during startup. When a brand new chip leaves the factory, it is in a
special power-saving mode that disables most functions on the chip to
save battery power. The chip is stuck in this mode until the first write
to the time registers, which automatically clears the special power-saving
mode and starts the oscillator.
Also, the day-of-week register in this chip counts 1-7, not 0-6, so write
the values accordingly.
These changes are based on the patch submitted by Brian Scott, but I
elimated warnings since this condition is expected, and added some comments,
and so in general blame me for any mistakes.
PR: 223642
Don't enable the oscillator when it is found to be stopped at init time,
just let the first setting of valid time start it. But still report a dead
battery if it's stopped at init time.
Don't force the chip into 24hr mode, just cope with whatever mode it is
already in.
Schedule the clock_settime() callbacks to align the RTC clock to top of
second when setting it.
I2C real-time clock (RTC).
The DS3231 has an integrated temperature-compensated crystal oscillator
(TXCO) and crystal.
DS3231 has a temperature sensor, an independent 32kHz output (which can be
turned on and off by the driver) and another output that can be used as
interrupt for alarms or as a second square-wave output, which frequency and
operation mode can be set by driver sysctl(8) knobs.
Differential Revision: https://reviews.freebsd.org/D1016
Reviewed by: ian, rpaulo
Tested on: Raspberry pi model B
