Duilio Foschi wrote:

> Are there uPs with clock/calendar as an internal feature ?
> Is it advisable to use an external clock/calendar chip anyway ?

  The chief advantage is that the purpose-built clock chips have
extremely low current consumption.  We are talking applications where
the device is battery powered and the main CPU is shut down much of the
time, aren't we?

  The larger PICs, AVRs and others have a low-power secondary counter
to operate from a 32,768 Hz "watch" crystal (you must carefully match
the crystal parameters to the circuit to keep proper time), so that the
chip is woken by an interrupt each second, counts the seconds and goes
back to sleep.

  This process is limited in effectiveness by the time taken to start
up, during which the chip is of course drawing its full "running"
current, and the residual current consumption of the "sleep" mode.  An
external clock chip may be used to allow the MCU completely shut down,
perhaps even be cut off from the supply if this achieves lower current,
and you do not have these current "burps" each second.
--
  Cheers,
        Paul B.