Changes

==Propagation Delays==

The propagation delay of a digital device is the time it takes for the signal to change at the output in response to a change at the input. Each output pin of a digital device has a propagation delay associated with it. The propagation delay of a pin can be affected by the circuit's default fixed delay, the circuit's default delay class, the specific pin's overriding delay or delay class, and the device model's maximum, minimum and typical delays.

The delay actually involves two values, a rise time and a fall time. TPLH (the low-to-high propagation delay) is the time from the input change time to the output change time when the output is going from low to high. TPHL (the high-to-low propagation delay) is the time from the input change time to the output change time when the output is going from high to low. Propagation delays are important because they determine the speed of your circuit, and in circuits with feedback and flip flops and registers, propagation delays can even affect whether or not your circuit works. A digital device's delays can be accessed from its property dialog.

An example showing the delays for the 7404 inverter is shown in the figure on the right. In this case, TPLH = 8ns and TPHL = 12ns. You can see that as the input value changes from 0 to 1 at t = 20ns, the output value changes from 1 to 0 at t = 20 + TPLH = 28ns. subsequently, as the input value changes from 1 to 0 at t = 40ns, the output value changes from 0 to 1 at t = 40 + TPHL = 52ns.

Many digital parts store their states regardless of what is happening at their inputs until a specific input changes value in a specific direction. The 7474 D flip flop is such a device. Its D input can change, but the output will not change until the clock pin rises from zero to one. We call the activating change in the clock pin a "Trigger". For the trigger to work, the circuit must be stable (i.e., inputs must not change) for a specific time period before the trigger occurs. This time is called the "Setup Time". After the trigger, the inputs must be stable for a time period so that the new state is not disturbed. This post-trigger time period is called the "Hold Time". If either the set up or hold time specifications are violated, the program notifies the user and sets the device's output to indeterminate.

==Initial Circuit State==