MPC992

TIMING SOLUTIONS

BR1333 — REV 5

5

MOTOROLA

a parallel specified crystal used in a series resonant mode

will exhibit an oscillatory frequency a few hundred ppm lower

than the specified value. For most processor implementa–

tions a few hundred ppm translates into kHz inaccuracies, a

level which does not represent a major issue.

Figure 2 shows an optional series capacitor in the crystal

oscillator interface. The on–board oscillator introduces a

small phase shift

in the overall loop which causes the

oscillator to operate at a frequency slightly slower than the

specified crystal. The series capacitor is used to compensate

the loop and allow the oscillator to function at the specified

crystal frequency. If a 100ppm type error is not important, the

capacitor can be left off the PCB. For more detailed

information, order Motorola Application Note AN1579/D.

Figure 2. Recommended Crystal Interface

XTAL2

CTUNE

(Optional)

XTAL1

MPC992

Table 1. Crystal Specifications

Parameter

Value

Crystal Cut

Fundamental AT Cut

Resonance

Series Resonance*

Frequency Tolerance

±75ppm at 25°C

Frequency/Temperature Stability

±150ppm 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)

Power Supply Filtering

The MPC992 is a mixed analog/digital product and as

such it 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 MPC992 provides separate

power supplies for the digital circuitry (VCCI) 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 MPC992.

Figure 3 illustrates a typical power supply filter scheme.

The MPC992 is most susceptible to noise with spectral

content in the 10kHz 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 MPC992. 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 3 must have a resistance of 10–15

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.

Figure 3. Power Supply Filter

VCCA

VCC

MPC992

0.01

µF

22

µF

0.01

µF

3.3V

RS=10–15Ω

A higher level of attenuation can be achieved by replacing

the resistor with an appropriate valued inductor. A 1000

µH

choke will show a significant impedance at 10KHz

frequencies and above. Because of the current draw and the

voltage that must be maintained on the VCCA pin a low DC

resistance inductor is required (less than 15

Ω). Generally the

resistor/capacitor filter will be cheaper, easier to implement

and provide an adequate level of supply filtering.

The MPC992 provides sub–nanosecond output edge

rates and thus a good power supply bypassing scheme is a

must. The important aspect of the layout for the MPC992 is

low impedance connections between VCC and GND for the

bypass capacitors. Combining 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 MPC992 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