PXD20RM Datasheet, PDF (225/1628 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller

the MC_CGM also has an output clock enable register (see Section 8.3.3.1.2, Output Clock Enable
Register (CGM_OC_EN)) which contains the output clock enable/disable control bit.
8.4 Oscillators
8.4.1 Pierce oscillator (FXOSC)
8.4.1.1 Introduction
The Pierce oscillator (FXOSC) module provides a robust, low-noise and low-power clock source. The
module is operated from the internally generated V_{DDE_B} supply rail (3.3 V) and require the minimum
number of external components. It is designed for optimal start-up margin with typical crystal oscillators.
8.4.1.2 Features
The FXOSC contains circuitry to dynamically control current gain in the output amplitude. This ensures
a signal with low harmonic distortion, low power and good noise immunity.
• High noise immunity due to input hysteresis
• Low RF emissions with peak-to-peak swing limited dynamically
• Transconductance (gm) sized for optimum start-up margin for typical oscillators
• Dynamic gain control eliminates the need for external current limiting resistor
• Integrated resistor eliminates the need for external bias resistor in loop controlled Pierce mode.
• Low power consumption:
— Operates from externally provided power V_{DDE_B} (3.3 V)
— Amplitude control limits power
• Clock monitor
8.4.1.3 Modes of operation
Two modes of operation exist:
1. Loop controlled Pierce (LCP) oscillator
2. External square wave mode
8.4.1.4 Block diagram
Figure 8-26 shows a block diagram of the FXOSC.
Freescale Semiconductor
PXD20 Microcontroller Reference Manual, Rev. 1
Preliminary—Subject to Change Without Notice
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