Changes

Digital circuit simulation is a time domain simulation. Furthermore, it is an event-driven simulation. It means that the simulation engine waits for changes in the input(s) of the circuit and then updates the digital state of various nodes in response to those state changes. You can run a time domain simulation in three different modes: '''Step''', '''Walk''' or '''Run'''. "Stepping" increments time a single time step at a time. "Walking" is faster than stepping and increments time by multiple steps at a time determined using a "Walk Factor". "Running" is equivalent to the analog live simulation and continues indefinitely until you pause it or stop it and reset. The first thing you always do in a [[Digital Simulation|digital simulation]] is to set the time step size.

Set the "'''Time Step'''" to 20n, leave the "'''Walk Factor'''" at the default value of 2 and close the dialog. Now start the [[Digital Simulation|digital simulation]] by "Stepping" your digital circuit. Click the "'''Step'''" [[File:b2Step_Tool.png]] button of the '''[[Main toolbar|Main Toolbar]]''' or select "Step" from the "'''Simulate Menu'''" or simply use the keyboard shortcut {{key|Ctrl+H}}. The simulation time will increase to 20ns, and the output’s value will change to 0. Note that all your three inputs have initial values of 0. The state of your circuit is shown in the figure below on the left. Next, change the value of Input 1 to 1 and leave the Input 2 and Input 3 at 0. During a live simulation, you can change the values of inputs by clicking on their {{key|Up Arrow}} and {{key|Down Arrow}} buttons at their right end. Note that by changing the input values, the output doesn't change immediately. This is because the engine is waiting for you to step the time to compute the next state of the circuit. Click the "'''Step'''" [[File:b2Step_Tool.png]] button or type {{key|Ctrl+H}} one more time. The simulation time increases to 40ns, but the output’s value stays at 0. This is expected as a three-input AND circuit requires all three inputs to be 1 to have an output of 1. The state of the circuit at t = 40s is shown below in the middle figure. Finally, change the values of Input 2 and Input 3 to 1. Step the simulation to 60ns. This time, the output changes to 1 as shown below in the figure on the right. Note that at each time step, the state of all the wires including the output of the NAND gate is displayed on the circuit. The binary states are shown in small black box labels. You can also tell the state of a wire from its color. Blue is 0 and red is 1.