Chapter 5
Oscillator (OSC)
5.1 Introduction
The oscillator module consist of three types of oscillator circuits:
•
•
•
Internal oscillator
RC oscillator
32.768kHz crystal (x-tal) oscillator
The reference clock for the CGM and other MCU sub-systems is selected by programming the mask
option register located at $FFCF.
The reference clock for the timebase module (TBM) is selected by the two bits, OSCCLK1 and OSCCLK0,
in the CONFIG2 register.
The internal oscillator runs continuously after a POR or reset, and is always available. The RC and crystal
oscillator cannot run concurrently; one is disabled while the other is selected; because the RC and x-tal
circuits share the same OSC1 pin.
NOTE
The oscillator circuits are powered by the on-chip V
regulator,
REG
therefore, the output swing on OSC1 and OSC2 is from V to V
.
SS
REG
Figure 5-1. shows the block diagram of the oscillator module.
5.2 Clock Selection
Reference clocks are selectable for the following sub-systems:
•
CGMXCLK and CGMRCLK — Reference clock for clock generator module (CGM) and other MCU
sub-systems other than TBM and COP. This is the main reference clock for the MCU.
•
OSCCLK — Reference clock for timebase module (TBM).
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor
71
FREESCALE [ Freescale ]
