Posted on May 17, 2011 at 14:25I'm not a USB expert, but from what I understood this is happening here:
USB Bulk Transport (which the CDC example uses) uses the following mechanism to determine when a ''bulk'' of data is complete:
When a ''short packet'', i.e. a packet containing less bytes than the max. endpoint size, is received, the Windows CDC driver (usbser.sys) considers the transfer complete and sends the data to the application (Hyperterminal or others). Whenever a completely full packet is received (64 Bytes) it expects more data to come and polls the device for more data as fast as it can.
To get the Windows driver to consider the transfer as ''finished'', you need to send a zero-length packet after sending a 64-Byte packet. USB_Sil_Write('''',0);
That is actually specified in the USB specification, chapter 5.8.3:
When a bulk IRP involves more data than can fit in one maximum-sized
data payload, all data payloads are required to be maximum size except for the last data payload, which will
contain the remaining data. A bulk transfer is complete when the endpoint does one of the following:
• Has transferred exactly the amount of data expected
• Transfers a packet with a payload size less than wMaxPacketSize or transfers a zero-length packet
Hope this helps.
Jens
