Freescale Semiconductor MC68HC908MR32 Manual Do Utilizador

Freescale Semiconductor

71

required to reduce the frequency error. Therefore, the slower the reference the longer it takes to make 
these corrections. This parameter is also under user control via the choice of crystal frequency, f

XCLK

.

Another critical parameter is the external filter capacitor. The PLL modifies the voltage on the VCO by 
adding or subtracting charge from this capacitor. Therefore, the rate at which the voltage changes for a 
given frequency error (thus change in charge) is proportional to the capacitor size. 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 

Also important is the operating voltage potential applied to V

DDA

. 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.

As described in 

, the external filter capacitor, C

F

, is critical 

to the stability and reaction time of the PLL. The PLL is also dependent on reference frequency and supply 
voltage. The value of the capacitor must, therefore, be chosen with supply potential and reference 
frequency in mind. For proper operation, the external filter capacitor must be chosen according to this 
equation:

For acceptable values of C

FACT

. For the value of V

DDA

, 

choose the voltage potential at which the MCU is operating. If the power supply is variable, choose a value 
near the middle of the range of possible supply values.

This equation does not always yield a commonly available capacitor size, so round to the nearest 
available size. If the value is between two different sizes, choose the higher value for better stability. 
Choosing the lower size may seem attractive for acquisition time improvement, but the PLL can become 
unstable. Also, always choose a capacitor with a tight tolerance (

±20 percent or better) and low 

dissipation.

The actual acquisition and lock times can be calculated using the equations here. These equations yield 
nominal values under these conditions:

•

Correct selection of filter capacitor, C

F

, see 

•

Room temperature operation

•

Negligible external leakage on C

GMXFC

•

Negligible noise

C

F

C

FACT

V

DDA

f

RDV

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