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Keeping the base address of this structure in a register allows us to use offsets to load individual fields from it, without needing their individual addresses. However, the ldr instruction can only use immediate offsets relative to the low registers (r0-r7), or the stack pointer (r13). Low registers are in short supply and are needed for other instructions which can only use r0-r7, so we use the stack pointer here. It's safe to do this because we do not use the stack. There are no function calls, interrupt handlers or push/pop instructions in the M0 code. This change saves four cycles by eliminating loads of the addresses for the offset & tx registers, plus a further two by eliminating the need to stash one of these addresses in r8.
The primary firmware source code for USB HackRF devices is hackrf_usb. Most of the other directories contain firmware source code for test and development. The common directory contains source code shared by multiple HackRF firmware projects. The cpld directory contains HDL source for the CPLD. The firmware is set up for compilation with the GCC toolchain available here: https://developer.arm.com/open-source/gnu-toolchain/gnu-rm/downloads Required dependency: https://github.com/mossmann/libopencm3 If you are using git, the preferred way to install libopencm3 is to use the submodule: $ cd .. $ git submodule init $ git submodule update To build and install a standard firmware image for HackRF One: $ cd hackrf_usb $ mkdir build $ cd build $ cmake .. $ make $ hackrf_spiflash -w hackrf_usb.bin If you have a Jawbreaker, add -DBOARD=JAWBREAKER to the cmake command. If you have a rad1o, use -DBOARD=RAD1O instead. It is possible to use a USB Device Firmware Upgrade (DFU) method to load firmware into RAM. This is normally only required to recover a device that has had faulty firmware loaded, but it can also be useful for firmware developers. For loading firmware into RAM with DFU you will need: http://dfu-util.sourceforge.net/ To start up HackRF One in DFU mode, hold down the DFU button while powering it on or while pressing and releasing the RESET button. Release the DFU button after the 3V3 LED illuminates. A .dfu file is built by default when building firmware. Alternatively you can use a known good .dfu file from a release package. Load the firmware into RAM with: $ dfu-util --device 1fc9:000c --alt 0 --download hackrf_usb.dfu