On Fri, 12 Nov 2004, omar kfitz wrote: > > There is no way to make a host controller passive other than turning it > > off. > > Do u have convincing reasons for that?
One of the things a host controller does is send out SOF (Start Of Frame) packets once every millisecond (faster for high-speed). If you look at the documentation for the UHCI (Universal Host Controller Interface) you'll see that there's no way to prevent it from sending those packets other than stopping it entirely. Also, a host controller initiates data transfers by sending SETUP, IN, and OUT packets. It will not transfer data when it receives one of these packets -- such a packet will always be treated as an error. Another problem is that devices are supposed to send STALL packets sometimes, but host controllers never do. Maybe your supervisor won't regard these reasons as convincing. In fact, maybe he's not inclined to believe anything I say. In that case all you can do is show him the published specifications. > Actually my supervisor provided me with an A to A USB cable that doesn't > respect USB norms. Where did he get the cable? I never heard of such a thing -- no existing hardware would be able to use it. > First of all, he suggested that I should try to cut > the Vbus wire to overcome the power problem. But I found out that the > Vbus is not only used to supply power but also involved in the process > where the host detects that some hardware has been attached ( even > though I couldn't really understand how it does so. It has something to > do with resistors... It'd be great if someone could clarify me on this > point too :) ). Vbus isn't used in detecting when a device attaches. Instead what happens is that both the D+ and D- wires are normally kept at a low voltage by the host controller. When a device connects it pulls one of them to a high voltage -- the same voltage as on Vbus, but Vbus itself isn't used in the signalling, only D+ and D-. > So once I told him about that he still suggested to cut > the Vbus and simulate whatever is used to signal the device attachement > on the D+ and D- wires. There's no way to make a host controller raise the voltage on D+ or D-. > > What is in software? Do you mean that USB won't run without software > > support? Yes, that's correct. However it doesn't mean that changing the > > software can magically change the hardware. > > What I meant is that WIndows 95 or 98 doesn't recognize when a USB is > plugged (hot plug) in while XP does even though we are using the same > hardware. The point here is that the detection of the USB plug is > controlled by the software which is here the Operating System. It's controlled by _both_ the software and the hardware. If either one is wrong then USB won't work. Alan Stern ------------------------------------------------------- This SF.Net email is sponsored by: InterSystems CACHE FREE OODBMS DOWNLOAD - A multidimensional database that combines robust object and relational technologies, making it a perfect match for Java, C++,COM, XML, ODBC and JDBC. www.intersystems.com/match8 _______________________________________________ [EMAIL PROTECTED] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
