The NXP PCF8563 is a battery backed I2C clock at the fixed address 0x51,
common on RISC-V and ARM boards, and NuttX had no driver for it. The
nearest part in the tree, the PCF85263, has a different register map.
The driver follows the shape of the external I2C clocks already here,
ds3231 and pcf85263: board logic calls pcf8563_rtc_initialize() once the
bus exists, and the chip then answers up_rtc_getdatetime() and
up_rtc_settime() for the system clock.
Three behaviours are deliberate.
The top bit of the seconds register means the oscillator has stopped since
the time was last set, so every register after it holds whatever it stopped
on. A read in that state returns -ENODATA rather than the contents: a
caller told the time is unknown can act on that, one told a wrong time
cannot. A flat backup cell and a clock that has never been set both arrive
here.
The top bit of the month register marks a century rollover, but which value
means which century is a convention that parts disagree on, so no century
is read from it. The chip is treated as a clock for 2000 to 2099, the bit
is written back as it was read, and a date outside those years is refused
rather than stored as one that would read back different. This is what the
mainline Linux driver does.
The counters are stopped across a write so a carry cannot land between the
seconds and the minutes, and restarted even when the write failed, since
the alternative is leaving the clock stopped.
The alarm and countdown timer are not implemented; their registers are
defined.
Assisted-by: Claude:claude-opus-5
Signed-off-by: Justin Hammond <justin@dynam.ac>
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