.0 port, are they use only D+ and D- to transfer data?

besides, if both PC and devices is support USB 3.0, are they use only SSTX and SSRX to transfer data? Or use D+, D-, SSTX and SSRX?

Thanks in advance. Hope will get detailed reply soon..sorry for my bad English.

[Barry Twycross]

The USB 3 (SSTX+/- and SSRX+/-) and the USB 2 parts (D+/-) run independently. Only one is ever active at once. There are rules about when a device changes from one to another. Basically a device tries to configure a USB 3 link if available. If the USB 3 doesn't work the device then falls back to USB 2. Its possible that a device can change speed if there is a USB reset. Only one link is ever active. Each link also has totally different attach detection. The USB 2 portion has the usual Vbus detection, a USB 3 link doesn't need Vbus and is constantly in Rx detect sending out common mode probes on SSTX+/-.

Hubs are the exception, they have both the USB 2 and USB 3 links active at once. A USB 3 hub is compound of the USB 2 hub with transaction translators and a SuperSpeed hub. The two parts work almost indepenently of each other.

[ChongHan]

hi thanks for your reply.

so mean that, if i connect USB 2.0 device to USB 3.0 port, the link to use for data transaction is D+ and D-, otherwise if same USB 3.0 in both devices and PC port will use SSTX and SSRX?

Between the usb reset can change the speed?

[Barry Twycross]

That is correct.

If a USB 2 device is connected, it knows nothing of the SuperSpeed lines, it doesn't have any superSpeed lines, so everything happens over the USB 2 lines (D+/-).

A USB reset gives a device a chance to reconsider its links. A USB 2 device has no SuperSpeed lines, so nothing can happen there.

A SuperSpeed capable device can end up using the USB 2 lines (D+/-) if its SuperSpeed links don't train successfully. If its in that state and its reset, it may again try to train its SuperSpeed links. So it may then reattach as a SuperSpeed device.

If a SuperSpeed capable device did train its links and connect at SuperSpeed and is subsequently reset (I think this needs to be a Warm Reset, not Hot reset), it again tries to train its links. If the training fails, it will then fall back to USB 2.

This is all covered in the LTSSM (Link Taining and Status State Machine), section 7.5 of the spec. Fig 7-14 is particularly useful for working all this out.

[ChongHan]

Thank you for helping me. I will try to understand more detail on link training and SSM.