Freescale Semiconductor, Inc.
Clock Functions
Limp-Home and Fast STOP Recovery modes
During this power up sequence, after the POR pulse falling edge, the
VCO supplies the limp-home clock frequency to the 13-stage counter, as
the BCSP output is forced high and MCS is forced low. XCLK, BCLK and
MCLK are forced to be PCLK, which is supplied by the VCO at fVCOMIN
.
The initial period taken for the 13-stage counter to reach 4096 defines
the internal reset period.
If the clock monitor indicates the presence of an external clock during the
internal reset period, limp-home mode is de-asserted and the 13-stage
counter is then driven by EXTALi clock. After the 13-stage counter
reaches a count of 4096 XCLK cycles, the internal reset is released, the
13-stage counter is reset and the MCU exits reset normally using
EXTALi clock.
However, if 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 will
leave the reset state in limp-home mode. The LHOME flag is set and
LHIF limp-home interrupt request is set, to indicate it is not operating at
the desired frequency. Then after yet another 4096 XCLK cycles
followed regularly by 8192 XCLK cycles (corresponding to the 13-stage
counter timing out), a check of the clock monitor status is performed.
When the presence of an external clock is detected limp-home mode is
exited generating a limp-home interrupt if enabled.
CAUTION: The clock monitor circuit can be misled by the 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. To ensure that this situation does not
occur, the external Reset period should be longer than the oscillator
stabilisation time - this is an application dependent parameter.
With the VDDPLL supply voltage at VSS level, the PLL module and
hence limp-home mode are disabled, the device will remain effectively
in a static state whilst there is no activity on EXTALi. The internal reset
period and MCU operation will execute only on EXTALi clock.
NOTE: The external clock signal must stabilise within the initial 4096 reset
counter cycles.
MC68HC912DG128 — Rev 3.0
Technical Data
Clock Functions
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