The AVR chips (bizarrely) start an ADC conversion when entering Idle
mode. This behavior can cause the ADC to be busy when a sample is
required. Worse, if a series of events cause the cpu to enter and
leave Idle mode with a timing similar to the ADC checking rate then it
can cause the ADC to show as busy for extended periods. This could
cause high MCU load and possibly lead to a "Rescheduled timer in the
past" shutdown.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>