Intel Pentium 4 Processor in the 423-pin package EMI Guideline

Intel® Pentium® 4 Processor in the 423-pin package EMI Guideline

CLK - clock trace

CLK' - clock bar

trace

H-field

caused by

Iclk

Iclk'

H-field

caused by

Iclk'

Figure 3. Cancellation of H-fields Through Inverse Currents

Differential clocking can also reduce the amount of noise coupled to other traces, which improves

signal quality and reduces EMI. I/O signals are particularly important because they often leave the

system chassis (serial and parallel ports, keyboards, mouse, etc) and will radiate noise that has been

induced onto them. A single-ended clock’s return path is usually a reference plane, which is shared by

other signals/traces. When noise is created on a single-ended clock, the noise will appear on the

reference plane and may be coupled to I/O traces. A differential clock’s return path is the clock-bar

signal/trace, which is more isolated than the reference plane and minimizes potential I/O trace

coupling.

For best results, the trace lengths and routing of the clock lines must be closely matched and spacing

between the two traces should be kept as small as possible. This will minimize loop area and maximize

H-field cancellation. In addition, the real and parasitic terminations of each signal of a differential pair

should be the same. Also, the skew between the signal level transitions on the two lines must be small

compared to the rise time of the level transitions.

Placing ground traces on the outside of the differential pair may further reduce emissions. Intermediate

vias to ground may be needed to reduce the opportunity for re-radiation from the ground traces

themselves. Distance between vias should be less than ¼ of a wavelength of the 5

harmonic of the

processor core frequency.

2.3

PCI Bus Clock Control

Experimental data has indicated a reduction in EMI may be possible by disabling the clocks to unused

(and therefor unterminated) PCI slots. CK00, the clock chip that has been specified and designed for

Pentium® 4 processors, supports individual control of the various PCI clocks. Designers have the

option to enable or disable individual PCI clocks depending upon their specific system configuration

requirements. Refer to the CK00 Clock Synthesizer Design Guidelines for details on how to configure

the PCI clocks.

2.4

Heatsink Effects

Heatsink grounding may be an effective way to reduce system EMI emissions. Noise coupled from the

processor package to the heatsink may cause it to act as an antenna and re-radiate the noise. Heatsink

size, shape, fin pattern, orientation and material may all impact its ability to reradiate the high