Revision 1.1
81
www.national.com
G
SuperI/O Module
(Continued)
External Elements
Choose C
1
and C
2
capacitors (see Figure 4-5 on page 80)
to match the crystal’s load capacitance. The load capaci-
tance C
L
“seen” by crystal Y is comprised of C
1
in series
with C
2
and in parallel with the parasitic capacitance of the
circuit. The parasitic capacitance is caused by the chip
package, board layout and socket (if any), and can vary
from 0 to 10 pF. The rule of thumb in choosing these
capacitors is:
C
L
= (C
1
* C
2
) / (C
1
+ C
2
) + C
PARASITIC
Example:
Crystal C
L
= 10 pF, C
PARASITIC
= 8.2 pF
C
1
= 3.6 pF, C
2
= 3.6 pF
Oscillator Tuning
The oscillator starts to generate 32.768 KHz pulses to the
RTC after about 100 msec from when V
BAT
is higher than
V
BATMIN
(2.4V) or V
SB
is higher than V
SBMIN
(3.0V). The
oscillation amplitude on the X32O pin stabilizes to its final
value (approximately 0.4V peak-to-peak around 0.7V DC)
in about 1 s.
C
1
can be trimmed to achieve precisely 32.768 KHz. To
achieve a high time accuracy, use crystal and capacitors
with low tolerance and temperature coefficients.
4.5.2.2
32.768 KHz can be applied from an external clock source,
as shown in Figure 4-6.
External Oscillator
Connections
Connect the clock to the X32I ball, leaving the oscillator
output, X32O, unconnected.
Signal Parameters
The signal levels should conform to the voltage level
requirements for X32I, of square or sine wave of 0.0V to
V
CORE
amplitude. The signal should have a duty cycle of
approximately 50%. It should be sourced from a battery-
backed source in order to oscillate during power-down.
This assures that the RTC delivers updated time/calendar
information.
4.5.3
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 4-7.
Timing Generation
Bits [6:4] (DV[2:0]) of the CRA register control the following
functions:
Normal operation of the divider chain (counting).
Divider chain reset to 0.
Oscillator activity when only V
BAT
power is present
(backup state).
The divider chain can be activated by setting normal opera-
tional mode (bits [6:4] of CRA = 01x or 100). The first
update occurs 500 msec after divider chain activation.
Bits [3:0] of the CRA register select one of the fifteen taps
from the divider chain to be used as a periodic interrupt.
The periodic flag becomes active after half of the pro-
grammed period has elapsed, following divider chain acti-
vation.
See Table 4-18 on page 87 for more details.
Figure 4-6. External Oscillator Connections
Figure 4-7. Divider Chain Control
32.768 KHz
Clock Generator
Internal
External
CLKIN
(X32I)
X32O
NC
To other
modules
Battery
V
BAT
B
1
C
F
C
F
OUT
POWER
C
F
= 0.1
μ
F
R
1
R
2
R
2
= 30 K
R
1
= 30 K
3.3V square wave
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
X32I
X32O
To other
modules
Reset
4