Datasheet

Clock Selection
MC68HC908AP Family Data Sheet, Rev. 4
Freescale Semiconductor 73
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 setting the STOP_ICLKDIS
bit in CONFIG2.
5.2.2 TBM Reference Clock Selection
The timebase module reference clock (OSCCLK) is selected by configuring two bits in the CONFIG2
register, at $001D. See Chapter 3 Configuration & Mask Option Registers (CONFIG & MOR).
NOTE
The RCCLK or XCLK is only available if that clock is selected as the CGM
reference clock, whereas the ICLK is always available.
Table 5-1. CGMXCLK Clock Selection
OSCSEL1 OSCSEL0 CGMXCLK OSC2 Pin Comments
0 0 — — Not used
0 1 ICLK
f
BUS
Internal oscillator generates the CGMXCLK.
1 0 RCCLK
f
BUS
RC oscillator generates the CGMXCLK.
Internal oscillator is available after each POR or
reset.
1 1 XCLK
Inverting
output of
X-TAL
X-tal oscillator generates the CGMXCLK.
Internal oscillator is available after each POR or
reset.
Address: $001D
Bit 7654321Bit 0
Read:
STOP_
ICLKDIS
STOP_
RCLKEN
STOP_
XCLKEN
OSCCLK1 OSCCLK0
00
SCIBD-
SRC
Write:
Reset:00000000
Figure 5-3. Configuration Register 2 (CONFIG2)
Table 5-2. Timebase Module Reference Clock Selection
OSCCLK1 OSCCLK0 Timebase Clock Source
0 0 Internal oscillator (ICLK)
0 1 RC oscillator (RCCLK)
1 0 X-tal oscillator (XCLK)
1 1 Not used