Tricky USB device.

[Bernd Walter]

On Fri, Apr 08, 2005 at 04:47:43PM -0700, Maksim Yevmenkin wrote:
> Bernd Walter wrote:
> >On Fri, Apr 08, 2005 at 06:12:33PM -0400, David Gilbert wrote:
> >
> >>Bernd> Has this device multiple interfaces?  e.g. one HID and
> >>another Bernd> as described.  I often thought about getting ugen
> >>working at Bernd> interface level too.
> >>
> >>Here's the output of udesc_dump on it.  Right now, using the
> >>current version of libusb (not the version from ports), I can use 
> >>usb_interrupt_write(dev, 1, "MK255", 5, 0) to send data to it ---
> >>and the data is sent --- at least lights on the USB hub flash.  If
> >>I replace '1' with anything else, it doesn't accept it.  However,
> >>it doesn't seem to have opened the relays.
> >
> >Yes - you must use 1 - there is only one out-endpoint. 0x81 is for
> >receiving data and endpoint 0 is the mandandory control endpoint. 
> >Interrupt Endpoints are not variable in size. Both interrupt
> >endpoints are 8 Bytes, so you must read and write exact 8 Bytes per
> >transfer - 5 shouldn't work for USB compliant devices.
> 
> hmmm... i was always confused about bMaxPacketSize. i was thinking that 
> it limits the size of one usb transaction, and it could take several usb 
> transactions to transfer one data packet.

It is a bit more complicated.
For control endpoints packets transfers that are bigger than one packet
can be transfered using multiple packets using a shortened last packet,
which can be even of 0 length if the transfer exactly fits in packets.
For bulk endpoints things can be handled specific to the protocol
requirements - e.g. most serials don't track transfer borders.
We have interrupt endpoints - you are right smaller than max packets
are allowed - just checked the specs.
The remaining is the same as for bulk endpoints, but interrupt endpoint
are different in bus time calculations.


> for example i have a bluetooth usb dongle that has
> 
>         Standard Endpoint Descriptor:
>           bLength          7
>           bDescriptorType  05
>           bEndpointAddress 81 (in)
>           bmAttributes     03 (Interruput)
>           wMaxPacketSize   16
>           bInterval        1
> 
> and i certanly can receive data packets from this endpoint that are more 
> (and less) then 16 bytes in size. so, i would guess (and i might be 
> wrong) that it is ok to send/receive data packets that are not equal to 
> bMaxPacketSize in size.

As corrected above - you are really allowed to have smaller packets.
But you can't have larger ones - however you can transfer multiple
packets in one transaction, but this is not optimal speedwise as
interrupt endpoints are laid out in a specific timeline.
bInterval=1 means one packet per 1ms will be transfered and not more.
Doing a transfer with e.g. 2 packets will take 1ms longer - even
if the bus is idle in the meantime.
This is because interrupt endpoints get garantied bus time.

-- 
B.Walter                   BWCT                http://www.bwct.de
bernd at bwct.de                                  info at bwct.de