EMBEDDED Intel486™ PROCESSOR HARDWARE REFERENCE MANUAL
10-18
There are several techniques which can be employed to further minimize the effects caused by an
impedance mismatch during the layout process:
1.
Impedance matching.
2.
Daisy chaining.
3.
Avoidance of 90° corners.
4.
Minimization of the number of vias.
10.3.2.1
Impedance Matching
Impedance matching is the process of matching the impedance of the source or load with that of
the trace and it is accomplished with a technique called termination. The reflection, overshoot and
undershoot of signals are reduced by terminating the remote end of the transmission line from the
source. The terminating impedance combines with the destination input circuitry to produce a
load that closely matches the characteristic impedance of the line (board traces have characteristic
impedances in the range of 30 ohms to 200 ohms).
The calculation of characteristic impedance was already discussed. Impedance of the printed cir-
cuit board backplane connectors have the impedance in the same range as the traces (i.e., 30–200
ohms).
Depending upon the length of the conductors or when using twisted pairs of coaxial cable in place
of printed circuit traces, the characteristic impedance of a backplane may change. Backplane im-
pedance is also affected by the number of boards plugged into the backplane.
Need for Termination
The transmission line should be terminated when the t
pd
exceeds one-third of t
r
(risetime). If the
t
pd
≥
1/3 t
r
(rise time), the line can be left un-terminated, provided the capacitive coupling be-
tween the traces does not cause electromagnetic interference.
Termination thus eliminates impedance mismatches, increases noise immunity, suppresses
RFI/EMI and helps to ensure that signals reach their destination with minimum distortion. There
are five methods for terminating traces on the board:
1.
Series
2.
Parallel
3.
Thevenin
4.
AC
5.
Active
Terminations usually cost money, because they require additional components and power. In the
case of passive terminations, extra drivers are needed to deliver more current to the line. In case
of active terminations extra power is needed, which increases the power dissipation of the system.
Series Termination
One way of controlling ringing on longer lines is with the series termination technique also
known as damping. This is accomplished by placing a resistor in series with the transmission line
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