Issue link: https://resources.pcb.cadence.com/i/1180526
PSpice User Guide Digital device modeling October 2019 406 Product Version 17.4-2019 © 1999-2019 All Rights Reserved. the flip-flops (QA, QB, QC, QD, and QDBAR). The flip-flops are primitive devices themselves and are not part of the logic expression. The outputs are the eight J-K data inputs to the flip-flops, RCO, the four data lines used internal to the logic expression (A, B, C, D), and the seven control lines: CLK, CLKBAR, EN, ENT, ENP, CLRBAR, and LOADBAR. The schematic representation of the device shows buffers on every input signal of the model, while the logic diagram of the device in the data book shows buffers or inverters on only the CLRBAR_I, CLK_I, and LOADBAR_I signals. We have added buffers to the inputs to minimize the insertion of A-to-D interfaces when the device is driven by analog circuitry. The best example is the CLK signal. With the buffer in place, if the CLK signal is analog, one A-to-D interface device will be inserted into the circuit by the simulator. If the buffer was not present, then an interface device would be inserted at the CLK pin of each of the flip-flops. The buffers have no delay associated with them, but by minimizing the number of A-to-D interfaces, we speed up the mixed-signal simulation by reducing the number of necessary calculations. For situations where the device is only connected to other digital nodes, the buffers have no effect on the simulation. The D0_GATE, shown in the listing, is a zero-delay primitive gate timing model. For most TTL modeling applications, this only serves as a place holder and is not an active part of the model. Its function has been replaced by the PINDLY primitive. The D0_GATE model can be found in the library file DIG_IO.LIB. For a more detailed description of digital primitives, see the Digital Devices chapter in the online PSpice Reference Guide. IO_STD, shown in the listing, is the standard I/O model. This determines the A-to-D and D-to-A interface characteristics for the subcircuit. The device contains family-specific information, but the models have been created for nearly all of the stock families. The various I/O models can be found in the library file DIG_IO.LIB. The logic expressions themselves are straightforward. The first nine are buffering the input signals from outside the subcircuit. The rest describe the logic of the actual device up to the flip-flops. By tracing the various paths in the design, you can derive each of the logic equations.