• 20 characters Serial Number
• Unique ID for your device
• Enabled/Disabled by software
Adding to the VID (Vendor ID) and the PID (Product ID), the USB device may need a unique serial number. The AVR ® USB products offer you this feature and you do not need any external data to build this serial number. All you need is to read the unique ID provided in the AVR embedded flash.
A familiarity with USB Software Library for AT90USBxxx Microcontrollers (Doc
7675, Included in the CD-ROM & Atmel ® website) and the USB specification
(http://www.usb.org) is assumed.
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In the USB firmware package provided by Atmel, you have a flash driver library available in the flash_drv.h file. The macro Flash_read_sn(pos) allows you to read the byte from the position “pos”. Calling this macro 10 times using the position 0 to 9 will provide you a unique ID with 10 bytes (The position order does not matter) . These
10 bytes will be used to generate the serial number.
Once you have collected the 10 bytes, the serial number can be built for your USB device. This serial number will contain 20 characters. Each byte will be transformed on two characters. For example the byte 0x1A will generate two characters “1” and
“A” (see the figure below).
During the enumeration process, the host asks for the USB descriptors information which includes the string descriptors. One of these string descriptors contains the
Serial number if available (It is not mandatory to provide a serial number to the host).
This request is managed by the usb_standard_request.c
file. Please note that the serial number generation can be enabled or disabled in your firmware.
To enable or disable the serial number generation, you have to set the correct configuration in the conf_usb.h
header file, see below:
#define USB_DEVICE_SN_USE ENABLE
#define USE_DEVICE_SN_UNIQUE ENABLE
If USB_DEVICE_SN_USE is disabled, no serial number will be sent to the host. If this parameter is enabled, two cases are possible:
¾ USB_DEVICE_SN_UNIQUE is disabled:
In this case, the default serial number (will be the same for all devices) will be set.
This default serial number is defined in the usb_descriptors.h header file:
2
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#define USB_SN_LENGTH 0x05
#define USB_SERIAL_NUMBER \
{ Usb_unicode('0') \
,Usb_unicode('0') \
,Usb_unicode('0') \
,Usb_unicode('0') \
,Usb_unicode('0') \
}
¾ USB_DEVICE_SN_UNIQUE is enabled:
In this case, the unique serial number will be set and sent to the host as described above.
The figures below show you the device proprieties window provided by the device manager (open your device manager and double click on the HID device to get this window, you may have many HID devices connected to your PC, please select the correct one, you can check with the demo name “AVR USB MOUSE DEMO”) for each case:
9 Unique serial number assigned:
3

9 Default number:
9 No serial number:
4
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Please note that the serial number showed in the below figure is set by Windows
® and nothing has been sent by the device. This serial number is not stored in your
USB device.
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5

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