FM31T372/374/376/378-G
The serial number is located in registers 11h to 18h.
The lock bit is SNL, register 0Bh bit 7. Setting the
SNL bit to „1‟ disables writes to the serial number
registers, and
the SNL bit cannot be cleared.
Real-Time Clock (TCXO) Operation
The real-time clock is a timekeeping function that
can be battery or capacitor backed for continuous
operation. The RTC is operated by a temperature
compensated crystal oscillator (TCXO) based on an
embedded 32.768 kHz crystal.
The RTC consists of an oscillator, clock divider, and
a register system for user access. It divides down the
32.768 kHz time-base and provides a minimum
resolution of seconds (1Hz). Static registers provide
the user with read/write access to the time values. It
includes registers for seconds, minutes, hours,
day-of-the-week, date, months, and years. A block
diagram (Figure 8) illustrates the RTC function.
The user registers are synchronized with the
timekeeper core using R and W bits in register 00h
described below. Changing the R bit from „0‟ to „1‟
transfers timekeeping information from the core into
holding registers that can be read by the user. If a
timekeeper update is pending when R is set, then the
core will be updated prior to loading the user
registers. The registers are frozen and will not be
updated again until the R bit is cleared to „0‟. R is
used for reading the time.
Setting the W bit to „1‟ locks the user registers.
Clearing it to „0‟ causes the values in the user
registers 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 2.4V 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
F-RAM 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.
Rev. 1.1
Apr. 2011
Figure 8. 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 9. V
BAK
(min.) vs. Temperature
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 80µA 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. There is a Fast Charge
mode which is enabled by the FC bit (register 0Bh,
bit 5). In this mode the trickle charger current is set to
approximately 1 mA, allowing a large backup
capacitor to charge more quickly.
In the case where no battery is used, the V
BAK
pin
should be tied to V
DD
.
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
approximately 2.4V.
Page 7 of 26
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