MC12429

TIMING SOLUTIONS

7

MOTOROLA

AC CHARACTERISTICS (TA = 0° to 70°C, VCC = 3.3V to 5.0V ±5%)

Symbol

Characteristic

Min

Max

Unit

Condition

FMAXI

Maximum Input Frequency

S_CLOCK

Xtal Oscillator

10

10

20

MHz

Note 3.

FMAXO

Maximum Output Frequency

VCO (Internal)

FOUT

200

25

400

400

MHz

Note 4.

tLOCK

Maximum PLL Lock Time

10

ms

tjitter

Period Deviation (Peak–to–Peak)

±25

ps

Note 4.,

See Applications Section

ts

Setup Time

S_DATA to S_CLOCK

S_CLOCK to S_LOAD

M, N to P_LOAD

20

20

20

ns

th

Hold Time

S_DATA to S_CLOCK

M, N to P_LOAD

20

20

ns

tpwMIN

Minimum Pulse Width

S_LOAD

P_LOAD

50

50

ns

Note 4.

tr, tf

Output Rise/Fall

FOUT

300

800

ps

20%–80%, Note 4.

3. 10MHz is the maximum frequency to load the feedback devide registers. S_CLOCK can be switched at higher frequencies when used as a

test clock in TEST_MODE 6.

4. 50

Ω to VCC – 2.0V pulldown.

APPLICATIONS INFORMATION

Using the On–Board Crystal Oscillator

The MC12429 features a fully integrated on–board crystal

oscillator to minimize system implementation costs. The

oscillator is a series resonant, multivibrator type design as

opposed to the more common parallel resonant oscillator

design. The series resonant design provides better stability

and eliminates the need for large on chip capacitors. The

oscillator is totally self contained so that the only external

component required is the crystal. As the oscillator is

somewhat sensitive to loading on its inputs the user is

advised to mount the crystal as close to the MC12429 as

possible to avoid any board level parasitics. To facilitate

co–location surface mount crystals are recommended, but

not required. Because the series resonant design is affected

by capacitive loading on the xtal terminals loading variation

introduced by crystals from different vendors could be a

potential issue. For crystals with a higher shunt capacitance it

may be required to place a resistance across the terminals to

suppress the third harmonic. Although typically not required it

is a good idea to layout the PCB with the provision of adding

this external resistor. The resistor value will typically be

between 500 and 1K

Ω.

The oscillator circuit is a series resonant circuit and thus

for optimum performance a series resonant crystal should be

used. Unfortunately most crystals are characterized in a

parallel resonant mode. Fortunately there is no physical

difference between a series resonant and a parallel resonant

crystal. The difference is purely in the way the devices are

characterized. As a result a parallel resonant crystal can be

used with the MC12429 with only a minor error in the desired

frequency. A parallel resonant mode crystal used in a series

resonant circuit will exhibit a frequency of oscillation a few

hundred ppm lower than specified, a few hundred ppm

translates to kHz inaccuracies. In a general computer

application this level of inaccuracy is immaterial. Table 1

below specifies the performance requirements of the crystals

to be used with the MC12429.

Table 1. Recommended Crystal Specifications

Parameter

Value

Crystal Cut

Fundamental AT Cut

Resonance

Series Resonance*

Frequency Tolerance

±75ppm at 25°C

Frequency/Temperature Stability

±150pm 0 to 70°C

Operating Range

0 to 70

°C

Shunt Capacitance

5–7pF

Equivalent Series Resistance (ESR)

50 to 80

Ω

Correlation Drive Level

100

µW

Aging

5ppm/Yr (First 3 Years)

* See accompanying text for series versus parallel resonant

discussion.