Oscillator Module (OSC)
MC68HC908QL4 MC68HC908QL3 MC68HC908QL2 Data Sheet, Rev. 4
108
Freescale Semiconductor
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.
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.
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.
Bit 7
6
5
4
3
2
1
Bit 0
Read:
OSCOPT1
OSCOPT0
ICFS1
ICFS0
ECFS1
ECFS0
ECGON
ECGST
Write:
Reset:
0
0
1
0
0
0
0
0
= Unimplemented
Figure 11-4. Oscillator Status and Control Register (OSCSC)
OSCOPT1
OSCOPT0
Oscillator Modes
0
0
Internal oscillator (frequency selected using ICFSx bits)
0
1
External oscillator clock
1
0
External RC
1
1
External crystal (range selected using ECFSx bits)
ICFS1
ICFS0
Internal Clock Frequency
0
0
4.0 MHz
0
1
8.0 MHz
1
0
12.8 MHz — default reset condition
1
1
25.6 MHz