EMBEDDED Intel486™ PROCESSOR HARDWARE REFERENCE MANUAL
10-22
determine the capacitor value. If the value of RC time constant is small, the RC circuit acts as an
edge generator and creates overshoot and undershoot. Increasing the capacitor value reduces the
overshoot and undershoot, but it increases power consumption. As a rule of thumb, the RC time
constant should be greater than twice the delay line. The power dissipation of the AC termination
is a function of the frequency.
Active Termination
These terminations consist of resistors that are connected between the inputs and outputs of a
buffer driver as shown in Figure 10-17.
Figure 10-17. Active Termination
The main advantage of this technique is that it can tolerate large impedance variations and this
tolerance is valuable when three-state drivers are connected to backplane buses. However, the ter-
minations are costly, and the signals that are produced are not as clean as other terminations. A
common solution is to place active terminations at both ends of the bus. This helps to maintain
the uniform drive levels along the entire length of the bus, and it reduces EMI and ringing.
Table 10-1 shows the comparisons of different termination techniques.
Beyond matching impedances, there are other techniques that can help avoid reflections. These
are discussed in the following sections.
Table 10-1. Comparison of Various Termination Techniques
Termination
# of Extra
Components
R
L
Power Consumption Prop Delay
Series 1 Z
0
- Z
OUT
Low Yes
Parallel 1 Z
0
High No
Thevenin 2 2Z
0
High No
AC
†
22Z
0
Medium No
Active 1 2Z
0
Medium No
PCBs in
backplane
Connectors
One line of backplane bus
Active Termination Active Termination
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