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.
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