MC908QT4MPE datasheet(92/184 Pages) FREESCALE

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Oscillator Module (OSC)

MC68HC908QY/QT Family Data Sheet, Rev. 5

Freescale Semiconductor

11.3.3 XTAL Oscillator

The XTAL oscillator circuit is designed for use with an external crystal or ceramic resonator to provide an

accurate clock source. In this configuration, the OSC2 pin is dedicated to the external crystal circuit. The

OSC2EN bit in the port A pullup enable register has no effect when this clock mode is selected.

In its typical configuration, the XTAL oscillator is connected in a Pierce oscillator configuration, as shown

in Figure 11-2. This figure shows only the logical representation of the internal components and may not

represent actual circuitry. The oscillator configuration uses five components:

•Crystal, X1

•

Fixed capacitor, C1

•

Tuning capacitor, C2 (can also be a fixed capacitor)

•

Feedback resistor, R

•

Series resistor, R

S (optional)

NOTE

The series resistor (R

S) is included in the diagram to follow strict Pierce

oscillator guidelines and may not be required for all ranges of operation,

especially with high frequency crystals. Refer to the crystal manufacturer’s

data for more information.

Figure 11-2. XTAL Oscillator External Connections

SIMOSCEN

XTALCLK

(1)

MCU

FROM SIM

OSC2

OSC1

÷ 2

BUSCLKX2

BUSCLKX4

TO SIM

Note 1.

S can be zero (shorted) when used with higher-frequency crystals. Refer to manufacturer’s

data. See Chapter 16 Electrical Specifications for component value recommendations.

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