MC912DG128A Motorola, MC912DG128A Datasheet - Page 141

• Manufacturer Part Number: MC912DG128A
• Description: Microcontrollers
• Manufacturer: Motorola

Limp-Home and Fast STOP Recovery modes
Clock Loss during
Normal Operation
7-clock
MOTOROLA
If the crystal frequency is not available due to a crystal failure or a long
crystal start-up time, the MCU system clock can be supplied by the VCO
at its minimum operating frequency, f
called Limp-Home Mode and is only available when the VDDPLL supply
voltage is at VDD level (i.e. power supply for the PLL module is present).
Upon power-up, the ability of the system to start in Limp-Home Mode is
restricted to normal MCU modes only.
The Clock Monitor circuit (see section Clock Monitor) can detect the loss
of EXTALi, the external clock input signal, regardless of whether this
signal is used as the source for MCU clocks or as the PLL reference
clock. The clock monitor control bits, CME and FCME, are used to
enable or disable external clock detection.
A missing external clock may occur in the three following instances:
The ‘no limp-home mode’ bit, NOLHM, determines how the MCU
responds to an external clock loss in this case.
With limp home mode disabled (NOLHM bit set) and the clock monitor
enabled (CME or FCME bits set), on a loss of clock the MCU is reset via
the clock monitor reset vector. This is the same behavior as standard
M68HC12 circuits without PLL or operation with VDDPLL at VSS level.
With limp home mode enabled (NOLHM bit cleared) and the clock
monitor enabled (CME or FCME bits set), on a loss of clock, the PLL
VCO clock at its minimum frequency, f
clock, allowing the MCU to continue operating.
The MCU is said to be operating in “limp-home” mode with the forced
VCO clock as the system clock. PLLON and BCSP (‘bus clock select
PLL’) signals are forced high and the MCS (‘module clock select’) signal
•
•
•
During normal clock operation.
At Power-On Reset.
In the STOP exit sequence
Clock Functions
Limp-Home and Fast STOP Recovery modes
VCOMIN
VCOMIN
. This mode of operation is
,
is provided as the system
MC68HC912DT128A Rev 2.0
Clock Functions
141