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Chapter 4 Clock Description
MPC5602P Microcontroller Reference Manual, Rev. 4
Freescale Semiconductor
87
4.7
XOSC external crystal oscillator
The external crystal oscillator (XOSC) operates in the range of 4 MHz to 40 MHz. The XOSC digital
interface contains the control and status registers accessible for the external crystal oscillator.
Main features are:
•
Oscillator clock available interrupt
•
Oscillator bypass mode
4.7.1
Functional description
The crystal oscillator circuit includes an internal oscillator driver and an external crystal circuitry. The
XOSC provides an output clock to the PLL or it is used as a reference clock to specific modules depending
on system needs.
The crystal oscillator can be controlled by the ME:
•
Control by ME module. The OSCON bit of the ME_xxx_MCRs controls the powerdown of
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 put to power down state and software has to switch on when required.
Whenever the crystal oscillator is switched on from off state, OSCCNT counter starts and when it reaches
the value EOCV[7:0] × 512, oscillator clock is made available to the system. Also an interrupt pending bit
I_OSC of OSC_CTL register is set. An interrupt will be 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 external clock applied on EXTAL pin and the oscillator status is forced to ‘1’. The bypass
configuration is independent of the powerdown mode of the oscillator.
shows the truth table of different configurations of oscillator.
Table 4-2. Crystal oscillator truth table
ENABLE
BYP
XTALIN
EXTAL
CK_OSCM
XOSC Mode
0
0
No crystal,
High Z
No crystal,
High Z
0
Power down, IDDQ
x
1
x
Ext clock
EXTAL
Bypass, XOSC disabled
1
0
Crystal
Crystal
EXTAL
Normal, XOSC enabled