76cf98c35b
By using the phase jitter between the DFLL48M clock and the FDPLL96M clock. Even if both use the same reference source, they have a different jitter. SysTick is driven by FDPLL96M, the us counter by DFLL48M. As a random source, the us counter is read out on every SysTick and the value is used to accumulate a simple multiply, add and xor register. According to tests it creates about 30 bit random bit-flips per second. That mechanism will pass quite a few RNG tests, has a suitable frequency distribution and serves better than just the time after boot to seed the PRNG.
Port of MicroPython to Microchip SAMD MCUs
Supports SAMD21 and SAMD51. For each supported device there is a
subdirectory in the boards/ directory.
The entry point for the specific port documentation is at https://docs.micropython.org/en/latest/samd/quickref.html, which also shows the assignment of IO-Functions to pins. The generic MicroPython documentation applies for anything not specific for the SAM port.
Due to the different flash sizes of SAMD21 and SAMD51 devices, the coverage of MicroPython modules differ. Use help("modules") to tell, which MicroPython modules are provided.
Build instructions
Before building the firmware for a given board the MicroPython cross-compiler must be built; it will be used to pre-compile some of the built-in scripts to bytecode. The cross-compiler is built and run on the host machine, using:
$ make -C mpy-cross
This command should be executed from the root directory of this repository. All other commands below should be executed from the ports/stm32/ directory.
An ARM compiler is required for the build, along with the associated binary
utilities. The default compiler is arm-none-eabi-gcc, which is available for
Arch Linux via the package arm-none-eabi-gcc, for Ubuntu via instructions
here, or
see here for the main GCC ARM
Embedded page. The compiler can be changed using the CROSS_COMPILE variable
when invoking make.
Next, the board to build must be selected. There is no default board. Any
of the names of the subdirectories in the boards/ directory is a valid board.
The board name must be passed as the argument to BOARD= when invoking make.
All boards require certain submodules to be obtained before they can be built.
The correct set of submodules can be initialised using (with
ADAFRUIT_ITSYBITSY_M4_EXPRESS as an example of the selected board):
$ make BOARD=ADAFRUIT_ITSYBITSY_M4_EXPRESS submodules
Then to build the board's firmware run:
$ make BOARD=ADAFRUIT_ITSYBITSY_M4_EXPRESS clean
$ make BOARD=ADAFRUIT_ITSYBITSY_M4_EXPRESS
The above command produces binary images in the
build-ADAFRUIT_ITSYBITSY_M4_EXPRESS/ subdirectory (or the equivalent
directory for the board specified).
Flashing the Firmware
Most SAMD21 and SAMD51 boards have a built in firmware loader. To start it, push
the reset button of the boards twice. The speed varies a little bit. If the
firmware loader starts, a drive will appear in the file manager of your PC.
Copy the created firmware.uf2 file to that drive. If the upload is finished, the
drive will disappear and the board will reboot.