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.
Figure 8. IBAK vs. VBAK Voltage
The minimum VBAK voltage varies linearly with
temperature. The user can expect the minimum VBAK
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
VBAK voltage has been characterized at -40°C and
+85°C but is not 100% tested.
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.
Figure 9. VBAK (min.) vs. Temperature
Trickle Charger
To facilitate capacitor backup, the VBAK pin can
optionally provide a trickle charge current. When the
VBC bit, register 0Bh bit 2, is set to „1‟ the VBAK pin
will source approximately 80µA until VBAK reaches
VDD or 3.75V whichever is less. In 3V systems, this
charges the capacitor to VDD 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.
Backup Power
The real-time clock/calendar is intended to be
permanently powered. When the primary system
power fails, the voltage on the VDD pin will drop.
When VDD is less 2.4V the RTC (and event counters)
will switch to the backup power supply on VBAK. 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.
In the case where no battery is used, the VBAK pin
should be tied to VDD
.
The IBAK current varies with temperature and voltage
(see DC parametric table). The following graph
shows IBAK as a function of VBAK. These curves are
useful for calculating backup time when a capacitor
is used as the VBAK source.
Although VBAK may be connected to VSS, this is not
recommended if the companion is used. None of
the companion functions will operate below
approximately 2.4V.
Rev. 1.1
Apr. 2011
Page 7 of 26
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
