Freescale Semiconductor MCF51QE128RM manual Mode Switching, Bus Frequency Divider, Stop

Internal Clock Source (S08ICSV3)

In FLL bypassed external mode, the ICSOUT clock is derived from the external reference clock. The external reference clock controls the FLL clock, and the FLL loop locks the FLL frequency to the FLL factor times the external reference frequency, as selected by the RDIV bits. The ICSLCLK is available for BDC communications, and the external reference clock is enabled.

12.4.1.6FLL Bypassed External Low Power (FBELP)

The FLL bypassed external low-power (FBELP) mode is entered when all the following conditions occur:

•CLKS bits are written to 10.

•IREFS bit is written to 0.

•BDM mode is not active and LP bit is written to 1.

In FLL bypassed external low power mode, the ICSOUT clock is derived from the external reference clock and the FLL is disabled. The ICSLCLK is not available for BDC communications. The external reference clock is enabled.

12.4.1.7Stop

Stop mode is entered when the MCU enters a stop state. In this mode, all ICS clock signals are static except in the following cases:

ICSIRCLK is active in stop mode when all the following conditions occur:

•IRCLKEN bit is written to 1

•IREFSTEN bit is written to 1

ICSERCLK is active in stop mode when all the following conditions occur:

•ERCLKEN bit is written to 1EREFSTEN bit is written to 1

12.4.2Mode Switching

The IREF bit can be changed at anytime, but the actual switch to the newly selected clock is shown by the IREFST bit. When switching between FLL engaged internal (FEI) and FLL engaged external (FEE) modes, the FLL begins locking again after the switch is completed.

The CLKS bits can also be changed at anytime, but the actual switch to the newly selected clock is shown by the CLKST bits. If the newly selected clock is not available, the previous clock remains selected.

The DRS bits can be changed at anytime except when LP bit is 1. If the DRS bits are changed while in FLL engaged internal (FEI) or FLL engaged external (FEE), the bus clock remains at the previous DCO range until the new DCO starts. When the new DCO starts the bus clock switches to it. After switching to the new DCO ,the FLL remains unlocked for several reference cycles. After the selected DCO startup time is over, the FLL is locked. The completion of the switch is shown by the DRST bits.

12.4.3Bus Frequency Divider

The BDIV bits can be changed at anytime and the actual switch to the new frequency occurs immediately.

MCF51QE128 MCU Series Reference Manual, Rev. 3

256 Freescale Semiconductor

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