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Index > OS Construction > USB Get Descriptor

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BAiC



Joined: 22 Mar 2011
Posts: 272
Location: California
BAiC
does anyone here have experience with (real) devices that only report a partial Device Descriptor? I'm working on USB drivers (both UHCI and EHCI) which work for the Full Speed and High Speed devices that I have. that said; one of the devices (on the system board I presume given there are no other devices plugged in) simply refuses to give up more than 10 bytes. the device reports itself as a Full Speed yet the Max Packet field in the partial descriptor is only 8.

- Stefan

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Post 10 Sep 2015, 08:26
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BAiC



Joined: 22 Mar 2011
Posts: 272
Location: California
BAiC
just to be clear on a few things:

I have tried breaking up the IN portion of the request into multiple TDs (one TD for every 8 bytes). when it hits the second it either fails outright (Actual Length: 0x7FF [in other words: zero]) or it returns 2 more bytes before stalling (this was the 10byte reference).

I tried sending in a request for a single IN transaction at 64bytes just to see what would happen and I ended up with a successful short packet (no error). the result was mostly expected (I tested after seeing the MaxPacketSize field).

- Stefan

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Post 10 Sep 2015, 09:12
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BAiC



Joined: 22 Mar 2011
Posts: 272
Location: California
BAiC
perhaps a better name for bMaxPacketSize0 is bOnlyPacketSize0. as a workaround I requested only the first 8bytes then used bOnlyPacketSize0 to retrieve the rest. turns out the device that wasn't responding fully is the card reader.

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Post 10 Sep 2015, 11:45
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