Freescale Semiconductor MC68HC08KH12 manual 111

8.9.3 Choosing a Filter Capacitor

As described in 8.9.2 Parametric Influences on Reaction Time, the external filter capacitor, CF, 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:

 VDDA

CF = CFACT ------------

fRDV

For the value of VDDA, 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 may become unstable. Also, always choose a capacitor with a tight tolerance (±20 percent or better) and low dissipation.

8.9.4 Reaction Time Calculation

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

• Correct selection of filter capacitor, CF (See 8.9.3 Choosing a Filter Capacitor.)

•Room temperature operation

•Negligible external leakage on CGMXFC

•Negligible noise

The K factor in the equations is derived from internal PLL parameters. KACQ is the K factor when the PLL is configured in acquisition mode, and KTRK is the K factor when the PLL is configured in tracking mode. (See 8.4.4 Acquisition and Tracking Modes.) Reaction time is based on

MC68HC(7)08KH12 — Rev. 1.1	Advance Information
Freescale Semiconductor	111