
8.0 Real-Time Clock (RTC)
(Continued)
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P
Signal Parameters
The signal levels must conform to the voltage level requirements for 32KCLKIN/32KX1 stated in Section 11.2 on page 233.
The signal must have a duty cycle of approximately 50%. To oscillate during power-down, the signal must be sourced from
a battery-backed source. This assures that the RTC delivers updated time/calendar information.
8.2.5
The timing generation function divides the 32.768 KHz clock by 2
15
to derive a 1 Hz signal, which serves as the input for the
seconds counter. This is performed by a divider chain composed of 15 divide-by-two latches, as shown in Figure 41.
Timing Generation
Figure 41. Divider Chain Control
Bits 6-4 (DV2-0) of the CRA register control the following functions:
G
Normal operation of the divider chain (counting).
G
Divider chain reset to 0.
G
Oscillator activity when only V
BAT
power is present (backup state).
The divider chain can be activated by setting Normal Operation mode (bits 6-4 of CRA = 010b). The first update occurs
500 ms after divider chain activation.
Bits 3-0 of the CRA register select one the of 15 taps from the divider chain to be used as a periodic interrupt. The periodic
flag becomes active after half of the programmed period has elapsed, following divider chain activation.
See Section 8.3.13 on page 154 for more details.
8.2.6
Timekeeping
Data Format
Time is kept in BCD or binary format, as determined by bit 2 (DM) of Control Register B (CRB), and in either 12 or 24-hour
format, as determined by bit 1 of this register.
Note: When changing the above formats, re-initialize all the time registers.
Daylight Saving
Daylight saving time exceptions are handled automatically, as described in the RTC Control Register B (CRB) in Section
8.3.2 on page 150.
Leap Years
Leap year exceptions are handled automatically by the internal calendar function (every four years, February is extended to
29 days).
32.768 KHz
2
1
2
2
2
3
2
13
2
14
2
15
1 Hz
Divider Chain
DV2 DV1 DV0
6
5
CRA Register
Oscillator
Enable
32KX1 /
32KCLKIN
32KX2
To other
modules
Reset
4