FM33256B
Document Number: 001-86213 Rev. *C
Page 9 of 39
is set, then the core will be updated prior to loading the user
registers. The user registers are frozen and will not be updated
again until the R bit is cleared to a '0'.
The W bit is used to write new time/date values. Setting the W
bit to a '1' stops the RTC and allows the timekeeping core to be
written with new data. Clearing it to '0' causes the RTC to start
running based on the new values loaded in the timekeeper core.
The RTC may be synchronized to another clock source. On the
8th clock of the write to register 00h (W = ‘0’), the RTC starts
counting with a timebase that has been reset to zero
milliseconds.
Note: 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 than 2.5 V, 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
Electrical Characteristics
table).
Figure 10
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.
The minimum V
BAK
voltage varies linearly with temperature. The
user can expect the minimum V
BAK
voltage to be 1.23 V at
+85 °C and 1.90 V at -40 °C. The tested limit is 1.55 V at +25 °C.
Note
The minimum V
BAK
voltage has been characterized at
-40 °C and +85 °C but is not 100% tested.
Figure 9. Real-time Clock Core Block Diagram
32.768 kHz
crystal
Oscillator
Clock
Divider
Update
Logic
512 Hz or
Square Wave
Seconds
7 bits
MInutes
7 bits
Hours
6 bits
Date
6 bits
Months
5 bits
Years
8 bits
Days
3 bits
User Interface Registers
1 Hz
W
R
OSCEN
CF
Figure 10. I
BAK
vs. V
BAK
Voltage
I
BAK
(
µ
A)
V
BAK
(V)
Figure 11. V
BAK
(min.) vs Temperature
Temperature (°C)
V
BAK
m
in.
(V)
