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PSpice User Guide Digital device modeling October 2019 405 Product Version 17.4-2019 © 1999-2019 All Rights Reserved. Digital primitives Primitives in the simulator are devices or functions which are compiled directly into the code. The primitives serve as fundamental building blocks for more complex macro models. There are two types of primitives in the simulator: gate level and behavioral. A gate level primitive normally refers to an actual physical device (such as buffers, AND gates, inverters). A behavioral primitive is not an actual physical device, but rather helps to define parameters of a higher level model. Just like gate level primitives, behavioral primitives are intrinsic functions in the simulator and are treated in much the same manner. They are included in the gate count for circuit size and cannot be described by any lower level model. In our 74160 example (see The TTL Data Book from Texas Instruments for schematic and description), the four J-K flip-flops are the four digital gate level primitives. While flip-flops are physically more complex than gates in terms of modeling, they are defined on the same level as a gate (for example, flip-flops are a basic device in the simulator). Since all four share a common Reset, Clear, and Clock signal, they can be combined into one statement as an array of flip-flops. They could just as easily have been written separately, but the array method is more compact. See the Digital Devices chapter in the online PSpice Reference Guide for more information. Logic expression (LOGICEXP primitive) Looking at the listing in 74160 example on page 412 and at the schematic representation of the 74160 subcircuit, you can see that there are three main parts to the subcircuit. Following the usual header information, .SUBCKT keyword, subcircuit name, interface pin list, and parameter list is the LOGICEXP primitive. It contains everything in the component that can be expressed in terms of simple combinational logic. The logic expression device also serves to buffer other input signals that will go to the PINDLY primitive. In this case, LOGICEXP buffers the ENP_I, ENT_I, CLK_I, CLRBAR_I, LOADBAR_I, and four data signals. See the Digital Devices chapter in the online PSpice Reference Guide for more information. For our 74160 example, the logic expression (LOGICEXP) has fourteen inputs and twenty outputs. The inputs are the nine interface input pins in the subcircuit plus five feedback signals that come from