Re: [libusb] USB 3.0 bulk EP bursts

[dineshkumar m. <dsp...@gm...>]

Thanks for your valuable comments. Yes, Pixel clock is 100Mz. I agree that
if there are enough buffers we can handle the burst and maintain average
data rate. Let me try allocating more buffers.
Thanks again.


On Wed, Jul 31, 2013 at 9:12 PM, Tim Roberts <ti...@pr...> wrote:

> dineshkumar muthusamy wrote:
> > Yes, you are correct. I have not taken blanking and sync timings into
> > account. Streaming a VGA image @ 60fps is not a problem.
> >
> > since 2 bytes transferred at 100MHz, I computed as 200MB/s as peak
> > throughput. Also data is not transferred for completed duration of a
> > second because of frame rate. This is the reason I was only thinking
> > about peak throughput to transfer 16KB that is generated.
>
> For a burst rate, that's true, but cameras are not continuous.  They are
> very bursty.  You get a burst during a scanline, then a pause for
> horizontal blank and sync, then another burst.  Eventually, there's a
> long pause during vertical blank and sync.  As long as you have some
> buffering to ride through that, the average bandwidth is much less.
>
>
> > For a larger images e.g. 2500X2000 pixels (1 pix=2 bytes) the 16KB
> > buffer is filled at higher rate than VGA image.
>
> So, your pixel clock is 100 MHz?  At 2500x2000, that's roughly 20 frames
> per second.  May I ask what sensor you are using?
>
>
> > This in turn has to be transmitted by USB 3.0 EndPoint. But this does
> > not happen and buffer over run happens as camera continues to stream
> > irrespective of data is read at otherend ( i.e. USB 3.0 end pint)
> >
> > Hence I thought device is stream the data but host is not able to read
> > the data at same rate.
> > Could you please comment on this?
>
> Well, you've pretty much heard the possibilities.  Either your device
> hardware can't keep up, or the host controller hardware can't keep up,
> or the CPU is too underpowered to keep up
>
> --
> Tim Roberts, ti...@pr...
> Providenza & Boekelheide, Inc.
>
>
>
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-- 
Best regards,
Dineshkumar M.