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32580B
External Elements
Choose C
and C
capacitors (see Figure 5-5 on page
1
2
111) to match the crystal's load capacitance. The load
capacitance C
"seen" by crystal Y is comprised of C
L
series with C
and in parallel with the parasitic capacitance
2
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
= (C
* C
) / (C
+ C
L
1
2
1
Example:
Crystal C
= 10 pF, C
L
PARASITIC
C
= 3.6 pF, C
= 3.6 pF
1
2
Oscillator Startup
The oscillator starts to generate 32.768 KHz pulses to the
RTC after about 100 msec from when V
V
(2.4V) or V
is higher than V
BATMIN
SB
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
can be trimmed to achieve precisely 32.768 KHz. To
1
achieve a high time accuracy, use crystal and capacitors
with low tolerance and temperature coefficients.
5.5.2.2
External Oscillator
32.768 KHz can be applied from an external clock source,
as shown in Figure 5-6.
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
amplitude. The signal should have a duty cycle of
CORE
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.
5.5.2.3
Timing Generation
The timing generation function divides the 32.768 KHz
15
clock by 2
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 5-7.
Bits [6:4] (DV[2:0]) of the CRA Register control the follow-
ing functions:
• Normal operation of the divider chain (counting).
• Divider chain reset to 0.
• Oscillator activity when only V
(backup state).
112
) + C
2
PARASITIC
= 8.2 pF
is higher than
BAT
(3.0V). The
SBMIN
power is present
BAT
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.
in
1
Bits [3:0] of CRA select one the of fifteen taps from the
divider chain to be used as a periodic interrupt. The peri-
odic flag becomes active after half of the programmed
period has elapsed, following divider chain activation.
See Table 5-20 on page 117 for more details.
V
BAT
C
F
POWER
C
B
F
1
Battery
Figure 5-6. External Oscillator Connections
1
2
32.768 KHz
X32I
Figure 5-7. Divider Chain Control
AMD Geode™ SC2200 Processor Data Book
SuperI/O Module
To other
modules
X32O
CLKIN
(X32I)
NC
R
2
R
1
3.3V square wave
OUT
R
32.768 KHz
Clock Generator
R
C
Divider Chain
2
3
13
14
15
2
2
2
2
2
Reset
DV2 DV1 DV0
6
4
5
CRA Register
To other
modules
X32O
Internal
External
= 30 KΩ
1
= 30 KΩ
2
= 0.1 μF
F
1 Hz
Oscillator
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