After firmware startup it's always in a valid range, even in the
unlikely case analog_init() is called twice.
This saves 4 bytes binary size without drawback.
On AVRs analog conversions were running in essentially
free-running mode, i.e. next conversion was being started
immediately after the previous one ended. This caused many
unnecessary interrupts.
Now, conversions are initiated manually by start_adc(). This
causes a single read on each of the registered channels. Once
all channels are read, conversions stop until the next call of
start_adc().
Markus Hitter
Robert Konklewski