A digital circuit must have at least a digital input and a digital output with one or more digital devices connected in between. The input/output ports and devices are connected through wires. Just like device pins, the wires, too, have a digital state at any instant of time. You control and set the state of the input devices. B2.Spice’s digital simulator then determines the state of the wires and other device pins including the digital outputs.
A digital circuit simulation is an event-driven, time-domain simulation. You can define an arbitrary time step for your live [[Digital Simulation|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. It is recommended to use a time step of 20ns, which is typical for many commercially available digital devices. 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.
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. 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. It takes a finite amount of time for the device to react to the state change. This is called the propagation delay. 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). As you connect and cascade many digital devices, these propagation delays add up and amount to a sizable delay time. In high-speed digital circuits, where state changes may occur very fast, the accumulated propagation delays may create serious operational problems.