STK17T88 Datasheet, PDF (15/28 Pages) List of Unclassifed Manufacturers – nvTime Event Data Recorder 32K x 8 AutoStore nvSRAM With Real-Time Clock

REAL TIME CLOCK OPERATION
STK17T88
REAL TIME CLOCK
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 regis-
ters 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 "0" are currently
not used and are reserved for future use by Simtek.
READING THE CLOCK
The user should halt internal updates to the real time
clock registers before reading clock data to prevent
reading of data in transition. Stopping the internal
register updates does not affect clock accuracy.
Write a “1” to the read bit "R" (in the Flags register at
0x7FF0) to capture the current time in holding regis-
ters. Clock updates do not restart until a “0” is written
to the read bit. The RTC registers can now be read
while the internal clock continues to run.
Within 20ms after a “0” is written to the read bit, all
real time clock registers are simultaneously updated.
SETTING THE CLOCK
Set the write bit “W” (in the Flags register at 0x7FF0)
to a "1" enable the time to be set. The correct day,
date and time can then be written into the real time
clock registers in 24-hour BCD format. The time writ-
ten is referred to as the "Base Time." This value is
stored in non-volatile registers and used in calcula-
tion of the current time. Reset the write bit to "0" to
transfer the time to the actual clock counters, The
clock will start counting at the new base time.
BACKUP POWER
The RTC is intended to keep time even when system
power is lost. When primary power, V_CC, drops
below V_SWITCH, the real time clock will switch to the
backup power supply connected to either the V_RTC-
cap ^{or V}RTCbat ^pin.
The clock oscillator uses a maximum of 300 nano-
amps at 2 volts to maximize the backup time avail-
able from the backup source.
You can power the real time clock with either a
capacitor or a battery. Factors to be considered
when choosing a backup power source include the
expected duration of power outages and the cost
and reliability trade-off of using a battery versus a
capacitor.
If you select a capacitor power source, connect the
capacitor to the V_RTCcap pin and leave the V_RTCbat
pin unconnected. Capacitor backup time values
based on maximum current specs are shown below.
Nominal times are approximately 3 times longer.
Capacitor Value
0.1 F
0.47 F
1.0 F
Backup Time
72 hours
14 days
30 days
A capacitor has the obvious advantage of being
more reliable and not containing hazardous materi-
als. The capacitor is recharged every time the power
is turned on so that the real time clock continues to
have the same backup time over years of operation
If you select a battery power source, connect the bat-
tery to the V_RTCbat pin and leave the V_RTCcap pin
unconnected. A 3V lithium is recommended for this
application. The battery capacity should be chosen
for the total anticipated cumulative down-time
required over the life of the system.
The real time clock is designed with a diode inter-
nally connected to the V_RTCbat pin. This prevents the
battery from ever being charged by the circuit.
STOPPING AND STARTING THE
RTC OSCILLATOR
The OSCEN bit in Calibration register at 0x7FF8
enables RTC oscillator operation. This bit is non-vol-
atile and shipped to customers in the “enabled” state
(set to 0) . OSCEN should be set to a 1 to preserve
battery life while the system is in storage . This will
turn off the oscillator circuit extending the battery life.
If the OSCEN bit goes from disabled to enabled, it
will typically take 5 seconds (10 seconds max) for
the oscillator to start.
Document Control #ML0024 Rev 1.7
15
March 2007