overview
the fm31xxx family combines a serial nonvolatile
ram with a real-time clock (rtc) and a processor
companion. the companion is a highly integrated
peripheral including a processor supervisor, a
comparator used for early power-fail warning,
nonvolatile event counters, and a 64-bit serial
number. the fm31xxx integrates these
complementary but distinct functions that share a
common interface in a single package. although
monolithic, the product is organized as two logical
devices, the fram memory and the
rtc/companion. from the system perspective they
appear to be two separate devices with unique ids on
the serial bus.
the memory is organized as a stand-alone 2-wire
nonvolatile memory with a standard device id value.
the real-time clock and supervisor functions are
accessed with a separate 2-wire device id. this
allows clock/calendar data to be read while
maintaining the most recently used memory address.
the clock and supervisor functions are controlled by
25 special function registers. the rtc and event
counter circuits are maintained by the power source
on the vbak pin, allowing them to operate from
battery or backup capacitor power when vdd drops
below an internally set threshold. each functional
block is described below.