RTCs with Constant-Voltage Trickle Charger
5C/DS12R87
An external 32.768kHz oscillator can also drive the
LOCAL GROUND PLANE (LAYER 2)
DS12R885. In this configuration, the X1 pin is connected
to the external oscillator signal and the X2 pin is left
unconnected.
X1
Clock Accuracy
The accuracy of the clock is dependent upon the accu-
CRYSTAL
X2
racy of the crystal and the accuracy of the match
between the capacitive load of the oscillator circuit and
the capacitive load for which the crystal was trimmed.
NOTE: AVOID ROUTING SIGNAL LINES
IN THE CROSSHATCHED AREA
(UPPER LEFT QUADRANT) OF
THE PACKAGE UNLESS THERE IS
A GROUND PLANE BETWEEN THE
SIGNAL LINE AND THE DEVICE PACKAGE.
Additional error is added by crystal frequency drift
caused by temperature shifts. External circuit noise cou-
GND
pled into the oscillator circuit can result in the clock run-
ning fast. Figure 2 shows a typical PC board layout for
isolation of the crystal and oscillator from noise. Refer to
Application Note 58: Crystal Considerations with Dallas
Real-Time Clocks (RTCs) for more detailed information.
Figure 2. Layout Example
used by the daylight saving function, so the value 1 is
defined as Sunday. The date at the end of the month is
automatically adjusted for months with fewer than 31
days, including correction for leap years.
The DS12R887 and DS12CR887 are calibrated at the
factory to an accuracy of 1 minute per month at
+25°C during data-retention time for the period tDR
.
Power-Down/Power-Up
Considerations
Before writing the internal time, calendar, and alarm reg-
isters, the SET bit in Register B should be written to logic
1 to prevent updates from occurring while access is
being attempted. In addition to writing the 10 time, calen-
dar, and alarm registers in a selected format (binary or
BCD), the data mode bit (DM) of Register B must be set
to the appropriate logic level. All 10 time, calendar, and
alarm bytes must use the same data mode. The SET bit
in Register B should be cleared after the data mode bit
has been written to allow the RTC to update the time and
calendar bytes. Once initialized, the RTC makes all
updates in the selected mode. The data mode cannot be
changed without reinitializing the 10 data bytes. Tables
2A and 2B show the BCD and binary formats of the time,
calendar, and alarm locations.
The real-time clock continues to operate regardless of
the V
input level, and the RAM and alarm memory
CC
locations remain nonvolatile. V
must remain
BACKUP
within the minimum and maximum limits when V
is
CC
not applied. When V
is applied and exceeds V
CC
PF
(power-fail trip point), the device becomes accessible
after t —if the oscillator is running and the oscillator
countdown chain is not in reset (Register A). This time
allows the system to stablize after power is applied. If
the oscillator is not enabled, the oscillator-enable bit is
enabled on power-up, and the device becomes imme-
diately accessible.
REC
Time, Calendar, and Alarm
Locations
The time and calendar information is obtained by read-
ing the appropriate register bytes. The time, calendar,
and alarm are set or initialized by writing the appropri-
ate register bytes. The contents of the 10 time, calen-
dar, and alarm bytes can be either binary or
binary-coded decimal (BCD) format.
The 24/12 bit cannot be changed without reinitializing the
hour locations. When the 12-hour format is selected, the
higher-order bit of the hours byte represents PM when it
is logic 1. The time, calendar, and alarm bytes are always
accessible because they are double-buffered. Once per
second the seven bytes are advanced by one second
and checked for an alarm condition.
If a read of the time and calendar data occurs during
an update, a problem exists where seconds, minutes,
hours, etc., may not correlate. The probability of read-
ing incorrect time and calendar data is low. Several
The day-of-week register increments at midnight, incre-
menting from 1 through 7. The day-of-week register is
____________________________________________________________________ 13
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