The device defines the maxPacketSize of an endpoint. This is the
maxPacketSize of the endpoint descripter described in Chapter 9 of
the USB spec.
When you issue a ReadPipeAsync, your request size should be a
multiple of the maxPacketSize of the endpoint, or you have the
potential to loose data if the device returns a maxPacketSize packet.
In USB bus terms -- the host controller issues an IN request, the
device responds with at most maxPacketSized bytes. The host
controller acknowledges the receipt of the data, and puts in the
buffer you provided when calling ReadPipe. If the data received is
less that MaxPacketSize (called a short packet), then the ReadPipe
request is returned with what data did arrive. If the request is
for more bytes, then another IN request will be issued by the host
controller.
Good Luck,
David Ferguson
USB Software Team
Apple Computer
At 2:29 PM +0000 1/17/05, b johnny wrote:
thks david! that help alot! sorry for bothering you again but i
don;t realli understand the part where u said
- Assuming your device is actually running at hi-speed, and still
returning short 64 byte packets, you could re-code your driver to
expect this. Issue multiple reads of 512 bytes, but deal with the
fact that they only return 64 bytes. You will need to use asynch
requests so that you can have multiple transactions outstanding at
one time. You probably need as many requests as the number of 64
byte packets your device can supply in 1ms.
i looked up at the ReadPipeAsync function documentation but i dun
think this function allows me to declare the MaxDataPacket read.
Thus, which method am i supposed to use if i am to define the
maxPacket Size on a bulk pipe???
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