Thanks I will look at the 2.0 spec. What would cause isochronous data to not arrive, or not arrive with good data?
errors. After all this is a serial bus running on a PC. If you want reliable data, bulk transfers will retry. If you want guaranteed bandwidth ISO works. Even a very tiny percentage error will eventually get you if you send enough data over a long enough time.
So is there no way to guarantee data and timing with USB (1.1 or 2.0)?
Nope, you can choose ONE of good data or good timing.
So syncing 2 USB busses is possible in Linux, but difficult as you explained. Can I use (micro)frame number? Are these numbers the same on two different root hubs?
Thats part of what is difficult. I doubt if anybody currently tries to sync frame numbers between controllers and various controller flavors of uhci, ohci and ehci. Similarly, you would have to sync the SOF start of multiple controllers. All in all, difficult and probably not worth it.
I would look at the device design. If you could buffer your data and then use it synced on say the next SOF packet, you might be able to get very tight timing between multiple devices. In this case being on a common usb bus would be necessary. If you need a continuous supply of data, maybe you could ignore bad iso packets and reuse previous data?
Good luck, Steve
Ted
Steve Calfee wrote:
> At 10:38 AM 5/29/03 -0700, Ted Huntington wrote: > >Is syncronizing 2 isochronous signals possible on 2 different hubs? on > >the same hub? I need to make sure that data from 1 USB port and data > >from a different USB port arrive at my circuit at the same time (to > >within 1us). For now I am only using Full Speed. > > > >thanks > >Ted > > > >- > > You need to read the spec for usb 2.0 at www.usb.org But here are some > quick comments. > > USB is a serial bus, so if the devices are on the same bus (and full > speed), each bit is 84 ns, so your packet would have to be less than 12 > bytes (including overhead) to be less than 1us. Probably not possible or > useful. Might be possible at high speed. > > You might be able to use two root hubs, so you have two usb busses, but > then they will need to be somehow synchronized... possible but not easy. > > Iso is an unreliable send, so you are not guaranteed that both packets will > always arrive and arrive with good data. The only guarantee is bandwidth. >
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