NXP Semiconductors MPC5606S - FXOSC External Oscillator

Clock Description
MPC5606S Microcontroller Reference Manual, Rev. 7
Freescale Semiconductor 209
8.5 FXOSC external oscillator
The FXOSC digital interface controls the external crystal oscillator (FXOSC). It holds control and status
registers that are accessible by software.
8.5.1 Main features
• External crystal oscillator (FXOSC) digital interface
• Oscillator clock available interrupt
• Oscillator bypass mode
• Output clock division factors ranging from 1,2,3....32
8.5.2 Functional description
The crystal oscillator circuit includes an internal oscillator driver and external crystal circuitry. It provides
an output clock that can be provided to PLL or used as a reference clock to specific modules depending on
system needs.
The crystal oscillator is controlled by the MC_ME module. The OSCON bit of ME_<xxx>_MCR registers
controls the power-down of the oscillator based on the current device mode, while S_OSC of ME_GS
register provides the oscillator clock available status.
After system reset, the oscillator is powered down, and software has to restart it when required. Whenever
the crystal oscillator is switched on from the off state, the OSCCNT counter starts. When it reaches the
value of EOCV[7:0] × 512, an oscillator clock is made available to the system. Also, an interrupt pending
bit, I_OSC of the OSC_CTL register, is set. An interrupt is generated if the interrupt mask bit M_OSC is
set.
The oscillator circuit can be bypassed by setting OSC_CTL[OSCBYP]. This bit can only be set by the
software. System reset is needed to reset this bit. In this bypass mode, the output clock has the same
polarity as the external clock applied on the EXTAL pin, and the oscillator status is forced to 1. The bypass
configuration is independent of the power-down mode of the oscillator.
Table 8-14 shows the truth table for different configurations of the oscillator.
Table 8-14. Truth table of crystal oscillator
ENABLE BYP XTAL EXTAL CK_OSCM OSC MODE
0 0 No crystal,
high
impedance
No crystal,
high
impedance
0 Power down,
IDDQ
x 1 x Ext clock EXTAL Bypass, OSC
disabled
1 0 Crystal Crystal EXTAL Normal, OSC
enabled
Gnd Ext clock EXTAL Normal, OSC
enabled