# Timing Diagram problem: confused about changes from 0 to 1?

#### Gonzalo Armbrust

Joined Jun 28, 2015
16
I'm a bit confused on how to interpret a timing diagram, let's say that you have A = B = C = 0, and D is set to 1, and each inverter has a delay of 1 ns and each gate has a delay of 2ns. C changes from 0 to 1 at a certain interval, let's say it's 2ns. Since the other variables are basically left as constants, do they add extra delay to the circuit since they go through inverters when c is the only thing that changes? I have an example drawn out in an attached picture, and also my attempt to solve it:

When C changes from 0 to 1 at 2ns, and it goes through the gate, as well as considering that D is going through an inverter ( D=0 now and C=1, making F =1), does this add extra delay? Meaning instead of 2ns to go through the gate for a change, it'll take 3 ns because of the inverter that D goes through along with the gate to get F?

#### WBahn

Joined Mar 31, 2012
25,911
If a signal is static, how could it add additional delay? How do you measure when that delay would start and why would you choose that point?

Look carefully at your signal H. Does it make sense that a 3-input AND gate will be HI given the initial states of signals E, F, and G? Why does it change at 4ns when that is the time that the first of its inputs changes?

The propagation delay is the time it takes for a change in the input to a gate to effect a change in the output of that gate. So to determine the output of one of the gates (not the inverters) at time T, you look at what its inputs were 2ns earlier.

So walk ALL the signals, including the unlabeled ones, through the timing diagram and the answer should become apparent.