Differential Clocking - Intel Pentium M Processor Design Manual

® ®
Intel Pentium M Processor and Intel
High-Speed Design Concerns
Figure 151. Impact of Spread Spectrum Clocking on Radiated Emissions
SCC
12.5.2

Differential Clocking

Differential clocking requires that the clock generator supply both clock and clock-bar traces.
Clock-bar has equal and opposite current as the primary clock, and is also 180 degrees out of
phase. To maximize the benefit of differential clocking, both clock lines must be routed parallel to
each other for their entire length. Devices connected to the clock must also be designed to accept
both the clock and clock-bar signals.
EMI reduction due to differential clocking is caused by H-field cancellation. Since H-field
orientation is generated by and is dependent upon current flow, two equal currents flowing in
opposite directions and 180 degrees out of phase may have their H-fields cancelled (see
Figure 152). Lower H-fields may result in reduced EMI radiation.
Figure 152. Cancellation of H-fields Through Inverse Currents
H-field
caused by
Iclk
220
®
E7501 Chipset Platform
Δ
(1-δ)f
nom
f
nom
CLK - clock trace
Iclk
Iclk'
non-SSC
CLK' - clock bar
trace
H-field
caused by
Iclk'
Design Guide