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C167CS

Derivatives

Clock Generation

User’s Manual

6-2

V2.0, 2000-07

6.1

Oscillator

The main oscillator of the C167CS is a power optimized Pierce oscillator providing an

inverter and a feedback element. Pins XTAL1 and XTAL2 connect the inverter to the

external crystal. The standard external oscillator circuitry (see Figure 6-2) comprises the

crystal, two low end capacitors and series resistor (

Rx2) to limit the current through the

crystal. The additional LC combination is only required for 3rd overtone crystals to

suppress oscillation in the fundamental mode. A test resistor (

RQ) may be temporarily

inserted to measure the oscillation allowance of the oscillator circuitry.

Figure 6-2

External Oscillator Circuitry

The on-chip oscillator is optimized for an input frequency range of 4 to 40 MHz.

An external clock signal (e.g. from an external oscillator or from a master device) may

be fed to the input XTAL1. The Pierce oscillator then is not required to support the

oscillation itself but is rather driven by the input signal. In this case the input frequency

range may be 0 to 50 MHz (please note that the maximum applicable input frequency is

limited by the device’s maximum CPU frequency).

Note: Oscillator measurement within the final target system is recommended to

determine the actual oscillation allowance for the oscillator-crystal system. The

measurement technique, examples for evaluated systems, and recommendations

are provided in a specific application note about oscillators (available via your

representative or WWW).

MCS04335

XTAL1

XTAL2

Rx2

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