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SAFFIRe Bootloader

The SAFFIRe bootloader implements the security and functionality of the device-side part of SAFFIRe and is one of two components your team must implement (the other being the host tools in /host-tools/). The Bootloader runs on a Tiva C TM4C123GH6PM microcontroller which is emulated using qemu-system-arm.

The Bootloader is split into a few files that may be of interest to you, although you are free to change any and all files in this directory as long as functional and build requirements are met:

  • bootloader.c: Implements the main functionality of the SAFFIRe Bootloader. It contains main() and handles commands from the host tools to run different parts of the avionic update and boot system.
  • uart.{c,h}: Implements a UART interface to the host, reading and writing raw bytes.
  • flash.{c,h}: Implements a driver for programming the Flash memory.

We have also included the Tivaware driver library for working with the microcontroller peripherals. You can find Tivaware in lib/tivaware and will find the following files to be of interest:

  • startup_gcc.c: Implements the system startup code, including initializing the stack and jumping to the main. There is a good chance that you will not need to change startup_gcc.c, but some advanced designs may require it.
  • bootloader.ld: The linker script to set up memory regions. There is a good chance that you will not need to change bootloader.ld, but some advanced designs may require it.
  • makedefs: The common definitions included when compiling the Tivaware library and your SAFFIRe bootloader. If you want to specific optimizations and compiler options to both Tivaware and the bootloader, add/change them here. Otherwise, those options can be added to bootloader/Makefile.

On Adding Crypto

To aid with development, we have included Makefile rules and example code for using tiny-AES-c (see line 46 of the Makefile and lines 21 and 207 of bootloader.c). You are free to use the library for your crypto or simply use build process as a template for another crypto library of your choice.

If you choose to use a different crypto library, we recommend using the following steps to integrate it into your system. NOTE: All added libraries must compile from the all rule of bootloader/Makefile to follow the functional requirements.

  1. Find a crypto library suitable for your embedded system. Make sure it does not require any system calls or dynamic memory allocation (i.e. malloc), as the bootloader runs on bare metal without an operating system
  2. Add the library to the bootloader/ directory either as a submodule in git or as a copy of the library source code
  3. Run the included tests of the library to verify it works properly on your machine before you integrate it with your code
  4. Add the directory path to the list of include paths (IPATH+=/path/to/crypto) in bootloader/Makefile
  5. Add the directory path to the list of source files (VPATH+=/path/to/crypto) in bootloader/Makefile
  6. Add each object file you wish to link to the LDFLAGS list (LDFLAGS+=${COMPILER}/source_file_name.o) in bootloader/Makefile
  7. Add each object file you wish to link to the all rule (all: ${COMPILER}/source_file_name.o) in bootloader/Makefile