
Oscillator (OSC)
Data Sheet
MC68HC908SR12MC68HC08SR12 — Rev. 5.0
104
Oscillator (OSC)
Freescale Semiconductor
7.3.1 CGM Reference Clock Selection
The clock generator module (CGM) reference clock (CGMXCLK) is the
reference clock input to the MCU. It is selected by programming two bits
in a FLASH memory location; the mask option register (MOR), at $FF80.
NOTE:
On the ROM device, the oscillator is selected by a ROM-mask layer at
factory.
NOTE:
The internal oscillator is a free running oscillator and is available after
each POR or reset. It is turned-off in stop mode by clearing the
STOP_ICLKEN bit in CONFIG2.
Address:
$FF80
Bit 7
654321
Bit 0
Read:
OSCSEL1 OSCSEL0
RRRRRR
Write:
Erased:
11111111
Reset:
UUUUUUUU
R= Reserved
Figure 7-2. Mask Option Register (MOR)
Table 7-1. CGMXCLK Clock Selection
OSCSEL1
OSCSEL0
CGMXCLK
OSC2 Pin
Comments
0
—
Not used
01
ICLK
fBUS
Internal oscillator generates the CGMXCLK.
1
0
RCCLK
fBUS
RC oscillator generates the CGMXCLK.
Internal oscillator is available after each POR
or reset.
11
XCLK
Inverting
output of
X-TAL
X-tal oscillator generates the CGMXCLK.
Internal oscillator is available after each POR
or reset.