
MC68HC908QL4  MC68HC908QL3  MC68HC908QL2 Data Sheet, Rev. 4
Freescale Semiconductor
101
Chapter 11 
Oscillator Module (OSC)
11.1  Introduction
The oscillator (OSC) module is used to provide a stable clock source for the MCU system and bus. 
The OSC shares its pins with general-purpose input/output (I/O) port pins. See 
Figure 11-1
 for port 
location of these shared pins. The OSC2EN bit is located in the port A pull enable register (PTAPUEN) 
on this MCU. See 
Chapter 12 Input/Output Ports (PORTS)
 for information on PTAPUEN register.
11.2  Features
The bus clock frequency is one fourth of any of these clock source options:
1.
Internal oscillator: An internally generated, fixed frequency clock, trimmable to 
± 
0.4%. There are 
four choices for the internal oscillator, 25.6 MHz, 12.8 MHz, 8 MHz or 4 MHz. The 12.8-MHz 
internal oscillator is the default option out of reset.
2.
External oscillator: An external clock that can be driven directly into OSC1. 
3.
External RC: A built-in oscillator module (RC oscillator) that requires an external R connection only. 
The capacitor is internal to the chip.
4.
External crystal: A built-in XTAL oscillator that requires an external crystal or ceramic-resonator. 
There are three crystal frequency ranges supported, 8–32 MHz, 1–8 MHz, and 32–100 kHz.
11.3  Functional Description
The oscillator contains these major subsystems:
Internal oscillator circuit
Internal or external clock switch control
External clock circuit
External crystal circuit
External RC clock circuit