Freescale Semiconductor ColdFire MCF52210 - Manual (Page 112)

Clock Module
MCF52211 ColdFire® Integrated Microcontroller Reference Manual, Rev. 2
6-18 Freescale Semiconductor
In PLL mode, the PLL operates in self-clocked mode (SCM) during reset until the input reference clock
to the PLL begins operating within the limits given in the electrical specifications.
If a PLL failure causes a reset, the system enters reset using the reference clock. Then the system clock
source changes to the PLL operating in SCM. If SCM is not functional, the system becomes static.
Alternately, if SYNCR[LOCEN] is cleared when the PLL fails, the system becomes static. If external reset
is asserted, the system cannot enter reset unless the PLL is capable of operating in SCM.
6.8.3 System Clock Generation
In normal PLL clock mode, the default system frequency is six times the reference frequency after reset.
The RFD[2:0] and MFD[2:0] bits in the SYNCR select the frequency multiplier. The LPD[3:0] field in the
LPDR register provides additional settings for dividing down the system clock (including when the PLL
is disabled) for low-power operation.
When programming the PLL, do not exceed the maximum system clock frequency listed in the electrical
specifications. Use this procedure to accommodate the frequency overshoot that occurs when the MFD bits
are changed:
1. Determine the appropriate value for the MFD and RFD fields in the SYNCR. The amount of jitter
in the system clocks can be minimized by selecting the maximum MFD factor that can be paired
with an RFD factor to provide the required frequency.
2. Write a value of 1 + RFD (from step 1) to the RFD field of the SYNCR.
3. Write the MFD value from step 1 to the SYNCR.
4. Monitor the LOCK flag in SYNSR. When the PLL achieves lock, write the RFD value from step
1 to the RFD field of the SYNCR. This changes the system clocks frequency to the required
frequency.
NOTE
Keep the maximum system clock frequency below the limit given in the
electrical characteristics.
6.8.4 PLL Operation
In PLL mode, the PLL synthesizes the system clocks. The PLL can multiply the reference clock frequency
by 4x to 18x, provided that the system clock frequency remains within the range listed in electrical
specifications. For example, if the reference frequency is 2 MHz, the PLL can synthesize frequencies of
8 MHz to 36 MHz. In addition, the RFD can reduce the system frequency by dividing the output of the
PLL. The RFD is not in the feedback loop of the PLL, so changing the RFD divisor does not affect PLL
operation.
Figure 6-12 shows the external support circuitry for the crystal oscillator with example component values.
Actual component values depend on crystal specifications.
The following subsections describe each major block of the PLL. Refer to Figure 6-12 to see how these
functional sub-blocks interact.