Olin,

There is an interesting tradeoff between ISO and Bulk in USB.

ISO can lose data because there is no error correction on bad packets.

Bulk can lose data because retries take time on the bus and new data is then
lost.

Ya just can't win.

I guess I'd use bulk, but make sure I had some combination of:

1: a really big FIFO

2: enough headroom on the bus to allow for retries.

Bob Ammerman
RAm Systems




----- Original Message -----
From: "Olin Lathrop" <olin_piclist@EMBEDINC.COM>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Monday, January 20, 2003 9:28 AM
Subject: Re: [PIC]: Interfacing a PIC to a computer via USB2.0 ?


> > Good idea on the single unit thing.  I guess I can program the chip to
> > demand the bus to itself ?-)
>
> Only a fraction of the bandwidth.  That is what isochronous transfer mode
> is.  You ask for a specific bandwidth and the controller either grants it
> or tells you to stick it.  I would use bulk mode and just make sure
> nothing else is active on the USB at the time.  That also alleviates the
> need to phase lock your sample rate to the USB isochronous rate.
>
> > As for the transfer, I think some chip with a buildt in fifo will do the
> > trick.
> > All three chips I'm looking at have this.
>
> Yeah, how big?
>
> > I mentioned oscilloscope as it will be possible to use the device as a
> PC
> > oscilloscope by incorperating triggering in the software instead of on
> > chip/device.  It would make electronics cheaper and simpler and allow
> for me
> > playing around with the transfer.
>
> In theory that should work as long as there is enough buffering everywhere
> so that the USB and the CPU can catch up as bandwidth and cycles are
> available.  I don't think this would be acceptable as a commercial product
> because it requires too much of both the USB and the CPU, but for a
> personal project it sounds interesting.  For a more robust approach, I
> would put a DSP on the device to perform the trigger detection, then
> upload only triggered traces.
>
>
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