nRF5340 - runtime change of iSerialNumber

[Gabriele]

Hi,
I wanted to change at runtime the iSerialNumber exposed by USB descriptor.
As Kenneth suggested (link), the solution has been to override the weak function in usb_descriptor.c

weak uint8_t *usb_update_sn_string_descriptor(void)
{
  snprintk(usb_serial_str, sizeof(usb_serial_str), USB_SERIALNUMBER_TEMPLATE,
    (uint32_t)(NRF_FICR->DEVICEADDR[1] & 0x0000FFFF)|0x0000C000,
    (uint32_t)NRF_FICR->DEVICEADDR[0]);

  return usb_serial_str;
}

with this one

uint8_t *usb_update_sn_string_descriptor(void)
{
  snprintk(usb_serial_str, sizeof(usb_serial_str), USB_SERIALNUMBER_TEMPLATE, sn_recovered_from_littlefs);

  return usb_serial_str;
}

The problem is that the override function is executed when the littlefs is still unmounted, thus sn_recovered_from_littlefs=0.
What's the storage location of the iSerialNumber "D0766E074450A9F0" the nRF5340 uses if untouched ?

[Susheel Nuguru]

It seems to be kept in global memory space which means it is different for different application. You can probably attempt to write a linker file to create a memory section and keep this structure defined in zephyr\subsys\usb\device\usb_descriptor.c

[Gabriele]

Hi Susheel

Could you please expand a bit more your idea ?

Consider that each board has a serial number, which is uniquely assigned at some stage of manufacturing. This serial number shall be persistently stored in flash, and assigned to iSerialNumber.  

[Susheel Nuguru]

If you want to persistently store the serial number in flash, then store it at know address (or key/value) location.

inside usb_update_sn_string_descriptor, you can load the value in that address location to see if there is any data saved. If there is none, save the serial number and use that instead of sn_recovered_from_littlefs. 

What's the storage location of the iSerialNumber "D0766E074450A9F0" the nRF5340 uses if untouched ?

Right now, it seems it lives in the .bss section allocated by the compiler. Unless you created a linker script to save string_descr (in zephyr\subsys\usb\device\usb_descriptor.c)  at a specified location, you do not have control on where that is saved.