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vladsol2009
vladsol2009
Associate II
August 7, 2017
Question

HAL CDC Extra byte in CDC_Receive_FS buffer

  • August 7, 2017
  • 1 reply
  • 952 views
Posted on August 07, 2017 at 20:40

I have created a simple project in cubemx, with usb fs device (CDC).

But i see an strange behavior: The buffer of CDC_Receive_FS contains (bytes sent from PC + 1 extra byte) 

On example, when sending 0x32 0x33 in buffer i got 0x32 0x33 0xee

0x32 0x34 ->  0x32 0x34 0xaf

0x32 0x35 -> 0x32 0x35 0x6e

etc..

Why can this happen?

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    1 reply

    Vangelis Fortounas
    Vangelis Fortounas
    Associate II
    August 7, 2017
    Posted on August 08, 2017 at 01:03

    Hello !!

    The CDC_Receive_FS function you mention, is called from lower layers and contains as parameter a pointer to received length.

    Is this received *length equal to 3? or as you wrote before , just contained in buffer?

    Your hardware is unknown to us, provide some information about it

    vladsol2009
    vladsol2009Author
    Associate II
    August 8, 2017
    Posted on August 08, 2017 at 04:48

    MCU is STM32F103R8T6

    UserRxBufferFS size is 64

    USB_OTG_MAX_EP0_SIZE is 64

    USB_OTG_FS_MAX_PACKET_SIZE is 64

    Received *Len contains ''5'', but *Buf contains 6 bytes.

    0690X00000603zpQAA.jpg

    With even number of bytes there is no problems:

    1 byte sent, 2 bytes in buffer, Len is 1

    2 bytes sent, 2 bytes in buffer, Len is 2 == OK

    3 bytes sent, 4 bytes in buffer, Len is 3

    4 bytes sent, 4 bytes in buffer, Len is 4

     == OK

    5 bytes sent, 6 bytes in buffer, Len is 5

    6 bytes sent, 6 bytes in buffer, Len is 6

     == OK

    7 bytes sent, 8 bytes in buffer, Len is 7

    8 bytes sent, 8 bytes in buffer, Len is 8

     == OK

    9 bytes sent, 10 bytes in buffer, Len is 9

    10 bytes sent, 10 bytes in buffer, len is 10

     == OK

    Vangelis Fortounas
    Vangelis Fortounas
    Associate II
    August 8, 2017
    Posted on August 08, 2017 at 13:35

    Hello again!!

    You wrote... '1 byte sent, 2 bytes in buffer, Len is 1

    2 bytes sent, 2 bytes in buffer, Len is 2 == OK

    3 bytes sent, 4 bytes in buffer, Len is 3'

    Two bytes in a 64 byte size buffer means nothing. the (buffer allways contains 64 bytes)

    Usually the buffers has as initial values per byte ,0x00

    With a 'normal' debuger is posible to see oly the bytes>0x20 . But this does not mean that the rest bytes don't exist.

    The internal structure of USB CDC modules is 'unknown' to the highest layer user.

    The knowledge of the user  must be , only , the number of bytes received and the buffer pointer to take theese determined bytes. The remaining of the buffer is not guaranteed to conforms with something determined or not.

    So, this the normal behaviour of this callback function.

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