
4-4
MCF5249UM
MOTOROLA
PLL Programming
4.2.1
PLL OPERATION
The input to the PLL is either the crystal clock, or the crystal clock divided by two. Selection is done by
CRSEL. The PLL divides this input frequency by a programmable division factor (PLLDIV+2). In the PLL
phase/frequency detector, this divided clock is compared with the VCO output clock divided by
(VCODIV+2). As a result, Fvco = Fin * (VCODIV+2)/(PLLDIV+2).
Note: The PLL lock counter is designed for worst case input frequency (Fin) of
33.8688MHz. This will result in the required 0.5 ns for the PLL to lock. Other Fin
frequencies can be used, however, the resulting lock time will be slightly longer.
In a second step, this VCO clock is divided by (VCOOUT * CPUDIV) to create the CPU clock PSTCLK.
The PLL has a PLL-bypass feature. When PLL bypass is written 0, the crystal clock is passed directly to
the CPU. When PLL bypass is written 1, CPU clock will be switched to PLL-generated values. The
switching is delayed until the PLL has been locked, and produces a stable clock output for CPU. The
processor can read the PLL lock status (bit 31 of PLLCR). The multiplexers that switch between PLL clock
and crystal clock is glitch-free, so no system reset is needed after switching this mux.
Note: It is important that before reprogramming the PLL division factors, users must
switch to PLL bypass mode. After reprogramming, users may immediately switch
back to PLL enabled mode. Switching back is delayed internally until the PLL is
locked.
4.2.2
PLL LOCK-IN TIME
Pll lock-in time is less than 10.0 ms.
4.2.3
PLL ELECTRICAL LIMITS
Due to implementation of the block, some limits apply to the PLL block. These limitations are shown in
Table 4-3 PLL Electrical Limits
NAME
MIN. FREQUENCY
MHZ
MAX FREQUENCY
MHZ
REASON
Fvco
200
400
PLL limitations
Fcpu
0
120 (144QFP)
140 (160 MAPBGA)
Max. operating frequency of device
Fin
5
50
PLL limitations
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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