[libhid-discuss] problems reading data

[Charles Lepple]

On Jan 8, 2010, at 8:55 AM, Tim Nicholson wrote:

> The Microchip URL where the download can be found is http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2651&param=en534494 
> . (The framework comes in the form of a zip file which,  
> unfortunately, is pretty large even by modern standards).

Is this the .zip file with a .exe in it? Or the legacy  
"LPCUSBDK_Labs_V3.zip" file?

> I have not made any changes to the demo source code as yet, as I was  
> just trying to interface with the standard demo software as a first  
> step.

If it's files from the .exe, and the source code license isn't too  
restrictive, can you post just the key files somewhere? (I don't have  
any Windows machines running here.)

> My final application will not be much more complex   as all I need  
> to do is detect some key presses and read the ADC on the PIC. The  
> ability to read and write single 64-byte buffers is all I am looking  
> for.  I will have a look at libusb, to see if I can make any sense  
> of that.

By the way, when I mention "libusb", I am referring to the legacy  
version, libusb-0.1.12. There is a new version out (1.0), but your  
embedded Linux host might not have packages for it.

You might want to compare the libhid source code and API documentation  
to that of libusb:

libhid: hid_opening.c File Reference

http://libusb.sourceforge.net/doc/

In the case of finding and opening a USB device, libhid is not doing  
much besides wrapping the libusb calls.

> My biggest problem is I am very new to USB and I am finding all the  
> terminology difficult to get my head around. All of the Microchip  
> texts and examples are designed for Windows developers which is fine  
> except my final application will be running on embedded Linux and I  
> have not really had anything to do with Windows software development  
> since the mid 90's. I have tried looking at the USB specifications  
> but trying to tie it altogether is not proving very easy - I am  
> suffering from information overload!
>
> Any pointers would be much appreciated. I just need to understand  
> what functions I need to call to establish a connection and transfer  
> data to and fro.

That's the thing - the answer to that question depends heavily on the  
firmware on the device.

For HID-class devices, there are two main paths: control transfers,  
which allow you to pick which report you want (moot in your case,  
since you only want either the input report or the output report, and  
the direction of transfer is encoded into the request), and interrupt  
transfers, which simply get the next available input* report from the  
device, or send a an output report to the device.

Again, since your report descriptor only has one report in each  
direction, and no Report ID fields, this simplifies things somewhat.

In an ideal world, the firmware would treat the control transfers and  
input transfers similarly, reading and writing the same buffers. When  
I was working with the PIC16C765, I don't think that was the case.

[*] Directions are all relative to the host computer: PIC-to-PC is  
input, and PC-to-PIC is output.

Also, you might search here to see if there is already some example  
code that could talk to the default firmware:

http://mcuee.blogspot.com/search/label/USB
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