CRPR problem with explanation - STA

by kamalnadh


Problem:- For the below problem time period=6ns,setup of FF2=0.2 . Solve the problem with out CRPR and with CRPR.

Let

•   FF1 and FF2 are flip flops.
•  A,B,C are the buffers.
•  C C is the combinational circuit.

In the above fig the buffers,flip flops,combinational circuit have two values.One is actual delay(min value) another is delay after adding derating(max value).

           To work the chip in any condition we are adding derating in the process of OCV.But adding more derating will also effect the speed of the chip.so by using CRPR method we are removing the derating for the common path.In the above fig A buffer is the common point for B and C buffer.

Without CRPR:-

Setup slack:-
               For calculating setup take arrival time is maximum value and required time is minimum value.
    Arrival time = buffer A delay(max value)+buffer B                                                   dealy(max value)+FF 1 delay(max value)+
                             CC delay(max value)
                          = 0.8+0.6+0.6+3.20
                          =5.20ns
   Requited time = buffer A delay(min value)+buffer C delay(min                                  value)+time period-setup of ff2
                           = 0.64+0.52+6-0.2
                           = 6.96ns
   Setup slack = Required time - Arrival time
                       = 6.96-5.20
                       = 1.76ns

Hold slack:-
                It is opposite to setup time.For calculating hold take arrival time is minimum value and required time is maximum value.

   Arrival time = buffer A delay(min value)+buffer B                                                   dealy(min value)+FF 1 delay(min value)+
                             CC delay(min value)
                         = 0.64+0.48+0.48+2.56
                         = 4.16ns
    Requited time = buffer A delay(max value)+buffer C delay(max                                  value)+setup of ff2+time period
                           = 0.8+0.65+0.2+0(for hold time period is 0)
                           = 1.65ns
     Hold time = Arrival time - required time
                      = 4.16-1.65
                      = 2.51ns

With CRPR:-
             In CRPR process we are removing the derating to common buffer.here the common buffer is A.so we are considering only one value i.e 0.64(min value).

Setup slack:-
              For calculating setup take arrival time is maximum value and required time is minimum value.except A buffer.take only min value of A buffer.
    Arrival time = Buffer A delay(min value)+buffer B                                                   delay(max value)+FF 1 delay(max value)+
                             CC delay(max value)
                         = 0.64+0.6+0.6+3.2
                         = 5.04ns

Requited time = Buffer A delay(min value)+buffer C delay(min                                  value)+time period-setup of ff2
                        = 0.64+0.52+6-0.2
                        = 6.96ns
Setup slack = Required time - Arrival time
                   = 6.96-5.04
                   = 1.92ns

Hold slack:-
                 It is opposite to setup time.For calculating hold take arrival time is minimum value and required time is maximum value.Except A buffer ,take only min value of A buffer.

   Arrival time = Buffer A delay(min value)+buffer B                                                   dealy(min value)+FF 1 delay(min value)+
                             CC delay(min value)
                        = 0.64+0.48+0.48+2.56
                        = 4.16ns
   Requited time = Buffer A delay(min value)+buffer C delay(max                                  value)+setup of ff 2+time period
                        = 0.64+0.65+0.2+0
                        = 1.49ns
    Hold slack = Arrival time - required time
                      = 4.16-1.49
                      = 2.67ns

Without CRPR the setup and hold values are:- 1.76ns and 2.51ns
With CRPR the setup and hold values are :- 1.92ns and 2.67ns
         
          From the above results it is clear that with the CRPR method both setup and hold is benefited.