MC908JL8CDWE Datasheet, PDF (79/212 Pages) Freescale Semiconductor, Inc – Microcontrollers

Chapter 6
Oscillator (OSC)
6.1 Introduction
The oscillator module provides the reference clocks for the MCU system and bus. Two oscillators are
running on the device:
Selectable oscillator — for bus clock
• Crystal oscillator (XTAL) — built-in oscillator that requires an external crystal or ceramic-resonator.
This option also allows an external clock that can be driven directly into OSC1.
• RC oscillator (RC) — built-in oscillator that requires an external resistor-capacitor connection only.
The selected oscillator is used to drive the bus clock, the SIM, and other modules on the MCU. The
oscillator type is selected by programming a bit FLASH memory. The RC and crystal oscillator cannot run
concurrently; one is disabled while the other is selected; because the RC and XTAL circuits share the
same OSC1 pin.
Non-selectable oscillator — for COP
• Internal oscillator — built-in RC oscillator that requires no external components.
This internal oscillator is used to drive the computer operating properly (COP) module and the SIM. The
internal oscillator runs continuously after a POR or reset, and is always available.
6.2 Oscillator Selection
The oscillator type is selected by programming a bit in a FLASH memory location; the mask option register
(MOR), at $FFD0.
(See 3.5 Mask Option Register (MOR).)
NOTE
On the ROM device, the oscillator is selected by a ROM-mask layer at
factory.
Address: $FFD0
Bit 7
6
5
4
3
2
1
Bit 0
Read:
OSCSEL
R
R
R
R
R
R
R
Write:
Erased: 1
1
1
1
1
1
1
1
Reset:
Unaffected by reset
Non-volatile FLASH register; write by programming.
R
= Reserved
Figure 6-1. Mask Option Register (MOR)
MC68HC908JL8/JK8 • MC68HC08JL8/JK8 • MC68HC908KL8 Data Sheet, Rev. 3.1
Freescale Semiconductor
79