OTA-DFU sometimes erases the application data unexpectedly

Question

Hi, 
 
 We are facing an issue where the application data stored using FDS is erased one in a while after OTA-DFU multiple times. 
 This issue does not occur all the time, but it appears as often as 5% of the times. 
 We are developing the customized application based on the ble_peripheral/ble_app_uart example with the following environment: 
 
 Target Chip: nRF52832 
 SDK: v16.0.0 
 Soft Device: s132 
 Compiler: IAR 
 
 For the DFU, I used secure_bootloader exmaple and combined these 2 application to crated 1 .zip file by nrfutil to upload via nRF Tools app (either iOS or Android). 
 I have made sure that the following parameters are matched: 
 
 // In nrf_dfu_types.h
DFU_APP_DATA_RESERVED CODE_PAGE_SIZE * 3

// In sdk_config.h
FDS_VIRTUAL_PAGES 3 
 This problem occurs regardless of the application size to be uploaded, i.e. it doesn't matter if it's single-bank or dual-bank update. 
 Is there any possible factor this issue to happen only sometimes? 
 If anyone can suggest the way to resolve or to reproduce the issue with 100% frequency, it would be so helpful! 
 
 Thank you.

tesc · Answer

Hi, 
 I had a more thorough look through the lines from the hex dump, and I see for instance that in one of the cases (pattern 1) there are two FDS data pages, but no FDS swap page. We have experienced some issues with FDS related to resets during garbage collection, although known issues should mostly have been fixed for SDK v16. In any case, I think this points in the direction of reset related issues. 
 Running garbage collection on startup (e.g. in initialization) is not a good idea, as resets often occur in close succession (e.g. battery powered with low power on battery leading to frequent brownouts, during automatic programming / testing situations, etc.) As garbage collection typically mean additional flash read/erase cycles, this contributes to flash wear. While garbage collection is designed not to break on reset, we did have some edge cases prior to SDK 16 where reset at a particular clock cycle during garbage collection would leave the device in a state similar to your "pattern 1" situation. 
 For "pattern 2", the first piece of data is {TAG_MAGIC, TAG_DATA}, no? Followed by the beginning of a record? 
 :10500000 DEC0ADDE FE011EF1 22220A00FFFFFFFF1F # @00075000 (16 bytes) 
 Regards, Terje

OTA-DFU sometimes erases the application data unexpectedly

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