Hello folks,
After struggling a long time with USB Devices I am now into developing software for an embedded host(NXP LPC2387). I am basing the work on NXP's library nxpUSBlib and I am currently trying to implement isochronous transfers to transfer audio. I have faced a problem which I don't know how to categorize, so maybe this isn't the right place to ask about it, though I hope I will get some clue what to do.
The problem is that the transfers must happen every frame(1 ms) and I seem unable to make that happen. In a PC it works that way. The library seems to do what I think is the right thing to do when queuing an IN-ITD, i.e getting the current frame number + 1 so that the IN transfer should be sent the following frame. But that is not what is happening, instead there is a frame in between before the IN transfer takes place, i.e it looks like it would be scheduled for current frame number + 2, not + 1. So when the data arrives one ms to late I repeat the ITD queuing and so the process is repeated resulting in one data transfer every 2nd transfer instead of every.
I have tried to change the code so it uses current frame number instead of current frame number + 1, but then no transfers at all take place, which I think is OK since the HC then sees the transfer as outdated. Also using current frame number + 2 gives just another frame in delay, i.e the transfer happens on the 3rd future frame.
I have also tried to queue two ITDs at the same time, using current frame number + 1 and +2. Doing so causes the first transfer to occur like before, after the 2nd frame, while the 2nd transfer then happens as expected in the next frame.
