On Friday 08 May 2009, Anders Montonen wrote: > If I remember the USB spec correctly, the lowest achievable round-trip > time (ie. one OUT transfer followed by one IN transfer) is two > milliseconds for low- and full-speed devices.
Depends on the implementation of the host controller driver and silicon errata. ISTR that OHCI on Linux was once two msec *per transfer* but had to add another msec to cope with some buggy silicon. So that realistically is closer to 5 msec, maybe 6 msec. UHCI doesn't have that annoying erratum (not many silicon vendors to commit such goofage!), so it's a bit quicker. And EHCI -- full speed through a highspeed hub -- can be much quicker than that. In theory, that round-trip could take less than one millisecond. But yes, the conclusion that USB should avoid synchronous calls is very accurate. _______________________________________________ Openocd-development mailing list Openocd-development@lists.berlios.de https://lists.berlios.de/mailman/listinfo/openocd-development
