MPC972 MPC973

MOTOROLA

TIMING SOLUTIONS

BR1333 — Rev 6

10

Using the On–Board Crystal Oscillator

The MPC972 features an on–board crystal oscillator to

allow for seed clock generation as well as final distribution.

The on–board oscillator is completely 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 MPC972

as possible to avoid any board level parasitics. To facilitate

co–location surface mount crystals are recommended, but

not required.

The oscillator circuit is a series resonant circuit as

opposed to the more common parallel resonant circuit, this

eliminates the need for large on–board capacitors. Because

the design is a series resonant design for the optimum

frequency accuracy a series resonant crystal should be used

(see specification table below). Unfortunately most of the

shelf crystals are characterized in a parallel resonant mode.

However a parallel resonant crystal is physically no different

than a series resonant crystal, a parallel resonant crystal is

simply a crystal which has been characterized in its parallel

resonant mode. Therefore in the majority of cases a parallel

specified crystal can be used with the MPC972 with just a

minor frequency error due to the actual series resonant

frequency of the parallel resonant specified crystal. Typically

a parallel specified crystal used in a series resonant mode

will exhibit an oscillatory frequency a few hundred ppm lower

than the spec ified value. For mos t proc e s s o r

implementations a few hundred ppm translates into kHz

inaccuracies, a level which does not represent a major issue.

Table 3. 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 Max

Correlation Drive Level

100

µW

Aging

5ppm/Yr (First 3 Years)

* See accompanying text for series versus parallel resonant

discussion.

The MPC972/973 is a clock driver which was designed to

generate outputs with programmable frequency relationships

and not a synthesizer with a fixed input frequency. As a result

the crystal input frequency is a function of the desired output

frequency. For a design which utilizes the external feedback

to the PLL the selection of the crystal frequency is straight

forward; simply chose a crystal which is equal in frequency to

the fed back signal.

Power Supply Filtering

The MPC972/973 is a mixed analog/digital product and

exhibits some sensitivities that would not necessarily be seen

on a fully digital product. Analog circuitry is naturally

susceptible to random noise, especially if this noise is seen

on the power supply pins. The MPC972/973 provides

separate power supplies for the output buffers (VCCO) and

the internal PLL (VCCA) of the device. The purpose of this

design technique is to try and isolate the high switching noise

digital outputs from the relatively sensitive internal analog

phase–locked loop. In a controlled environment such as an

evaluation board this level of isolation is sufficient. However,

in a digital system environment where it is more difficult to

minimize noise on the power supplies a second level of

isolation may be required. The simplest form of isolation is a

power supply filter on the VCCA pin for the MPC972/973.

Figure 12. Power Supply Filter

VCCA

VCC

MPC972/973

0.01

µF

22

µF

0.01

µF

3.3V

RS=5–10Ω

Figure 12 illustrates a typical power supply filter scheme.

The MPC972/973 is most susceptible to noise with spectral

content in the 1KHz to 1MHz range. Therefore the filter

should be designed to target this range. The key parameter

that needs to be met in the final filter design is the DC voltage

drop that will be seen between the VCC supply and the VCCA

pin of the MPC972/973. From the data sheet the IVCCA

current (the current sourced through the VCCA pin) is

typically 15mA (20mA maximum), assuming that a minimum

of 3.0V must be maintained on the VCCA pin very little DC

voltage drop can be tolerated when a 3.3V VCC supply is

used. The resistor shown in Figure 12 must have a

resistance of 5–10

Ω to meet the voltage drop criteria. The RC

filter pictured will provide a broadband filter with

approximately 100:1 attenuation for noise whose spectral

content is above 20KHz. As the noise frequency crosses the

series resonant point of an individual capacitor it’s overall

impedance begins to look inductive and thus increases with

increasing frequency. The parallel capacitor combination

shown ensures that a low impedance path to ground exists

for frequencies well above the bandwidth of the PLL.

Although the MPC972/973 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 schemes discussed in this section should be