Freescale Semiconductor, Inc.
Clock Functions
11.6.8 Pseudo-STOP
Pseudo-STOP is a low power mode similar to STOP where the external
oscillator is allowed to run (at reduced amplitude) whilst the rest of the
part is in STOP. This increases the current consumption over STOP
mode by the amount of current in the oscillator, but reduces wear and
mechanical stress on the crystal.
If the PSTP bit in the PLLCR register is set, the MCU goes into Pseudo-
STOP mode when a STOP instruction is executed.
Pseudo-STOP mode is exited the same as STOP with an external reset,
an external interrupt from IRQ or XIRQ, a Key Wake-Up interrupt from
port J or port H, or an MSCAN Wake-Up interrupt.
The effect of the DLY bit is the same as noted above in STOP Exit and
Fast STOP Recovery.
11.6.9 Pseudo-STOP exit in Limp Home mode with Delay
(NOLHM=0, CME=X, DLY=1)
When coming out of Pseudo-STOP mode with the NOLHM bit cleared
and the DLY bit set, the MCU goes into limp-home mode (regardless of
the state of the CME or FCME bits).
The VCO supplies the limp-home clock frequency to the 13-stage
counter (XCLK). The BCSP output is forced high and MCS is forced low.
After the 13-stage counter reaches a count of 4096 XCLK cycles, a
check of the clock monitor is performed and as the crystal oscillator was
kept running due to the Pseudo-stop mode, the MCU exits STOP
normally, using the EXTALi clock. In the case where a crystal failure
occurred during pseudo-stop, then the MCU exits STOP using the limp
home clock (fVCOMIN) 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 is performed. If the clock
monitor indicates the presence of an external clock, limp-home mode is
de-asserted, the LHOME flag is cleared and the LHIF limp-home
Technical Data
MC68HC912DG128 — Rev 3.0
Clock Functions
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