A synthesis question about "high-fanout nets"

[walter]

Hi All,

I am not sure if this is the right place to post the title. But I will
be very appreciated if any one can help.

When synthesis with Synopsys Design Compiler, I got the message :

Warning: Design 'ooo' contains 1 high-fanout nets. A fanout number of
1000 will be used for delay calculations involving these nets.
(TIM-134)

1. Is the warning quite important?
2. How can I identify the high-fanout nets?
3. How can I fix the warning?
I guess it might be reset or clock signal.
So I use
set_drive 0
set_dont_touch_network
on all clocks and reset
But still got the warning.

 

[Uwe Bonnes]

walter <cuteworm@wildmail.com> wrote:
: Hi All,

: I am not sure if this is the right place to post the title. But I will
: be very appreciated if any one can help.

: When synthesis with Synopsys Design Compiler, I got the message :

: Warning: Design 'ooo' contains 1 high-fanout nets. A fanout number of
: 1000 will be used for delay calculations involving these nets.
: (TIM-134)

: 1. Is the warning quite important?

For a high speed design mostly.

: 2. How can I identify the high-fanout nets?

Look at your design, look at the syntesis report and logs.

: 3. How can I fix the warning?
: I guess it might be reset or clock signal.
: So I use
: set_drive 0
: set_dont_touch_network
: on all clocks and reset

Try to use the FPGA family specific resources for that net. E.g. on Xilinx
for the reset line, you can use the STARTUP block and connect your generated
reset signal to that block.

Bye
--
Uwe Bonnes bon@elektron.ikp.physik.tu-darmstadt.de

Institut fuer Kernphysik Schlossgartenstrasse 9 64289 Darmstadt
--------- Tel. 06151 162516 -------- Fax. 06151 164321 ----------

 

[Alexander Gnusin]

cuteworm@wildmail.com (walter) wrote in message news:<36659e9e.0307011952.4e2bb49c@posting.google.com>...

Hi All,

I am not sure if this is the right place to post the title. But I will
be very appreciated if any one can help.

When synthesis with Synopsys Design Compiler, I got the message :

Warning: Design 'ooo' contains 1 high-fanout nets. A fanout number of
1000 will be used for delay calculations involving these nets.
(TIM-134)

1. Is the warning quite important?

DC uses the following formula to calculate net capacitance:
NET CAP = SUM OF ALL Cpins + Wire Cap, where:
Cpin is input capacitance of each driven pin;
Wire Cap is net capacitance, which is calculated from WLM using net
fanout and design area info. During Design optimization, DC need to
recalc NET CAP value capacitance every time it adds / removes net
loads.

However, for high fanout nets different formula is used:
NET CAP = high_fanout_net_threshold * high_fanout_net_pin_capacitance
+ Wire Cap
where high_fanout_net_threshold and high_fanout_net_pin_capacitance
are two initially predefined DC variables.
So, if net fanout exceeds high_fanout_net_threshold , NET CAP becomes
fixed and will not be recalculated every time DC makes incremental
changes.

2. How can I identify the high-fanout nets?
You may run "report_net_fanout -high_fanout" command (see help for

command options)

3. How can I fix the warning?
Use set_ideal_net or set_ideal_network commands to exclude net from

any optimizations.

I guess it might be reset or clock signal.
So I use
set_drive 0
set_dont_touch_network
on all clocks and reset
But still got the warning.

Regards,
Alexander Gnusin
www.TCLforEDA.net

 

[walter]

Execuse me for one more question.

I have tried but the result surprised me.
The high fan-out net I have been searched for these days are *Logic0*.

I am not sure if this is normal.

Or maybe something wrong , say coding style bugs?

And most of all, should I care about *Logic0*?



alexg@ottawa.com (Alexander Gnusin) wrote in message news:<a504dc86.0307020414.386f6bfb@posting.google.com>...

cuteworm@wildmail.com (walter) wrote in message news:<36659e9e.0307011952.4e2bb49c@posting.google.com>...
Hi All,

I am not sure if this is the right place to post the title. But I will
be very appreciated if any one can help.

When synthesis with Synopsys Design Compiler, I got the message :

Warning: Design 'ooo' contains 1 high-fanout nets. A fanout number of
1000 will be used for delay calculations involving these nets.
(TIM-134)

1. Is the warning quite important?

DC uses the following formula to calculate net capacitance:
NET CAP = SUM OF ALL Cpins + Wire Cap, where:
Cpin is input capacitance of each driven pin;
Wire Cap is net capacitance, which is calculated from WLM using net
fanout and design area info. During Design optimization, DC need to
recalc NET CAP value capacitance every time it adds / removes net
loads.

However, for high fanout nets different formula is used:
NET CAP = high_fanout_net_threshold * high_fanout_net_pin_capacitance
+ Wire Cap
where high_fanout_net_threshold and high_fanout_net_pin_capacitance
are two initially predefined DC variables.
So, if net fanout exceeds high_fanout_net_threshold , NET CAP becomes
fixed and will not be recalculated every time DC makes incremental
changes.



2. How can I identify the high-fanout nets?
You may run "report_net_fanout -high_fanout" command (see help for
command options)


3. How can I fix the warning?
Use set_ideal_net or set_ideal_network commands to exclude net from
any optimizations.

I guess it might be reset or clock signal.
So I use
set_drive 0
set_dont_touch_network
on all clocks and reset
But still got the warning.

Regards,
Alexander Gnusin
www.TCLforEDA.net