Posted on December 14, 2016 at 20:49First let me say that I am not a USB expert and have not used HAL or STM32CubeMX to generate any code so I may not be much use for answering questions. That said, I have modified an older ST example CDC application for flow control. Here is what I did:
In the file usbd_cdc_core.c there is a function usbd_cdc_DataOut()
This function contains a call:
APP_FOPS.pIf_DataRx(USB_Rx_Buffer, USB_Rx_Cnt);
This calls a user function that handles the received data. In that function I took the data and sent it to a comport routine that puts it into a circular buffer for output via an interrupt-driven uart (large enough to receive the entire packet). My application basically sent data received by the USB and sent it out a comport like a USB to serial adapter.
The next line down in usbd_cdc_DataOut() was:
DCD_EP_PrepareRx(pdev,CDC_OUT_EP,(uint8_t*)(USB_Rx_Buffer),CDC_DATA_OUT_PACKET_SIZE);
It is my understanding that this line causes the USB to quit NAKing incoming packets and allows it to receive more data. I commented this line out and replaced it with setting a flag that I created:
hold_usb_datarx = 1;
Also in usbd_cdc_core.c I added a function:
extern USB_OTG_CORE_HANDLE USB_OTG_dev;
void Enable_USB_data_rx(void)
{
if(hold_usb_datarx)
{
hold_usb_datarx = 0;
NVIC_DisableIRQ(OTG_FS_IRQn);
// Prepare Out endpoint to receive next packet
DCD_EP_PrepareRx(&USB_OTG_dev, //pdev,CDC_OUT_EP,(uint8_t*)(USB_Rx_Buffer),CDC_DATA_OUT_PACKET_SIZE);
NVIC_EnableIRQ(OTG_FS_IRQn);
}
}
Now in my main() loop I added:
if(!COM_data_to_xmit()) Enable_USB_data_rx();
COM_data_to_xmit() is simply a function that returns TRUE if the interrupt-driven uart has not yet transmitted all the data from the circular buffer. This call to Enable_USB_data_rx() probably could have been put in my uart transmit interrupt ISR, but I preferred not to modify that module so I put it in the main() loop. It could probably be optimized to re-enable the USB reception after the data in the circular buffer has dropped below a threshold which left enough room to handle another packet.
Good luck!
