Here we discuss the type of power information desired from a system model. Since the objective is to find weak points in the system power flow, only power extremes are considered. Power extremes are the cross talk and signal levels obtained when the worst possible combinations of device states and input power levels are assumed. The power extremes at detection points are of particular interest, because these are the locations where design flaws are expressed through signal misinterpretation. Detection points are places where signal power is interpreted. Detection points in a system may be control points where the signal level is used to control a device; at the input to a logic gate, for example. Or they may be output points where the power level is detected and conveyed to an output subsystem as a data stream. That distinction is irrelevant for this discussion, and we will refer to detection points in both of these cases. Consider the problem of correctly interpreting a bit stream at some arbitrary detection point. Figure 1 depicts a stream of 1's and 0's arriving at a detection point. PD represents the power detection threshold of the detector: that power level below which the detector detects a 0 and above which it detects a 1. PS1 and PS2 define a safety zone around PD. They are based on the uncertainty in PD and are established so that the chance of erroneous signal interpretation is negligible. By definition, the device will operate correctly as long as all 1's arriving at the detection point have power levels greater than PS1 and all 0's arriving at the detection point have power levels less than PS2. The weakest 1 arriving at the detection point under all conditions from all possible paths to the point is defined as P1min, and similarly, the strongest 0 is defined as P0max. Proper device operation can be ensured if the following relations are met:
It is also desirable to have information about Pmax, the maximum power level that can occur at the inputs to a given device. A power detector may provide erroneous results when the power of a logic 1 arriving at a detection point is too large; that is, when Pmax exceeds PD by some large amount. A second and more important reason for computing Pmax is that it makes the major contribution to cross talk, as discussed below. Knowledge of the power triple P0max, P1min, and Pmax at each device in a system permits the tracking of power levels throughout the entire system.