wasp-bootloader: a robust SoftDevice bootloader for PineTime

[JF002]

Here is a quick update on what I've found these last 2 days. These are mostly random notes I've taken while discovering how it's working. I hope they will be useful to anyone wanting to work on OTA updates and DFU


My firmware (https://github.com/JF002/Pinetime) works out of the box with wasp-bootloader. This is because they are built around the same version of Noric's BLE SoftDevice (BLE stack).


@SteveA (on the chat) tried his own firmware (https://github.com/xriss/timecake, which does not use the SoftDevice) without success for now : the bootloader just resets every 1/2 second.


I've also tried to run @Koen's (on the chat too) firmware (https://github.com/bosmoment/PineTime-apps) with no much luck either. This firmware uses Nimble bluetooth stack. I did some modification to the code and the linker script but it would also reset every 500ms. A quick analysis on GDB showed me that it would hardfault in cortex_vector_base() which is pretty bad...


I don't know what causes this issue. It could be a conflict with the softdevice and Nimble, as they both try to drive the BLE radio.
Also, the softdevice is accompanied by an MBR : a small block of code the receives all the IRQ and forward them to the bootloader, softdevice and the application. And I really don't know what happens when the MBR expect the softdevice to be there, but it's not there because the application does not use it...


On the technical side, wasp-bootloader uses GPRERET (General Purpose Retention registers) and UICR (User Information Configuration Register) to store state and other info even when the MCU is reset. It also provides a factory reset option, which is a quite good idea!


Here is the general workflow of the wasp-bootloader:

• Init board (watchdog, buttons, leds, task scheduler, timer and display)
  
• Init bootloader (from the lib nrf_bootloader, provided by NRF52-SDK)
  
• Init SoftDevice
  
• Start DFU and wait for completion / error / timeout
  
• Disable Softdevice
  
• Teardown board peripheral
  
• If there is a valid image : Jump to the image code
  
• else reset and start again
  

Some links:

• Nordic MBR, Bootloader and Softdevice : https://infocenter.nordicsemi.com/topic/...oader.html
  
• Nordic DFU : https://infocenter.nordicsemi.com/topic/...dules.html
  
• Nordic DFU protocol : https://infocenter.nordicsemi.com/topic/...sport.html