USB serial number size

[EThom.3]

As others before me, I wish to make my own serial number in a USB CDC device. This isn't the problem – I know how to do that.

However, I am puzzled about how the unique serial number is generated in the code auto-generated in STM32CubeIDE. Specifically the functions Get_SerialNum() and IntToUnicode() in usbd_desc.c.

/**
  * @brief  Create the serial number string descriptor
  * @PAram  None
  * @retval None
  */
static void Get_SerialNum(void)
{
  uint32_t deviceserial0;
  uint32_t deviceserial1;
  uint32_t deviceserial2;

  deviceserial0 = *(uint32_t *) DEVICE_ID1;
  deviceserial1 = *(uint32_t *) DEVICE_ID2;
  deviceserial2 = *(uint32_t *) DEVICE_ID3;

  deviceserial0 += deviceserial2;

  if (deviceserial0 != 0)
  {
    IntToUnicode(deviceserial0, &USBD_StringSerial[2], 8);
    IntToUnicode(deviceserial1, &USBD_StringSerial[18], 4);
  }
}

/**
  * @brief  Convert Hex 32Bits value into char
  * @PAram  value: value to convert
  * @PAram  pbuf: pointer to the buffer
  * @PAram  len: buffer length
  * @retval None
  */
static void IntToUnicode(uint32_t value, uint8_t * pbuf, uint8_t len)
{
  uint8_t idx = 0;

  for (idx = 0; idx < len; idx++)
  {
    if (((value >> 28)) < 0xA)
    {
      pbuf[2 * idx] = (value >> 28) + '0';
    }
    else
    {
      pbuf[2 * idx] = (value >> 28) + 'A' - 10;
    }

    value = value << 4;

    pbuf[2 * idx + 1] = 0;
  }
}

The three 32-bit values retrieved in lines 12 - 14 above, are together the Unique device ID of the microcontroller (UID[95:0]. For me, it would make sense to use all 96 bits or 24 hex digits to generate a unique ID. As the descriptor is supposed to be written the UTF-16, this would be a string of 48 bytes. I have read elsewhere, that the maximum number of bytes for any descriptor is 126 + 2 bytes. Yet, the constant USB_SIZ_STRING_SERIAL constant in usbd_desc.h limits the descriptor to 24 + 2 bytes, i.e. 12 UTF-16 characters.

To "fit" the 24 hex digits into half the size, the code "shrinks" the UID by adding UID[31:0] and UID[95:64] together, for the first 8 digits, and then uses half of UID[63:32] for the remaining 4.

Can anyone explain why? Am I missing something? Can the serial number really only be 12 hex digits in length?

[TDK]

It was an implementation decision to have serial numbers of that length, not a USB requirement/restriction. Serial numbers can be longer but become unwieldy at some point.

The odds of a serial number conflict with 12 hex digits are quite small (1:281 trillion).

[Pavel A.]

It can be longer than 12 characters. Maybe some old host systems have problems with longer strings.

[waclawek.jan]

> The odds of a serial number conflict with 12 hex digits are quite small (1:281 trillion).

That would hold if they would be random. They are not.

OTOH, the examples are just that, examples. Nobody really cares.

JW

[EThom.3]

Okay, thanks a lot gentlemen. I appreciate it.