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Question regarding virtual COM port communication using CDC

pass3master
Associate III

‎2024-03-17 05:13 PM

This is a question regarding virtual COM port communication using CDC.
A STM32F412 Discovery with CDC installed is connected to a Windows PC, and 70 bytes of data is sent from the PC via serial communication.
However, when receiving with CDC_Receive_FS, it is divided into 64 bytes of data and 6 bytes of data.
If I change the Define definition, can I receive 70 bytes of data without being split?

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3 REPLIES 3
TDK
Guru

‎2024-03-17 07:51 PM

No, 64 bytes is the limit on the packet size in the USB FS protocol. It cannot be changed. 

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pass3master
Associate III
In response to TDK

‎2024-03-17 08:44 PM

Thank you for your response.
I see, I understand. Does this constraint naturally affect CDC_Transmit_FS as well?

If I do CDC_Transmit_FS(&sendData[0], 70); and then run serialport1.Read(receiveData,0,70) on the PC side (C#), it seems to be received in one time...

Does this mean that it just looks that way?

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TDK
Guru
In response to pass3master

‎2024-03-18 05:33 AM

Yes, transmitted data is also subject to the same limitation. Packets are created in 64-byte chunks after the call to CDC_Transmit_FS.

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