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Re: How to handle USB Isochronous data Transfer?

Title: Re: How to handle USB Isochronous data Transfer?
Dear all,
 
    Thank you Mr. Mark Cookson for your responce. Actually my device has MaxPacketSize of
300 Bytes in Alternate Setting 3 which start streaming out data when this alternate setting has been set.
From your reply It is pretty clear that bytes per frame must be set Max Packet size of the opened Isoc Pipe.
what should be the typical number of frames that the USBIsocRead call must be active? Does this number
of frames depends on any factor or can we make the IsocRead for any number of frames? Does it required
to wait for 1msec after the USBIsocRead?
 
Thanks and Regards,
Sivaram Prasad
 
----- Original Message -----
Sent: Tuesday, February 22, 2005 11:02 PM
Subject: Re: How to handle USB Isochronous data Transfer?

For isoc reads, you specify the maximum amount of data you want per frame, and how many frames you want to read.  You give a buffer, and a memory descriptor for that buffer, along with the list of frames and bytes per frame to read, and IOUSBFamily does the rest.

If you don’t know how many bytes will come each frame (which it sounds like you don’t), then you have to pick some upper maximum (preferably other than 1023, but that is the maximum isoc frame size in USB 1.1).  Remember that IOUSBFamily puts the data where it would have gone if the full amount had been transferred in the frame before.  So, if you ask for 1023 bytes, but only get 100, the next block of data will be put at offset 1024 in your buffer, not 101, so you’ll have to coalesce the data together, if that’s important, because there will be lots of gaps with garbage in your buffer.

--
Mark Cookson
M-Audio, a division of Avid
225 Locust St.
Hudson, WI 54016

On 2/22/05 10:56 AM, "Nakkala Sivaram Prasad" <email@hidden> wrote:
Dear all,

    Could anybody suggest how to handle the USB isochronous data transfers.
In case of USB bulk data transfers (USB Bulk Read) we specify the amount of
data to read and the buffer pointer to hold the data. I would like to know how
to read in case of  Isochronous data transfer.

   In case of USB DDK 1.5.5f (Mac OS 9) we have an API USBIsocRead to read
data from an isochronous pipe. Here I want to read data for certain number of
frames. I am able to supply the starting frame number where actual read should
start and the number of frames the call must be active. How to handle the situation
if i don't want to specify the amount data required? Here intension is to read the
data for certain number of frames without specifying the amount of data required.

Thanks and Regards,
Sivaram Prasad

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