Below is what I've done to measure the interval between USB_HostCdcDataSend and its callback on TWR-65 USB EHCI Host.
1. Connect the K65 USB EHCI Host with FTDI device.
2. Send data packet to device via USB_HostCdcDataSend and set the time as T1.
3. Once the callback comes, set the time as T2.
4. T2 - T1 is nearly 2ms. Moreover, if there happens to be an IN transfer between the USB_HostCdcDataSend and its callback, then the value of (T2 - T1) would be nearly 5ms.
That's totally too long a cost for real time systems. However, the FTDI device can response to the PC host within 1ms. I suspect it is an issue in USB low level driver.
BTW, I didn't see any NAK after calling USB_HostCdcDataSend.
Can anyone help to verify this issue, or share some performance benchmark in this case?