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INITIALIZATION AND SYSTEM REQUIREMENTS
12.6.7
High Frequency Design Considerations
At high signal frequencies and/or with fast edge rates, the transmission line properties of signal
paths in a circuit must be considered. Transmission line effects and crosstalk become significant in
comparison to the signals. These errors can be transient and therefore difficult to debug. In this
section, some high-frequency design issues are discussed; for more information, consult a
reference on high-frequency design.
12.6.8
Line Termination
Input voltage level violations are usually due to voltage spikes that raise input voltage levels above
the maximum limit (overshoot) and below the minimum limit (undershoot). These voltage levels can
cause excess current on input gates, resulting in permanent damage to the device. Even if no damage
occurs, many devices are not guaranteed to function as specified if input voltage levels are exceeded.
Signal lines are terminated to minimize signal reflections and prevent overshoot and undershoot.
Terminate the line if the round-trip signal path delay is greater than signal rise or fall time. If the
line is not terminated, the signal reaches its high or low level before reflections have time to
dissipate and overshoot or undershoot occurs.
For the i960 Jx processor, two termination methods are attractive: AC and series. An AC
termination matches the impedance of the trace, there by eliminating reflections due to the
impedance mismatch.
Series termination decreases current flow in the signal path by adding a series resistor as shown in
Figure
12-11. The resistor increases signal rise and fall times so that the change in current occurs
over a longer period of time. Because the amount of voltage overshoot and undershoot depends on
the change in current over time (V = L di/dt), the increased time reduces overshoot and
undershoot. Place the series resistor as close as possible to the signal source. AC termination is
effective in reducing signal reflection (ringing). This termination is accomplished by adding an
RC combination at the signal's farthest destination
no DC load, the RC combination damps signal transients.
Selection of termination methods and values is dependent upon many variables, such as output
buffer impedance, board trace impedance and input impedance.
12-38
(Figure
12-12). While the termination provides
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