MC68HC12 Datasheet, PDF (166/478 Pages) Motorola, Inc – The MC68HC912DT128A microcontroller unit (MCU) is a 16-bit device composed of standard on-chip peripherals including a 16-bit central processing unit

Clock Functions
12.6.2 No Clock at Power-On Reset
The voltage level on VDDPLL determines how the MCU responds to an
external clock loss in this case.
With the VDDPLL supply voltage at VDD level, any reset sets the Clock
Monitor Enable bit (CME) and the PLLON bit and clears the NOLHM bit.
Therefore, if the MCU is powered up without an external clock, limp-
home mode is entered provided the MCU is in a normal mode of
operation.
VDD
Power-On Detector
EXTALi
Clock Monitor Fail
(Slow EXTALi)
Limp-Home
13-stage counter
(Clocked by XCLK)
BCSP
Internal reset
0 --> 4096
0 --> 4096
Reset: BCSP = 0
SYSCLK
PLLCLK (L.H.)
EXTALi
SYSCLK
(Slow EXTALi)
PLLCLK (Software check of Limp-Home Flag)
Figure 12-4. No Clock at Power-On Reset
EXTALi
Technical Data
166
Clock Functions
MC68HC912DT128A — Rev 4.0
MOTOROLA