MC68HC908AP16CFB datasheet(126/440 Pages) MOTOROLA

126 / 440 page

Clock Generator Module (CGM)

Data Sheet

MC68HC908AP Family — Rev. 2.5

126

MOTOROLA

8.8 Acquisition/Lock Time Specifications

The acquisition and lock times of the PLL are, in many applications, the

most critical PLL design parameters. Proper design and use of the PLL

ensures the highest stability and lowest acquisition/lock times.

8.8.1 Acquisition/Lock Time Definitions

Typical control systems refer to the acquisition time or lock time as the

reaction time, within specified tolerances, of the system to a step input.

In a PLL, the step input occurs when the PLL is turned on or when it

suffers a noise hit. The tolerance is usually specified as a percent of the

step input or when the output settles to the desired value plus or minus

a percent of the frequency change. Therefore, the reaction time is

constant in this definition, regardless of the size of the step input. For

example, consider a system with a 5 percent acquisition time tolerance.

If a command instructs the system to change from 0Hz to 1MHz, the

acquisition time is the time taken for the frequency to reach

1MHz

±50kHz. 50kHz = 5% of the 1MHz step input. If the system is

operating at 1MHz and suffers a –100kHz noise hit, the acquisition time

is the time taken to return from 900kHz to 1MHz

±5kHz. 5kHz = 5% of

the 100kHz step input.

Other systems refer to acquisition and lock times as the time the system

takes to reduce the error between the actual output and the desired

output to within specified tolerances. Therefore, the acquisition or lock

time varies according to the original error in the output. Minor errors may

not even be registered. Typical PLL applications prefer to use this

definition because the system requires the output frequency to be within

a certain tolerance of the desired frequency regardless of the size of the

initial error.

8.8.2 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.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MC68HC908AP16CFB Datasheet(PDF) 126 Page - Motorola, Inc

MC68HC908AP16CFB Datasheet(HTML) 126 Page - Motorola, Inc

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

Datasheet Download

Link URL