Datasheet

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DP83865
5.0 Design Guide (Continued)
5.6 Layout Notes on MAC Interface
Trace Impedance
All the signal traces of MII and GMII should be impedance
controlled. The trace impedance reference to ground is 50
Ohms. Uncontrolled impedance runs and stubs should be
kept to minimum.
5.6.1 MII, GMII, and RGMII Interfaces
MII and GMII are single ended signals. The output of these
signals are capable of driving 35 pF under worst condi-
tions. However, these outputs are not designed to drive
multiple loads, connectors, backplanes, or cables.
Termination Requirement
The purpose of the series termination is to reduce reflec-
tions and to improve the signal quality. The board designer
should evaluate the reflection and signal integrity to deter-
mine the need for the termination in each design. As a gen-
eral rule, if the trace length is less than 1/6 of the
equivalent length of the rise and fall times, the series termi-
nation is not needed. The following is an example of calcu-
lating the signal trace length.
The rise and fall times of GMII are in the order of 500 ps for
RX_CLK, and GTX_CLK. Propagation Delay = 170 ps/inch
on a FR4 board. Equivalent length of rise time = (1/6) Rise
time (ps) / Delay (ps/inch) = (1/6) *(500/ 170) = 0.5 inch.
Thus, series termination is not needed for traces less than
0.5 inch long.
The value of the series termination depends on the driver
output impedance and the characteristic impedance of the
PCB trace. Termination value Rs = characteristic imped-
ance Zo - driver output impedance Ro.
5.7 Twisted Pair Interface
The Twisted Pair Interface consists of four differential
media dependent I/O pairs (MDI_A, MDI_B, MDI_C, and
MDI_D). Each signal is terminated with a 49.9 Ω resistor.
Figure 18 shows a typical connection for channel A. The
circuitry of channels A, B, C, and D are identical. The MDI
signals are directly connect to 1:1 magnetics. To optimize
the performance, National specifies the key parameters for
the magnetics. Please refer to Section 5.13.2.
The following is a layout guide line for the MDI section.
— Place the 49.9 Ω 1% termination resistors as close as
possible to the PHY. Place a 0.01 µF decoupling capac-
itor for each channel between 2.5V plane and ground
close to the termination resistor. Place a 0.01 µF decou-
pling capacitor for each port at the transformer center
tab.
— All the MDI interface traces should have a charateristic
impedance of 50 Ohms to the GND or 2.5V plane. This
is a strict requirement to minimize return loss.
— Each MDI pair should be placed as close as possible in
parallel to minimmize EMI and crosstalk. Each member
of a pair should be matched in length to prevent mis-
match in delay that would cause common mode noise.
— Ideally there should be no crossover or via on the signal
paths.
Figure 17. Signal crossing a plane split
Do NOT cross plane split
GND or power plane
Figure 18. Twisted Pair/Magnetics interface (Channel A)
1000 pF
75 Ω
V
DDA
= 2.5 V
TD4-
TCT4
TD4+
MX4+
MX4-
MCT4
MDI_A+
MDI_A-
A+
A-
B+
B-
C+
C-
D+
D-
1
2
3
6
4
5
7
8
Chassis Ground
3 kV
PULSE H-5007
RJ-45
49.9 Ω
49.9 Ω
DP83865
V
DDA
= 2.5 V
0.01 uF
0.01 uF
75 Ω
MCT1
Circuit Ground
50-Ohm controlled impedance with respect to VDD or GND
50-Ohm controlled impedance with respect to chassis GND