Trace Routing
Generic Trace routing considerations for final layout are described in the LAN On Motherboard
(LOM) Design guide Application Note (AP-391). 82559 specific layout information can be found
in the 82559 Printed Circuit Board (PCB) Design Application Note (AP-399). Critical layout issues
are covered in the following section for completeness. Critical signal traces should be kept as short
as possible to decrease the likelihood of being affected by high frequency noise from other signals,
including those propagated through power and ground planes. Capacitive loading, which is caused
by the signal trace, can also be reduced by keeping the traces as short as possible. Maximum separation
between differential pairs should be no more than one tenth of an inch as illustrated in the
figure below.
6.5 Signal Terminations
A single 100 W (1%) resistor is used to terminate the transmit differential (TDP/TDN) pair. For the
receive differential (RDP/RDN) pairs, a 120 W (1%) resistor was utilized. The 120 W resistor was
used in the layout of this design because of the improvements in receive performance, the designer
may want to experiment with values from 100 W to 120 W on the receive side. They should be
placed as close to the 82559 as possible.
6.5.1 Termination Plane
Resistors are used to terminate noise from the unused inputs of both the RJ45 connector and the
magnetics module to the termination plane. The netname TERMPLANE (for termination plane) is
provided as a guide to the termination plane. A termplane is a plane fabricated into the printed
circuit board (PCB) substrate. This plane, which has no DC termination, acts like a capacitive path
for the coupled noise.
Termination Plane Capacitance
It is recommended that the termination plane capacitance equal a minimum value of 1500 pF. This
helps reduce the amount of cross-talk on the differential pairs (TDP/TDN and RDP/RDN) from the
unused pairs of the RJ45. Pads may be placed for additional capacitance, which may be required
due to failure of electrical fast transient testing. If 1500pf is not achievable, then the series capacitor
may need to be populated.
6.6 Critical Dimensions
There are two critical dimensions that must be considered during the layout phase of an 82559
LOM implementation. These dimensions are identified in Figure 2 as A and B:
Distance A: Magnetics to RJ45 (Priority 1)
The distance labeled “A” in Figure 2 should be given the highest priority in board layout. The
distance between the magnetics module and the RJ45 connector should be kept to less than one
inch of separation. The following trace characteristics are important and should be observed:
1. Differential Impedance: The differential impedance should be 100 W. The single ended trace
impedance will be approximately 50 W; however, the differential impedance can also be
affected by the spacing between the traces.
2. Trace Symmetry: Differential pairs (such as TDP and TDN) should be routed with consistent
separation and with exactly the same lengths and physical dimensions (for example, width).
Caution: Asymmetrical and unequal length traces in the differential pairs contribute to common mode noise.
This can degrade the receive circuit’s performance and contribute to radiated emissions from the
transmit circuit.
If the 82559 must be placed further than a couple of inches from the RJ45 connector, distance B
can be sacrificed. Keeping distance A as short as possible should be a priority.
Distance B: PHY to Magnetics (Priority 2)
Distance B from Figure 2 should also be designed to extend less than one inch between devices.
The high speed nature of the signals propagating through these traces requires that the distance
between these components are closely observed. In general, any section of traces that is intended
for use with high speed signals should observe proper termination practices.
Proper termination of signals can reduce reflections caused by impedance mismatches between
devices and traces. The reflections of a signal may have a high frequency component that may
contribute more EMI than the original signal itself. For this reason, these traces should be designed
to a 100 W differential value