MC68HC908GR4MP datasheet(87/286 Pages) FREESCALE

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Acquisition/Lock Time Specifications

MC68HC908GR8 • MC68HC908GR4 Data Sheet, Rev. 7

Freescale Semiconductor

7.8.2 Parametric Influences on Reaction Time

Acquisition and lock times are designed to be as short as possible while still providing the highest possible

stability. These reaction times are not constant, however. Many factors directly and indirectly affect the

acquisition time.

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 7.3.3 PLL Circuits, 7.3.6 Programming the PLL, and

7.5.6 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 7.8.3 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.

7.8.3 Choosing a Filter

As described in 7.8.2 Parametric Influences on Reaction Time, the external filter network is critical to the

stability and reaction time of the PLL. The PLL is also dependent on reference frequency and supply

voltage.

Figure 7-10 shows two types of filter circuits. In low-cost applications, where stability and reaction time of

the PLL are not critical, the three component filter network of Figure 7-10 (B) can be replaced by a single

capacitor, CF, shown in Figure 7-10 (A). Refer to Table 7-4 for recommended filter components at various

reference frequencies. For reference frequencies between the values listed in the table, extrapolate to the

nearest common capacitor value. In general, a slightly larger capacitor provides more stability at the

expense of increased lock time.

MC68HC908GR4MP Datasheet(PDF) 87 Page - Freescale Semiconductor, Inc