Datasheet

MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor 71
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
REG
regulator,
therefore, the output swing on OSC1 and OSC2 is from V
SS
to V
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).