Even if boards do not have a clock crystal. In that case, the clock
quality will be very poor.
Always having machine.RTC means that the date/time can be set in a way that
is consistent with other ports.
This commit also removes the special code in modutime.c for devices without
the RTC class.
Signed-off-by: Damien George <damien@micropython.org>
This RNG passes many of the Diehard tests and also the AIS31 test suite.
The RNG is quite slow, delivering 200bytes/s.
Tested on boards with and without a crystal.
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.
Allowing to increase the clock a little bit to 54Mhz. Not much of a gain,
but useful for generating a RNG entropy source from the jitter between
DFLL48M and FDPLL96M.
For compatibility with other ports. Code increase up to ~1250 bytes for
SAMD21. The feature is configurable via MICROPY_PY_MACHINE_PIN_BOARD_CPU
in case flash memory is tight.
Methods implemented are:
- rtc.init(date)
- rtc.datetime([new_date])
- rtc.calibration(value)
The presence of this class can be controlled by MICROPY_PY_MACHINE_RTC. If
the RTC module is used, the time module uses the RTC as well.
For boards without a 32kHz crystal, using RTC makes no sense, since it will
then use the ULP32K oscillator, which is not precise at all. Therefore, it
will by default only be enabled for boards using a crystal, but can be
enabled in the respective mpconfigboard.h.
The range is 1MHz - 48 MHz. Note that below 8 MHz there is no USB support.
The frequency will be set to an integer fraction of 48 MHz. And after
changing the frequency, the peripherals like PWM, UART, I2C, SPI have to be
reconfigured.
Current consumption e.g. of the Seeed Xiao board at 1 MHz is about 1.5 mA,
mostly caused by the on-board LED (green LED with 1k resistor at 3.3V).
Changes are:
- Set the feature level for each MCU: CORE features for SAMD21, and EXTRA
features for SAMD51.
- Remove all definitions that are included in the core feature level.
- Keep the default settings for feature level and float, to make the choice
obvious.
It can be enabled/disabled by a configuration switch. The code size
increase is 308 bytes, but it requires RAM space for buffers, the larger
UART object and root pointers.
