Advanced Clock Drivers Devices

Freescale Semiconductor

9

MPC9239

Layout Recommendations

The MPC9239 provides sub-nanosecond output edge

rates and thus a good power supply bypassing scheme is a

must. Figure 6 shows a representative board layout for the

MPC9239. There exists many different potential board

layouts and the one pictured is but one. The important aspect

of the layout in Figure 6 is the low impedance connections

good quality general purpose chip capacitors with good PCB

layout techniques will produce effective capacitor resonances

at frequencies adequate to supply the instantaneous

switching current for the MPC9239 outputs. It is imperative

that low inductance chip capacitors are used; it is equally

important that the board layout does not introduce back all of

the inductance saved by using the leadless capacitors. Thin

interconnect traces between the capacitor and the power

plane should be avoided and multiple large vias should be

used to tie the capacitors to the buried power planes. Fat

interconnect and large vias will help to minimize layout

induced inductance and thus maximize the series resonant

point of the bypass capacitors. Note the dotted lines circling

the crystal oscillator connection to the device. The oscillator

is a series resonant circuit and the voltage amplitude across

the crystal is relatively small. It is imperative that no actively

switching signals cross under the crystal as crosstalk energy

coupled to these lines could significantly impact the jitter of

the device. Special attention should be paid to the layout of

the crystal to ensure a stable, jitter free interface between the

crystal and the on—board oscillator. Although the MPC9239

has several design features to minimize the susceptibility to

power supply noise (isolated power and grounds and fully

differential PLL), there still may be applications in which

overall performance is being degraded due to system power

supply noise. The power supply filter and bypass schemes

discussed in this section should be adequate to eliminate

power supply noise related problems in most designs.

Figure 6. PCB Board Layout Recommendation

for the PLCC28 Package

Using the On-Board Crystal Oscillator

The MPC9239 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 MPC9239 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 MPC9239 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 12

below specifies the performance requirements of the crystals

to be used with the MPC9239.

* See accompanying text for series versus parallel resonant

discussion.

1

C2

CF

XTAL

C1

C1

= GND

= Via

Table 12. Recommended Crystal Specifications

Parameter

Value

Crystal Cut

Fundamental AT Cut

Resonance

Series Resonance*

Frequency Tolerance

±75 ppm at 25°C

Frequency/Temperature Stability

±150 pm 0 to 70°C

Operating Range

0 to 70

°C

Shunt Capacitance

5-7 pF

Equivalent Series Resistance (ESR)

50 to 80

Ω

Correlation Drive Level

100

µW

Aging

5ppm/Yr (First 3 Years)