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USB frames synchronous?

Started by John Speth October 9, 2007
Hi group-

Does anyone know if the USB frame messages sent to all USB devices connected 
to the same computer are guaranteed to be synchronous and in lock step?

Please excuse my limited knowledge of USB.  The reason I'm asking is that 
I'm looking into the implementation of a USB based sensor where, when 
muliple sensors are connected to the same computer, they must all sample the 
sensing voltage to within 10 msec of each other.  It seems to me that the 
frame message is a convenient heartbeat that I can use only if it's 
synchronous with other USB devices and there are no dropped or lost frame 
messages.

Thanks, John.


Hi John,

> Does anyone know if the USB frame messages sent to all USB devices connected > to the same computer are guaranteed to be synchronous and in lock step?
No, there's no guarantee for that. A computer can have several independant USB hosts, for example one integrated in the chipset and another one on the "pro edition" mainboard. There's no guarantee other than that each host complies to the USB spec.
jetmarc@hotmail.com wrote:

> A computer can have several independant USB hosts, for example one > integrated in the chipset and another one on the "pro edition" > mainboard. There's no guarantee other than that each host complies to > the USB spec.
And then throw downstream hubs into the mix... How would you identify the same frame on different devices anyway? IIRC you can't even broadcast to different devices??? Regards, -- Mark McDougall, Engineer Virtual Logic Pty Ltd, <http://www.vl.com.au> 21-25 King St, Rockdale, 2216 Ph: +612-9599-3255 Fax: +612-9599-3266