STK18TA8 Datasheet, PDF (18/28 Pages) List of Unclassifed Manufacturers – nvTime Event Data Recorder Serial Peripheral Interface nvSRAM QuantumTrap CMOS Nonvolatile Static RAM

STK18TA8
REAL TIME CLOCK OPERATION
nvTIME OPERATION
The STK18TA8 offers internal registers that contain
Clock, Alarm, Watchdog, Interrupt, and Control
functions. Internal double buffering of the clock and
the clock/timer information registers prevents
accessing transitional internal clock data during a
read or write operation. Double buffering also cir-
cumvents disrupting normal timing counts or clock
accuracy of the internal clock while accessing clock
data. Clock and Alarm Registers store data in BCD
format.
CLOCK OPERATIONS
The clock registers maintain time up to 9,999 years in
one second increments. The user can set the time to
any calendar time and the clock automatically keeps
track of days of the week and month, leap years and
century transitions. There are eight registers
dedicated to the clock functions which are used to set
time with a write cycle and to read time during a read
cycle. These registers contain the Time of Day in
BCD format. Bits defined as “X” are currently not used
and are reserved for future use by Simtek.
READING THE CLOCK
While the double-buffered RTC register structure
reduces the chance of reading incorrect data from the
clock, the user should halt internal updates to the
STK18TA8 clock registers before reading clock data
to prevent the reading of data in transition. Stopping
the internal register updates does not affect clock
accuracy.
The updating process is stopped by writing a “1” to
the read bit “R” (in the flags register at 0x1FFF0), and
will not restart until a “0” is written to the read bit. The
RTC registers can then be read while the internal
clock continues to run.
Within 20ms after a “0” is written to the read bit, all
STK18TA8 registers are simultaneously updated.
SETTING THE CLOCK
Setting the write bit “W” (in the flags register at
0x1FFF0) to a “1” halts updates to the STK18TA8
registers. The correct day, date and time can then be
written into the registers in 24-hour BCD format. The
time written is referred to as the “Base Time.” This
value is stored in nonvolatile registers and used in
calculation of the current time. Resetting the write bit
to “0” transfers those values to the actual clock
counters, after which the clock resumes normal
operation.
BACKUP POWER
The RTC in the STK18TA8 is intended for
permanently powered operation. Either the V_RTCcap or
V_RTCbat pin is connected depending on whether a
capacitor or battery is chosen for the application.
When primary power, V_cc, fails and drops below V_switch
the device will switch to the backup power supply.
The clock oscillator uses very little current, which
maximizes the backup time available from the backup
source. Regardless of clock operation with the
primary source removed, the data stored in nvSRAM
is secure, having been stored in the nonvolatile
elements as power was lost. Factors to be considered
when choosing a backup power source include: the
expected duration of power outages and the cost
trade-off of using a battery versus a capacitor.
During backup operation the STK18TA8 consumes a
maximum of 300 nanoamps at 2 volts. Capacitor or
battery values should be chosen according to the
application. Backup time values based on maximum
current specs are shown below. Nominal times are
approximately 3 times longer.
Capacitor Value
0.1 F
72 hours
Backup Time
0.47 F
14 days
1.0 F
30 days
Using a capacitor has the obvious advantage of
recharging the backup source each time the system is
powered up.
April 2005
18
Document Control #ML0028 rev 0.8