Motorola MC68HC908MR32 사용자 설명서

Clock Generator Module (CGM)

Acquisition/Lock Time Specifications

MC68HC908MR16/MC68HC908MR32 — Rev. 4.0

Advance Information

 

MOTOROLA

Clock Generator Module (CGM)

 133

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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