LibsUsbK buffers not being filled when using function UsbK_IsoReadPipe

Question

I'm trying to write some code to read from an Isochronous pipe using LibUsbK in Win32. I have successfully initialised the device into the correct state to send and receive Isochronous data and I can see data being sent over the USB in my hardware USB analyser, but the buffers I am receiving are always unfilled even though the analyser shows that there was data in the packets sent to the PC.

I'm new to LibUsbK and using Isochronous transfers though I'm not new to USB in general but I've been really struggling with this.

The code I'm using to read from the device is something like this...

UsbK_SelectInterface(usbHandle,1,0);
UsbK_SetAltInterface(usbHandle,1,0,1);

IsoK_Init(&isoCtx, ISO_PACKETS_PER_XFER, 0);
IsoK_SetPackets(isoCtx, ISO_PACKET_SIZE); // Size of each individual packet
OvlK_Init(&ovlPool, usbHandle, 4, 0);
OvlK_ResetPipe(usbHandle, 0x83);
OclK_Acquire(&ovlkHandle, ovlPool);
UsbK_IsoReadPipe(usbHandle, 0x83, inBuffer, sizeof(inBuffer), ovlkHandle, isoCtx);

while(!finished)
{
    if(OvlK_IsComplete(ovlkHandle)
    {
        fwrite(inBuffer, sizeof(inBuffer), 1, outFile);
        memset(inBuffer,0xcc,sizeof(inBuffer));
        OvlK_ReUse(ovlkHandle);
        UsbK_IsoReadPipe(usbHandle, 0x83, inBuffer, sizeof(inBuffer), ovlkHandle, isoCtx);
    {
}

If I put a breakpoint at the fwrite line then the inBuffer is always full of 0xCC - ie, not having been filled by the iso read.

I've checked all the error return values from the UsbK/OvlK function calls and they are all as they should be. I've checked my buffers are sufficiently big to receive the data.

I use very similar code to write to the ISO out pipe on endpoint 0x02 and that works perfectly, the only difference really between the code above and my write code is that the fwrite/memset commands are replaced with a call to a "fillbuffer" function that populates my outBuffer before calling UsbK_IsoWritePipe function.

I tried looking through any examples I could find in the samples and also online but struggled to understand/get them to work with my particular device.

Any suggestions or help greatly appreciated.

Answer 1

So it appears that the above code did work and I was being mislead by the fact that the debugger was interrupting the flow of things - I keep forgetting that trying to debug real time stuff can introduce it's own issues.

The first issue was that stepping through the code in the debugger was causing issues with the low level libusbk code capturing the usb packets and filling my buffers correctly - once I let it run full speed and found other ways to test the buffers I did actually find there was some data in there.

The second problem I had was that quite often the buffer was starting to be filled part way through only (and not always right from the start) so when I examined the data I was only printing the first part of the buffer to the console and so all I saw was 0xCC and I was therefore assuming it hadn't worked.

Once I realised that there was actually some data later in the buffer I just started looking through the buffer in packet sized chuncks, if the packet was completely contained of 0xCC I would skip it and move on, but if any of it was not 0xCC then I would treat it as a valid packet - this worked perfectly and I was successfully receiving all the data. I'm sure there's a more "proper" way of doing this, but it works for me now.