The rtcfifo module uses RTC memory to store sensor samples across deep-sleeps,
making it possible to batch up samples for less frequent uploads. This
component uses 9 RTC memory slots for control, and a variable number of
slots for sample storage (see rtcfifo.prepare() on how to control the
size/location of the latter).
When used together with the rtctime module, it also exposes the convenience
function rtcfifo.dsleep_until_sample() which can be used to easily take
readings on a regular basis without having to manually take into account
time spent awake to get an accurate sleep time.
The format used for storing samples is quite dense, and allows for 16 bits
of data in a fixed point format (per sample).
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