CP3BT26

11.1EXTERNAL 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.

C1

12MHz Crystal

C2

X1CKI

X1CKO

GND

DS189

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

Figure 5. Main Clock External Crystal Network

Table 22 shows the component specifications for the 32.768 kHz crystal network.

32.768 kHz Crystal

C1

X2CKI

X2CKO

C2

GND

DS215

Figure 6. Slow Clock External Crystal Network

Table 21 Component Values of the High Frequency Crystal Circuit

Component

Parameters

Values

Tolerance

 

 

 

 

Crystal

Resonance Frequency

12 MHz ± 20 ppm

 

 

Type

AT-Cut

 

 

Max. Serial Resistance

50

N/A

 

Max. Shunt Capacitance

7 pF

 

 

Load Capacitance

22 pF

 

 

 

 

 

Capacitor C1, C2

Capacitance

22 pF

20%

 

 

 

 

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National CP3BT26 manual External Crystal Network, Crystal Resonance Frequency, Type, Capacitor C1, C2 Capacitance