PXS20RM Datasheet, PDF (1146/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Oscillators
Table 35-2. RC_CTL field descriptions
Field
RCTRIM
Main RC trimming bits
Description
35.2 XOSC external oscillator
The external crystal oscillator (XOSC) works in the range of 4–40 MHz.
The XOSC digital interface contains control and status registers accessible to applications.
Main features are:
• Oscillator clock available interrupt
• Oscillator bypass mode
35.2.1 Functional description
The external crystal oscillator circuit includes an internal oscillator driver and an external crystal circuitry.
It provides an output clock that can be provided to the FMPLL or used as a reference clock to specific
modules depending on system needs.
The external crystal oscillator can be controlled by the MC_ME. The OSCON bit of ME_xxx_MCR
registers 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 powered down, and software must enable the oscillator when required.
Whenever the external crystal oscillator is switched on from the off state, OSCCNT counter starts and
when it reaches the value EOCV[7:0]*512, the oscillator clock becomes 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 XTALOUT pin and the oscillator status is forced to ‘1’. The bypass
configuration is independent of the powerdown mode of the oscillator.
Table 35-3 shows the truth table of different configurations of the oscillator.
Table 35-3. Truth table of the external crystal oscillator
OSCON
0
x
1
OSCBYP
0
1
0
XTALIN
XTALOUT XOSC_CLK
No crystal, Hiz No crystal, Hiz
x
Ext clock
Crystal
Crystal
Gnd
Ext clock
0
XTALOUT
XTALOUT
XTALOUT
XOSC MODE
Power down, IDDQ
Bypass, OSC Disabled
Normal, OSC Enabled
Normal, OSC Enabled
35-2
PXS20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor