Frequency-modulated phase locked loop (FMPLL)MPC5644A Microcontroller Reference Manual, Rev. 6562 Freescale Semiconductor17.2.3.1 Bypass mode with crystal referenceIn the bypass mode with crystal reference, the FMPLL is completely bypassed and the system clock isdriven from the crystal oscillator. The user must supply a crystal that is within the appropriate frequencyrange, the crystal manufacturer recommended external support circuitry, and short signal route from theMCU to the crystal.In bypass mode the PLL itself may or may not be running, depending on the state of the CLKCFG[1] bitof FMPLL_ESYNCR1, but the PLL output is not connected to the system clock. Consequently, frequencymodulation is not available. The predivider is also bypassed.Bypass mode with crystal reference is the default mode at reset if the PLLREF input is driven high. Afterreset, this mode can be entered by programming FMPLL_ESYNCR1[CLKCFG] as shown in Table 17-2.17.2.3.2 Bypass mode with external referenceThe bypass mode with external reference functions the same as bypass mode with crystal reference, exceptthat the system clock is driven by an external clock generator connected to the EXTAL pin, rather than acrystal oscillator. The input frequency range is the same and frequency modulation is not available.Bypass mode with external reference is the default mode at reset if the PLLREF input is driven low. Afterreset, this mode can be entered by programming FMPLL_ESYNCR1[PLLCFG] as shown in Table 17-2.17.2.3.3 Normal mode with crystal referenceIn the normal mode with crystal reference, the FMPLL receives an input clock frequency from the crystaloscillator and the predivider, and multiplies the frequency to create the FMPLL output clock. The usermust supply a crystal that is within the appropriate frequency range, the crystal manufacturerrecommended external support circuitry, and short signal route from the MCU to the crystal.In normal mode with crystal reference, the FMPLL can generate a frequency modulated clock or anon-modulated clock (locked on a single frequency). The modulation rate, modulation depth, outputdivider (RFD) and whether the FMPLL is modulating or not can be programmed by writing to the FMPLLregisters.17.2.3.4 Normal mode with external referenceThe normal mode with external reference functions the same as normal mode with crystal reference,except that the input clock reference to the FMPLL is driven by an external clock generator connected tothe EXTAL pin, rather than a crystal oscillator. The input frequency range is the same and frequencymodulation is available.17.3 External signal descriptionTable 17-3 lists external signals used by the FMPLL during normal operation.
