Understanding transition

What is transition 

                       It is the amount of time required to charge discharge the load capacitance  of a cell completely. 

          Every mosfet (standard cells are made of them ) can be viewed as capacitance.        A cell output capacitance is nothing but the summation of the capacitance of all the cells connected to its output in parallel + the net capacitances.

          The time required to switch ON or OFF the cell can be viewed as the time required to charge the capacitance.

            

To understand transition in an easier way

             Let us assume the standard cell or buffer as a motor which is filling up a tank which is analogous to capacitance. The time required to charge the capacitance is nothing but the time required to fill the tank completely. 

         The bigger the tank the more is the time required to fill the tank. Which means the more the capacitance, the more is the transition time or the time required to charge the capacitance. 

motor pumping water into a tank 

What is the disadvantage of having more transition ?

              A more transition would mean the motor (standard cell ) would have to spend lot of power as it has to switch on for a longer time. Which means we would be having more dynamic power consumption. 
               Having more transition would also mean that your arrival time of data increases. Hence meeting setup gets more tougher. Let us have a look at how transition is affecting setup

How to decrease transition then?

One way to decrease transition would be to increase the power of motor so that it could fill up the tank quickly. In practical on chip it is same as the increasing the drive strength of the buffer so that it could charge more capacitance quickly. 

Another way to decrease transition is to reduce the capacity of the tank that needs to be filled which is nothing but to reduce the output capacitance.  The capacitance can be reduced by inserting a buffer in the path so that it would see only have to charge the next cell and wire capacitance instead of seeing the  previous total high load.

It is like inserting another motor after certain length so that the first motor would only have to fill up small tank ( capacitance reduces because subsequent  wire capacitance reduces and load gets splitted) till the next motor and the subsequent motor would drive it from there. Now two motors share the effort and hence time required to fill the tank comes down.