Best approach to save data to flash while using Softdevice

[AqibK]

Hello,

I am using  nrf52840, SDK 17.0.2, softdevice s140 and also implemented OTA DFU in my application.

I want to save some data lets say admin code in flash. Once I burn the firmware to device it should be like below

 uint8_t ADMIN_CODE[6]  = {0xFF,0xFF,0xFF,0xFF,0xFF,0xFF};

During my application is running I want to change it to lets say ADMIN_CODE[6]  = {0x12, 0x56,0xFF,0xAA,0xFA,0xCD}. Then this becomes my admin code and when I power off and back on my ADMIN_CODE[6] should not go back to {0xFF,0xFF,0xFF,0xFF,0xFF,0xFF} but instead should remain same ie: {0x12, 0x56,0xFF,0xAA,0xFA,0xCD} And when I do reset then it should go back to  {0xFF,0xFF,0xFF,0xFF,0xFF,0xFF}.

I saw some questions like this already on forum, some suggested to use flash write using NVMC or UICR and some suggests to use pstorage or FDS. I am confused what is the best approach in my scenario. Also if you can guide to some example code of right approach, that would be great!

[Hung Bui]

Hi AqibK, 

Could you let me know if you used any flash library ? or you access flash directly ? 
Please note that you would need to erase flash before you can write new value to it (flash is only allowed to write once after erase). Or you can do what Jimmy suggested, to use fds library.