USB CDC in HS with STM32F405, Issue in sending data to PC.

[taz1000]

I'm running USB CDC in HS with STM32F405.

Receiving data from PC is very good.

But, sending data to PC has problem sometimes.

To send data, I'm using CDC_Transmit_HS ( *Buf, Len )

There is no error in the function at all. But, data is not sent to PC.

To see if there is any error in CDC_Transmit_HS ( ), I checked

the hierarchical calls in CDC_Transmit_HS ().

But, even when the data is not sent, there is no error inside at all.

- Is there any known issue or possible problem that I had made ?

- Is there any relationship between USB CDC IN functions and OUT functions ?

- EX: when OUT is disabled (by not sending USBD_CDC_ReceivePacket), IN with CDC_Transmit_HS doesn't work.

* Windows10 home

* Driver for USB CDC (from device manager) : System32/Drivers/usbser.sys MS 10.0.18362.1 2006-06-21

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Added #1.

The problem occurs when MCU sends 512 x n bytes to the host.

After an internet searching for few hours, I found articles that zero-length-packet (ZLP) is required after sending the multiple of 512 in HS mode. (in FS, it would be 64Bytes)

So, I updated CDC_Transmit_HS caller function to send ZLP if ((Len%512)==0)

( When sending ZLP CDC_Transmit_HS, return value should be checked. USBD_BUSY can be returned.)

After updating, sending any bytes of data into PC works fine.

Is this an appropriate way to fix this ?

Thanks.

[thanks4opensource · Accepted solution]

Yes, USB requires a ZLP if the last data packet in a sequence is exactly the maximum packet size for the endpoint. The end of the sequence is indicated by a short (less than max) size packet and zero works correctly.

Congratulations on finding this out after only a few hours of internet searching. It took me a lot longer than that.

I don't know what library you're using and if CDC_Transmit_HS function is part of it or from your own code. (Or if you modified CDC_Transmit_HS itself vs your calls to it). I know the ST library for the STM32F10xxx series before CubeMX generated the ZLP internally/automatically, and I'd be surprised if current CubeMX doesn't also.