The possible strengths are strong (s), resistive (r), high impedance (z), and initial unknown (?), in order by their strength, from strongest to weakest. The possible logic levels are high (1), low (0), and indeterminate (x). B2.Spice A/D supports 3 signal levels and 4 signal strengths, giving a total of 12 different values. In high-speed digital circuits, where state changes may occur very fast, the accumulated propagation delays may create serious operational problems.Įach wire or each device pin has both a logic level & a strength that together make its value at any instant of time. As you connect and cascade many digital devices, these propagation delays add up and amount to a sizable delay time. Digital devices may have different propagation delays for change of state from low to high (TPLH) or change of state from high to low (TPHL). It takes a finite amount of time for the device to react to the state change. When the input pin of a digital device changes its state, for example, from low to high, the device’s output pin does not change its state instantaneously. The binary states of 1 and 0 indeed represent high and low voltages (typically 3V/5V and 0V), respectively, at the pins of a digital device. Or you can initiate a digital source and define a preset binary data sequence as your input, and then run a transient analysis of your digital circuit over the duration of your data sequence. It is recommended to use a time step of 20ns, which is typical for many commercially available digital devices. You can define an arbitrary time step for your live digital simulation and increment the simulation time discretely, one step at a time, and manually change the state of the input(s) at each time step. B2.Spice’s digital simulator then determines the state of the wires and other device pins including the digital outputs.Ī digital circuit simulation is an event-driven, time-domain simulation. You control and set the state of the input devices. Just like device pins, the wires, too, have a digital state at any instant of time. The input/output ports and devices are connected through wires. In contrast, digital devices and circuits operate in a discrete domain, where the device pins and circuit nodes have a binary state of either HIGH (1) or LOW (0) at any given instant of time.Ī digital circuit must have at least a digital input and a digital output with one or more digital devices connected in between. These quantities can be varied continuously as a function of time. 4 Setting Up Digital Circuit SimulationsĪn Overview of Digital Circuit SimulationĪnalog devices and circuits operate in a continuous domain, where node voltages and branch currents can take arbitrary (positive or negative) values.3 How Does the Digital Simulation Engine Work?.2 Wire and Pin Values (Level & Strength).1 An Overview of Digital Circuit Simulation.
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