Master of Engineering Balram Sahu - Embedded Sensing ...

25 4.4. Behavioral Model#Threshold point of an output rising edge:output_threshold_pct_rise : 50.0;Typically 50% threshold is used for most standard cell libraries. The propagation delaycan be represented as:1. Output fall delay (T f ) : For example, the output of an inverter will fall if the inputis rising.2. Output rise delay (T r ) : For example, the output of an inverter will rise if the inputis falling.In practice, these two values are different and are defined separately in the library. Figure4.5 shows the definitions of these variables.Figure 4.5: Timing definitions in standard cell libraryIn timing models slew rate of waveform also plays a very important role. In practicallife, a cell is always driven by another cell which is having some slew rate (transitiondelay). In terms of exact calculation of propagation delay we should know the outputtransition delay of preceded cell. in standard cell library slew threshold setting is definedby following four variables, as shown in Figure 4.5: