Re: [libusb] Streaming Data to a Microcontroller Stutters (LibUSBK Bulk Mode)

[Tim R. <ti...@pr...>]

On 7/10/21 7:24 AM, Matt wrote:

>
> Thanks for the suggestion. Do you know of any LibUSB asynchronous 
> examples for bulk mode that I can use?

There are many.  Here's one. 
https://falsinsoft.blogspot.com/2015/02/asynchronous-bulk-transfer-using-libusb.html


> I just thought 512 was the maximum supported for bulk mode.

The maximum packet size for a bulk endpoint on a high-speed device must 
be 512 bytes, but that doesn't need to concern you.  A single TRANSFER 
can be much longer than that.  The host controller driver will chop your 
transfer up into packets for transmission across the USB wire. On most 
operating systems, a transfer can be many megabytes.

This is an important point, because you NEED to use this in order to 
maintain highest performance..  Remember that USB traffic is all 
scheduled in advance.  The host controller driver lays out the schedule 
for the next frame, then sends it to the hardware where it gets executed 
autonomously.  Once the schedule is committed, it can't be changed.  A 
high-speed frame (125 microseconds) has room for about 7kB.  If you send 
512 bytes at a time, you will only get one slot in that microframe.  But 
if you send, say, a 64kB buffer, then it will fill each microframe with 
your packets until the transfer is finished.  For a 64kB buffer, that's 
1.2 microseconds vs 16 milliseconds.

-- 
Tim Roberts, ti...@pr...
Providenza & Boekelheide, Inc.