Host

[stevef56]

Can a host force a port to a particular speed?

I want to design an embedded system where the host, usb trasceiver, and hub are on a single board and are all 2.0 compliant. But I want to force the ports on the hub to comunicate at full speed, even if a high speed device is plugged in. I want to do this so I can provide port isolation,

[Jan Axelson]

Use a 2.0 host that doesn't support high speed.

Not sure what you mean or what you want to accomplish by port isolation.

Jan

[stevef56]

Hi,

As I understand it if the port is 2.0 compliant and you plug in a high speed device the port will run high speed. If you plug in a full speed device it will run full speed. But for some ports I only want to allow full speed so that I can electrically isolate the port. Analog devices makes an IC that will provide the isolation but it can only work at low and full speed. I need electrical isolation because this is a medical device and sensor will be plugged into the usb port that have patient contact. An even better solution would be isolation that includes high speed. But 480Mbits/sec is just too fast.

Steve

[Jan Axelson]

Embedded hosts aren't required to support high speed.

See the OTG and Embedded Host Supplement. It's included in the zip file with the USB 2.0 spec.

Another option is to provide the isolation on the device side, downstream from the USB connection. See:

http://www.edn.com/contents/images/6347257.pdf

Jan

[stevef56]

Thanks Jan,

I'll look at the article but I don't think I can isolate on the device side because the device will be an off the shelf item where we don't have control of the design.

[Pat Crowe]

A high-speed capable host can force full-speed by refusing to acknowledge the high-speed chirp from the device during reset. Whether a given host chip can be set into such a mode would depend on the design of that host. So you would need to examine the data sheet to find out. If necessary a full-speed only capable host could be used (a number of PICs fit this description) as Jan suggested.

[Bret]

Another thing to keep in mind, FWIW.  Unless somebody is aware of something I'm not, you can't control the connection speed of individual ports on a hub (at least not on a "standard", off-the-shelf hub that follows the USB specifications).  If a high-speed hub is connected to a high-speed host, and you plug in a high-speed device, the device will automatically connect at high speed and there's no way to force it to a lower speed.  The only way to force a full-speed connection is to use a full-speed-only hub, or connect the hub to a full-speed-only port on the host, either of which forces ALL the ports on the hub to be full-speed-only.

So, even if you can get a host that is speed-adjustable on a per-port basis, the fact that you want to use a hub may change your mind.  Your best might be to go with a full-speed-only host, and maybe not even worry about a hub.

[Ron Hemphill]

It's been a while since I've looked at isolation, but aren't the ADI isolator chips for USB PERIPHERAL DEVICES (which do not include host root-port implementation, but would include hubs)?  You seem to be indicating that you plan to use the AD isolators for the "ports", but they're not intended for that since root ports are part of the host and must support HS.  Further, since you're planning to use off-the-shelf devices with your host, then you're obviously not planning to redesign each of the devices using the ADI isolators.

Therefore, the solution may be to simply design an internal FS hub on your host using the isolators, which could then become your isolated FS "ports" on the host.  I bet ADI even has a reference design for an isolated hub.