Negative gate delay - is it possible

As discussed in our post ‘propagation delay’, the difference in time from the input reaching 50% of the final value of the transition to that of the output is termed as propagation delay. It seems a bit absurd to have negative value of propagation delay as it provides a misinterpretation of the effect happening before the cause. Common sense says that the output should only change after input. However, under certain special cases, it is possible to have negative delay. In most of such cases, we have one or more of the following conditions:

i)                    A high drive strength transistor

ii)                   Slow transition at the input

iii)                 Small load at the output

Under all of the above mentioned conditions, the output is expected to transition faster than the input signal, and can result in negative propagation delay. An example negative delay scenario is shown in the figure below. The output signal starts to change only after the input signal; however, the faster transition of the output signal causes it to attain 50% level before input signal, thus, resulting in negative propagation delay. In other words, negative delay is a relative concept.

Figure 1: Input and output transitions showing negative input delay

Also read:

• Race conditions
• Why clock gating checks are needed
• Worst slew propagation
• Can a net have negative propagation delay