MC908QY8CDWE Datasheet, PDF (102/232 Pages) Freescale Semiconductor, Inc – Addendum to MC68HC908QB8, rev. 3

Oscillator Module (OSC)
11.8 Registers
The oscillator module contains two registers:
• Oscillator status and control register (OSCSC)
• Oscillator trim register (OSCTRIM)
11.8.1 Oscillator Status and Control Register
The oscillator status and control register (OSCSC) contains the bits for switching between internal and
external clock sources. If the application uses an external crystal, bits in this register are used to select
the crystal oscillator amplifier necessary for the desired crystal. While running off the internal clock source,
the user can use bits in this register to select the internal clock source frequency.
Bit 7
6
5
Read:
OSCOPT1 OSCOPT0
Write:
ICFS1
Reset:
0
0
0
= Unimplemented
4
ICFS0
0
3
ECFS1
0
2
ECFS0
0
1
ECGON
Bit 0
ECGST
0
0
Figure 11-4. Oscillator Status and Control Register (OSCSC)
OSCOPT1:OSCOPT0 — OSC Option Bits
These read/write bits allow the user to change the clock source for the MCU. The default reset
condition has the bus clock being derived from the internal oscillator. See 11.3.2.2 Internal to External
Clock Switching for information on changing clock sources.
OSCOPT1
0
0
1
1
OSCOPT0
0
1
0
1
Oscillator Modes
Internal oscillator (frequency selected using ICFSx bits)
External oscillator clock
External RC
External crystal (range selected using ECFSx bits)
ICFS1:ICFS0 — Internal Clock Frequency Select Bits
These read/write bits enable the frequency to be increased for applications requiring a faster bus clock
when running off the internal oscillator. The WAIT instruction has no effect on the oscillator logic.
BUSCLKX2 and BUSCLKX4 continue to drive to the SIM module.
ICFS1
0
0
1
1
ICFS0
0
1
0
1
Internal Clock Frequency
4.0 MHz — default reset condition
8.0 MHz
12.8 MHz
Reserved
MC68HC908QB8 Data Sheet, Rev. 3
100
Freescale Semiconductor