Setup and Hold time for flip flops - Part 2

This post is in continuation to our previous post on Setup and Hold time for flip flops.

Q.1) Calculate the setup slack for the below example :

         t_clk-q = 300ps ; t_setup = 100ps ; t_combo = 400ps ; t_clk = 1000ps

Answer) 

                From the above waveform, we can derive the following equation :

                  t_clk = t_clk-q + t_combo + t_setup + setup-slack

                  So, setup-slack = t_clk – t_clk-q – t_combo  - t_setup

                  Hence, setup-slack = 1000 – 300 – 400 -200

                                          = 100ps

Q.2) Calculate the minimum clock cycle time, for the above example.

Answer) For the minimum clock cycle time, the setup slack is zero.
              Hence,  t_clk = t_clk-q + t_combo + t_setup + setup-slack becomes,
                      t_{clk (min.)} = t_clk-q + t_combo + t_setup
                      t_{clk (min.)} = 300 + 400 + 200
                      t_{clk (min.)} = 900ps 

Q.3) Calculate the hold slack for the above example. (t_hold = 100ps)

Answer) 

                From the above waveform, the equation for the hold slack comes as :
                                    hold-slack = t_clk-q + t_combo – t_hold

                             hold-slack = 300 + 400 – 100

                                          = 600ps

Q.4) Explain the  worst case hold time scenario.

Answer) The following equation is an analysis of worst case hold time scenarios. In this case, two FFs are directly connected to each other, i.e. there is no combo delay.

As depicted in the diagram, output Q0 of FF0 is directly connected to D1 of FF1.

Since, for worst case :  t_combo = 0
So the equation becomes,

                                    hold-slack = t_clk-q – t_hold

Since, hold-slack > 0 , as to avoid hold violation.

                                    t_clk-q – t_hold > 0

                                    t_clk-q > t_hold

                             hold-slack = t_clk-q + t_combo – t_hold       

For Verilog timing tasks related to setup and timing checks,  please go through this link.