5.5 Logic Level Variation

A part of the noise budget is taken up by the inability of a logic driver to provide a voltage that reaches the limits of the power supply. The limits of voltage swing are called out in the manufacturer's specifications. For a logic 1, the highest loaded voltage is specified as VOH_MAX. The lowest maximum loaded voltage is VOH_MIN. The highest logic 0 is VOL_MAX, which defines the other limit to the logic voltage swing. The difference between VOH_MIN and VOL_MAX defines an envelope that must be considered in handling a noise budget. When parallel terminations are used the terminating resistor can be made intentionally high to compensate for a lack of voltage swing. In series terminations the resistor can be lowered to accommodate for the lack of voltage swing.

Series line terminations must take into account that the driver may have a finite source impedance. The source impedance plus the series resistor must match the line impedance. If the sum of the resistances is high then the logic level at the open end of the line will be low. If the sum of the resistances is low then the logic level at the open end of the line will be high. These adjustments are sometimes needed to keep errors within the assigned budget. In fast circuits, the location of the termination resistors is critical. Ideally, series resistors should be placed at the driver. Any lead length ahead of the resistor is a short transmission line that causes reflections that in effect adds to the rise time. If there is doubt about the permitted trace length then some sort of simulation is suggested. As stated earlier, a resistor is really a short section of lossy transmission line. The characteristic impedance of the line is modified by changes to the trace geometry required to mount the resistor.

If there are routing difficulties, the design rules may have to be adjusted. Making traces narrower may increase cross talk, reduce reliability, and make it difficult to meet the characteristic impedance requirements. Meeting all performance requirements and staying within cost constraints is often a difficult engineering problem.