What is the effect of computing pwr without setting toggle rate?

Hi,

I am using design compiler to estimate the power of a design. Switching activity is annotated by setting only the static probability value on all primary inputs and leaving design compiler to compute the toggle rate to itself. Is this the right way to estimate pwr of a design.

What is the effect of computing pwr without setting toggle rate and only setting the static probability.

Is it necessary to annotate both the static probability and toggle rate as part of annotating switching activity.

thx

Nikhil

toggle rate power

The best way to estimate power is to annotate switching activity from a netlist simulation of a design, into design compiler. The switching activity can be captured using vcd which can be converted into 'saif', so that the design compiler can use it.
Static probabilities will give you a power figure but it will be less accurate than using saif annotation.
Here is a tutorial explaining how to do power analysis using saif:
http://www.vlsiip.com/power

saif toggle rate

There is a default value for both the toggle rate and static probability. THe tool will use that

power_default_toggle_rate = "0.1"
power_default_static_probability = "0.5"


-cheers
vlsi_eda_guy

toggle rate analysis

Dear vlsi_eda_guy,

In reference to pwr estimation of a design by annotating only static probability and not toggle rate as part of switching activity annotation,

If we annotate the switching activity with only static probability by using the following cmd,

set_switching_activity [remove_from_collection [all_inputs] clk_p] -static_probability 0.5 -clock clk

(here clk_p is the clock port.)

then toggle rate is calculated as default toggle rate value/period of related clok i.e. 0.1/5 = 0.02.

How to compute toggle rate based only on static probability.

As we know toggle rate can be calculated from static probability [sp] as 2*sp*(1-sp).

Now, how to force power compiler to use the toggle rate value calculated from the annotated static probability.

One more thing, if we say
set_switching_activity [remove_from_collection [all_inputs] clk_p] -static_probability 0.5 -clk clk

Can static probability be propagated by setting static probability as part of set_switching_activity cmd.

thx

S. Nikhil