MC68HLC908QY4 Datasheet, PDF (89/182 Pages) Motorola, Inc – Microcontrollers

Chapter 11
Oscillator Module (OSC)
11.1 Introduction
The oscillator module is used to provide a stable clock source for the microcontroller system and bus. The
oscillator module generates two output clocks, BUSCLKX2 and BUSCLKX4. The BUSCLKX4 clock is
used by the system integration module (SIM) and the computer operating properly module (COP). The
BUSCLKX2 clock is divided by two in the SIM to be used as the bus clock for the microcontroller.
Therefore the bus frequency will be one forth of the BUSCLKX4 frequency.
11.2 Features
The oscillator has these four clock source options available:
1. Internal oscillator: An internally generated, fixed frequency clock, trimmable to ±5%. This 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 oscillator module (XTAL oscillator) that requires an external crystal or
ceramic-resonator.
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
11.3.1 Internal Oscillator
The internal oscillator circuit is designed for use with no external components to provide a clock source
with tolerance less than ±25% untrimmed. An 8-bit trimming register allows adjustment to a tolerance of
less than ±5%.
The internal oscillator will generate a clock of 4.0 MHz typical (INTCLK) resulting in a bus speed (internal
clock ÷ 4) of 1.0 MHz.
Figure 11-3 shows how BUSCLKX4 is derived from INTCLK and, like the RC oscillator, OSC2 can output
BUSCLKX4 by setting OSC2EN in PTAPUE register. See Chapter 12 Input/Output Ports (PORTS).
MC68HLC908QY/QT Family Data Sheet, Rev. 3
Freescale Semiconductor
89