M68ICS08GR datasheet(126/408 Pages) FREESCALE

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Clock Generator Module (CGMC)

Technical Data

MC68HC908GR8 — Rev 4.0

126

Clock Generator Module (CGMC)

MOTOROLA

The most critical parameter which affects the reaction times of the PLL

is the reference frequency, fRDV. This frequency is the input to the phase

detector and controls how often the PLL makes corrections. For stability,

the corrections must be small compared to the desired frequency, so

several corrections are required to reduce the frequency error.

Therefore, the slower the reference the longer it takes to make these

corrections. This parameter is under user control via the choice of crystal

frequency fXCLK and the R value programmed in the reference divider.

(See PLL Circuits, Programming the PLL, and PLL Reference Divider

Select Register.)

Another critical parameter is the external filter network. The PLL

modifies the voltage on the VCO by adding or subtracting charge from

capacitors in this network. Therefore, the rate at which the voltage

changes for a given frequency error (thus change in charge) is

proportional to the capacitance. The size of the capacitor also is related

to the stability of the PLL. If the capacitor is too small, the PLL cannot

make small enough adjustments to the voltage and the system cannot

lock. If the capacitor is too large, the PLL may not be able to adjust the

voltage in a reasonable time. (See Choosing a Filter.)

Also important is the operating voltage potential applied to VDDA. The

power supply potential alters the characteristics of the PLL. A fixed value

is best. Variable supplies, such as batteries, are acceptable if they vary

within a known range at very slow speeds. Noise on the power supply is

not acceptable, because it causes small frequency errors which

continually change the acquisition time of the PLL.

Temperature and processing also can affect acquisition time because

the electrical characteristics of the PLL change. The part operates as

specified as long as these influences stay within the specified limits.

External factors, however, can cause drastic changes in the operation of

the PLL. These factors include noise injected into the PLL through the

filter capacitor, filter capacitor leakage, stray impedances on the circuit

board, and even humidity or circuit board contamination.

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