Ports and Interfaces > USB

USB isochronous high bandwidth transfers

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ulrich.sorensen:
Hi,

I need to know if my USB host stack is working right (I have not written it myself).

The case is that I am reading from a device (webcam) with an IN endpoint with max packet sixe of 3072 bytes. On my analyzer I can see SOFs generated every 125us but an IN transaction is not generated for every SOF (see attachment).

Isn't that an error? I would expect an IN transaction after every SOF.

Best Regards,
Ulrich

[attachment deleted by admin]

Jan Axelson:
What is the endpoint's bInterval?

Jan

ulrich.sorensen:
0x01 = 125 us

Best Regards,
Ulrich

Jan Axelson:
The endpoint has requested a transaction every microframe.

A transfer can consist of multiple transactions.

The host should be able to provide the requested bandwidth (IN packets) for the transactions within a transfer.

There might be delays between the end of one transfer and the host's scheduling the next one. I don't know if that's what you're seeing.

Jan

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