FM3104/16/64/256
to be loaded into the timekeeper core. W is used for
writing new time values. Users should be certain not
to load invalid values, such as FFh, to the
timekeeping registers. Updates to the timekeeping
core occur continuously except when locked.
Backup Power
The real-time clock/calendar is intended to be
permanently powered. When the primary system
power fails, the voltage on the V
DD
pin will drop.
When V
DD
is less 2.5V, the RTC (and event counters)
will switch to the backup power supply on V
BAK
. The
clock operates at extremely low current in order to
maximize battery or capacitor life. However, an
advantage of combining a clock function with FRAM
memory is that data is not lost regardless of the
backup power source.
The I
BAK
current varies with temperature and voltage
(see DC parametric table). The following graph
shows I
BAK
as a function of V
BAK
. These curves are
useful for calculating backup time when a capacitor
is used as the V
BAK
source.
Trickle Charger
To facilitate capacitor backup the V
BAK
pin can
optionally provide a trickle charge current. When the
VBC bit, register 0Bh bit 2, is set to 1 the V
BAK
pin
will source approximately 15 µA until V
BAK
reaches
V
DD
or 3.75V whichever is less. In 3V systems, this
charges the capacitor to V
DD
without an external
diode and resistor charger. In 5V systems, it provides
the same convenience and also prevents the user from
exceeding the V
BAK
maximum voltage specification.
In the case where no battery is used, the V
BAK
pin
should be tied according to the following conditions:
For 3.3V systems, V
BAK
should be tied to V
DD
.
This assumes V
DD
does not exceed 3.75V.
For 5V systems, attach a 1 µF capacitor to V
BAK
and turn the trickle charger on. The V
BAK
pin
will charge to the internal backup voltage which
regulates itself to about 3.6V. V
BAK
should not
be tied to 5V since the V
BAK
(max) specification
will be exceeded. A 1 µF capacitor will keep
the companion functions working for about 1.5
second.
Although V
BAK
may be connected to V
SS
, this is not
recommended if the companion is used. None of the
companion functions will operate below about 2.5V.
Note: systems using lithium batteries should clear
the VBC bit to 0 to prevent battery charging. The
V
BAK
circuitry includes an internal 1 K
series
resistor as a safety element.
Figure 7. I
BAK
vs. V
BAK
Voltage
The minimum V
BAK
voltage varies linearly with
temperature. The user can expect the minimum V
BAK
voltage to be 1.23V at +85°C and 1.90V at -40°C.
The tested limit is 1.55V at +25°C. The minimum
V
BAK
voltage has been characterized at -40°C and
+85°C but is not 100% tested.
Figure 8. V
BAK
(min.) vs. Temperature
Rev. 2.1
Sept. 2011
Page 7 of 26
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
