Common path pessimism removal (CPPR)

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Amit
Posts: 15
Joined: Sat May 31, 2014 8:52 pm

Common path pessimism removal (CPPR)

Post by Amit » Sun Jun 22, 2014 4:27 pm

What is Common path pessimism removal (CPPR)? Why it is required?

arushi
Posts: 5
Joined: Wed Apr 09, 2014 11:59 am

Re: Common path pessimism removal (CPPR)

Post by arushi » Sun Aug 17, 2014 7:47 pm

Consider
the logic shown in Figure attached where the PVT conditions can vary along
the chip. The worst condition for setup check occurs when the launch clock
path and the data path have the OCV conditions which result in the largest
delays, while the capture clock path has the OCV conditions which result
in the smallest delays.
From the figure,
LaunchClockPath = 1.2 + 0.8 = 2.0
MaxDataPath = 5.2
CaptureClockPath = 1.2 + 0.86 = 2.06
Tsetup_UFF1 = 0.35

e.g We apply the following derating to the example of Figure attached
set_timing_derate -early 0.9
set_timing_derate -late 1.2


With these derating values, we get the following for setup
check:
LaunchClockPath = 2.0 * 1.2 = 2.4
MaxDataPath = 5.2 * 1.2 = 6.24
CaptureClockPath = 2.06 * 0.9 = 1.854
Tsetup_UFF1 = 0.35 * 1.1 = 0.385
In the setup check above, there is a discrepancy since the common clock path
(Figure attached) of the clock tree, with a delay of 1.2ns, is being derated differently
for the launch clock and for the capture clock. This part of the clock
tree is common to both the launch clock and the capture clock and should
not be derated differently. Applying different derating for the launch and
capture clock is overly pessimistic as in reality this part of the clock tree
will really be at only one PVT condition, either as a maximum path or as a
minimum path (or anything in between) but never both at the same time.
The pessimism caused by different derating factors applied on the common
part of the clock tree is called Common Path Pessimism (CPP) which
should be removed during the analysis. CPPR, which stands for Common
Path Pessimism Removal,
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Amit
Posts: 15
Joined: Sat May 31, 2014 8:52 pm

Re: Common path pessimism removal (CPPR)

Post by Amit » Sun Aug 17, 2014 9:49 pm

Thanks Aru ! For detail explantion.

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