The value given for machine.freq(f) is extend to the range of 1_000_000 to
200_000_000. Frequencies below 48 MHz will be forced to an integer
fraction of 48 MHz. At frequencies below 8 MHz USB is switched off. The
power consumption e.g. of ADAFRUIT_ITSYBITSY_M4_EXPRESS drops to about
1.5 mA at 1 MHz.
Since the peripheral frequency is dropped as well, timing e.g. of PWM,
UART, I2C and SPI is affected and frequency/baud rate has to set again
after a frequency change below 48 MHz.
Two new compile flags are:
MICROPY_HW_DFLL_USB_SYNC: Effective only if DFLL48 does not run from the
crystal. It will synchronize the DFLL48M clock with the USB's SOF pulse.
If no USB is connected, it will fall back to open loop mode. The DFLL48M
clock is then pretty precise, but with a higher clock jitter at SAMD51
devices.
MICROPY_HW_MCU_OSC32KULP: Effective only if the devics uses a crystal as
clock source. Run the MCU clock from the ULP 32kHz oszillator instead of
the crystal. This flag was added to cater for a interference problem of
the crystal and Neopixel/Debug pins at Adafruit FEATHER Mx boards, which
causes the board to crash. Drawback: ticks_ms() and time.time() vs. than
ticks_us() and the peripherals like PWM run at not synchronous clocks.
