Chapter 10
Oscillator Mode (OSC)
10.1 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
for port
location of these shared pins. The OSC2EN bit is located in the port A pull enable register (PTAPUEN)
on this MCU. See
for information on PTAPUEN register.
10.2 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 4-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.
10.3 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
10.3.1 Internal Signal Definitions
The following signals and clocks are used in the functional description and figures of the OSC module.
10.3.1.1 Oscillator Enable Signal (SIMOSCEN)
The SIMOSCEN signal comes from the system integration module (SIM) and disables the XTAL oscillator
circuit, the RC oscillator, or the internal oscillator in stop mode. OSCENINSTOP in the configuration
register can be used to override this signal.
MC68HC908QC16 • MC68HC908QC8 • MC68HC908QC4 Data Sheet, Rev. 5
Freescale Semiconductor
97
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
