PC87591E Datasheet, PDF (358/437 Pages) National Semiconductor (TI) – LPC Mobile Embedded Controllers

Host-Controlled Modules and Host Interface (Continued)
Signal Parameters
The signal levels should conform to the voltage level requirements for 32KCLKIN/32KX1, stated in Section 7 on page 372.
The signal should have a duty cycle of approximately 50%. To oscillate during power-down, it should be sourced from a
battery-backed source. This guarantees that the RTC delivers updated time/calendar information.
6.2.5 Timing Generation
The timing generation function divides the 32.768 KHz clock by 2¹⁵ 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 115.
Divider Chain
123
222
13 14 15
2 2 2 1 Hz
Reset
DV2 DV1 DV0
654
CRA Register
32.768 KHz
To other
modules
Oscillator
Enable
32KX1 /
32KCLKIN
32KX2
Figure 115. Divider Chain Control
Bits 6-4 (DV2-0) of CRA register control the following functions:
● _{Normal operation of the divider chain (counting)}
● _{Divider chain reset to 0}
● _{Oscillator activity when only VBAT power is present (backup state).}
The divider chain can be activated by setting normal operational mode (bits 6-4 of CRA = 010₂). The first update occurs
500 ms after divider chain activation.
Bits 3-0 of 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 programed period has elapsed, following divider chain activation.
See “RTC Control Register A (CRA)” on page 367 for more details.
6.2.6 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 6.2.15 on page 363.
Leap Years
Leap year exceptions are handled automatically by the internal calendar function. Every four years, February is extended to
29 days. Year 2000 is a leap year.
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Revision 1.07