Posted on June 11, 2012 at 19:35> Both ends think that everything is fine but no data is transfered.
USB2.0 hub speeds up response of bulk transfers. Usually, it's fine. But it sometimes causes timing problem on firmware, and data drop may occur.
Here is an excerpt from ST VCP example,
\STM32F\STM32_USB-FS-Device_Lib_V3.3.0\Project\Virtual_COM_Port\src\usb_endp.c
void EP3_OUT_Callback(void)
{
uint16_t USB_Rx_Cnt;
/* Get the received data buffer and update the counter */
USB_Rx_Cnt = USB_SIL_Read(EP3_OUT, USB_Rx_Buffer);
/* USB data will be immediately processed, this allow next USB traffic being
NAKed till the end of the USART Xfer */
USB_To_USART_Send_Data(USB_Rx_Buffer, USB_Rx_Cnt);
#ifndef STM32F10X_CL
/* Enable the receive of data on EP3 */
SetEPRxValid(ENDP3);
#endif /* STM32F10X_CL */
}
EP3_OUT_Callback() is called by the stack, when PC writes to the device over the VCP.
1) This routine reads out data from the endpoint (USB_SIL_Read)
2) It processes the data (USB_To_USART_Send_Data)
3) And then, the routine arms the endpoint again (SetEPRxValid)
If your firmware swaps the order of 2) and 3), data drop may occur. While your firmware processes the last data, the endpoint accepts next data from host. Your firmware isn't aware of new data.
Tsuneo
