The S130 softdevice is now mandatory, independent of if BLE is activated or not, the flash is filled as follow:

+--------------+ 256k
|     MBS      |    Write protected
+--------------+ 252k
|  Bootloader  |
+--------------+ 232k
|              |
|              |
|              |
|              |
|              |
|  Firmware    |
+--------------+ 108K
|              |
|              |
|              |
|              |
|              |
|              |
|  Softdevice  |
+--------------+ 4K
|     MBR      |    Write protected
+--------------+ 0
  • MBR Softdevice Master Boot Record.
  • Softdevice S130 Bluetooth stack
  • Firmware This firmware
  • Bootloader Bluetooth/Shockburst bootloader
  • MBS Master Boot Switch

Boot sequence:

 MBR ----> MBS ----> Bootloader ----> Firmware

The MBR is part of the Softdevice. It boots the CPU and jumps to MBS. The MBR contains methods to start the Softdevice and can flash Softdevice and bootloader.

The MBS handles the ON/OFF button and communicates the duration of the press to the bootloader so that the bootloader knows what to boot. The reason for the MBS is to allow updating the bootloader over the air while still having a write-protected piece of software that can start the STM32 in USB DFU mode for recovery (the STM32 has access to the NRF51 SWD programming port). The boot switch is as follow:

Press time Blue LED state Program booted
Short Still Firmware
Long (>3s) Slow blink Bootloader
Very long (>5s) Fast blink Stays in MBS and power STM32 in USB DFU mode

The bootloader, if selected, starts the STM32 in bootloader mode and initializes both BLE and Shockburst (ESB) radio. It can flash everything but MBR and MBS. It also acts as a bridge to the STM32 bootloader.

If not selected, the bootloader jumps to the firmware.