Freescale Semiconductor, Inc.
Clock Functions
12.6.5 Executing the STOP instruction without Limp Home mode, clock monitor enabled
(NOLHM=1, CME=1, DLY=X)
If the NOLHM bit and the CME (or FCME) bits are set, a clock monitor
failure is detected when a STOP instruction is executed and the MCU
resets via the clock monitor reset vector.
12.6.6 STOP exit in Limp Home mode with Delay
(NOLHM=0, CME=X, DLY=1)
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, a normal STOP exit sequence requires
the DLY bit to be set to allow for the crystal stabilization period.
With the 13-stage counter clocked by the VCO (at f
), following a
VCOMIN
delay of 4096 XCLK cycles at the limp-home frequency, if the clock
monitor indicates the presence of an external clock, the limp-home mode
is de-asserted and the MCU exits STOP normally using EXTALi clock.
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.
Advance Information
156
68HC(9)12D60 — Rev 4.0
MOTOROLA
Clock Functions
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