[Little Brosers] DFU, Flash, and Heroku

Hello everyone. I’ve been working on various things for our project, here is a summary.


After struggling a lot, I succeeded performing a DFU in bootloader mode (in this mode, there is no app in the flash). Now I work on making a DFU in buttonless mode, where the DFU is performed with a GATT service with the application running.

We need to flash the bootloader and the softdevice to make it work. When there is a reset, the bootloader checks the apps present in the internal flash, it computes their crc32 and compare it to the checksum stored in a special internal flash page named bootloader_settings_page. So now when we flash our app, we also need to write the good checksum in this place. The program nrfutil can generate a hex file (a text file, basically with a list of address and their data) with the content of the bootloader settings page, and we can juste merge it with the application with the nordic mergehex program to make it work. I still need to write a script for the merge, and also handle the keys in the CI (with a protected secret key).


I made a basic code that check if the SPI external flash (AT45DB321E) is present and stop the execution otherwise. For that I used the “Manufacturer and Device ID Read”, I just send an opcode, and the flash has to answer correctly 5 bytes. 

After having made a board configuration header, and after having received the drop boards, I tested it, but it was not working. The logic analyser showed that the CS pin was not working and was kept low. I tested it on a second Drop, and surprisingly, it started working for some times, and then failed. At this time I suspected a hardware problem, for example the flash receives too much voltage and is destroyed. Hopefully with the help of Antony, we realised that the CS pin was by default used for NFC, and unlike other GPIO pins, needed to be explicitly configured as GPIO. I think it works one time because there is an undefined behaviour if the CS wire is always kept low.


We migrate the certification authority to Heroku because my 15€ router didn’t work correctly (after ~20 hours of up time, it wasn’t connected to the internet anymore, and I had to plug out and plug in the RJ45 wire, and it would work again).

The heroku migration was not so easy, the reason was that heroku has ephemeral filesystems, it was generating the keys at each app startup (often in the free version of Heroku). So I put all the keys in environnement variables. Heroku also doesn’t provide database, I need to find a free MongoDB Heroku plugin to make it work, and Heroku ask for your CB even if it’s free. I updated the continuous deployment, it is now easier (I was doing a git filter-branch to keep only the ca folder history before) and use a ruby util named dpl (

I also put the Heroku API_KEY  as a protected secret variable in GitLab (Accessible only in the master branch, otherwise my coworkers could just write a CI script “echo $API_KEY” in one of their branch and would be able to steal the API_KEY).

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