National CP3BT26 - Manual (Page 52)

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CP3BT26
11.1 EXTERNAL CRYSTAL NETWORK
An external crystal network is connected to the X1CKI and
X1CKO pins to generate the Main Clock, unless an external
clock signal is driven on the X1CKI pin. A similar external
crystal network may be used at pins X2CKI and X2CKO for
the Slow Clock. If an external crystal network is not used for
the Slow Clock, the Slow Clock is generated by dividing the
fast Main Clock.
The crystal network you choose may require external com-
ponents different from the ones specified in this datasheet.
In this case, consult with National’s engineers for the com-
ponent specifications
The crystals and other oscillator components must be
placed close to the X1CKI/X1CKO and X2CKI/X2CKO de-
vice input pins to keep the printed trace lengths to an abso-
lute minimum.
Figure 5 shows the external crystal network for the X1CKI
and X1CKO pins. Figure 6 shows the external crystal net-
work for the X2CKI and X2CKO pins. Table 21 shows the
component specifications for the main crystal network, and
Table 22 shows the component specifications for the 32.768
kHz crystal network.
Figure 5. Main Clock External Crystal Network
Figure 6. Slow Clock External Crystal Network
C1
12 MHz
Crystal
C2
GND
X1CKI
X1CKO
DS189
C1
32.768 kHz
Crystal
C2
GND
X2CKI
X2CKO
DS215
Table 21 Component Values of the High Frequency Crystal Circuit
Component Parameters Values Tolerance
Crystal Resonance Frequency
Type
Max. Serial Resistance
Max. Shunt Capacitance
Load Capacitance 12 MHz ± 20 ppm
AT-Cut
50
Ω
7 pF
22 pF
N/A
Capacitor C1, C2 Capacitance 22 pF 20%