MC912DG128ACPVE Datasheet, PDF (170/478 Pages) Freescale Semiconductor, Inc – Upward compatible with M68HC11 instruction set

Clock Functions
Freescale Semiconductor, Inc.
Where the crystal start-up time is longer than the initial count of 4096
XCLK cycles, or in the absence of an external clock, the MCU recovers
from STOP following the 4096 count in limp-home mode with both the
LHOME flag set and the LHIF limp-home interrupt request set to indicate
it is not operating at the desired frequency. Each time the 13-stage
counter reaches a count of 4096 XCLK cycles, a check of the clock
monitor status is performed.
When the presence of an external clock is detected, limp-home mode is
exited and the LHOME flag is cleared. This sets the limp-home interrupt
flag and if enabled by the LHIE bit, the limp-home mode interrupt is
requested.
CAUTION:
The clock monitor circuit can be misled by EXTALi clock into reporting a
good signal before it has fully stabilised. Under these conditions,
improper EXTALi clock cycles can occur on SYSCLK. This may lead to
a code runaway.
12.6.7 STOP exit in Limp Home mode without Delay (Fast Stop Recovery)
(NOLHM=0, CME=X, DLY=0)
Fast STOP recovery refers to any exit from STOP using DLY=0.
If the NOLHM bit is cleared, then the CME (or FCME) bit is masked when
a STOP instruction is executed to prevent a clock monitor failure. When
coming out of STOP mode, the MCU goes into limp-home mode where
CME and FCME signals are asserted.
When using a crystal oscillator, it is possible to exit STOP with the DLY
bit cleared. In this case, STOP is de-asserted without delay and the MCU
will execute software in limp-home mode, giving the crystal oscillator
time to stablise.
CAUTION: This mode is not recommended since the risk of the clock monitor
detecting incorrect clocks is high.
Technical Data
170
MC68HC912DT128A — Rev 4.0
Clock Functions
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