Physical Design Flow I:Netlist In & Floorplanning

Netlist In
The first stage in physical design flow is reading in the netlist and the constraints to your tool of choice. Let us see what kinds of files we are dealing with here. I have used both Cadence and Synopsys tools extensively, so those are what I will base my examples on. However, every tool uses pretty much the same flow and even the same format files.

1. Gate Level Netlist
   Once you choose a process and a library, a synthesis tool will translate your RTL into a collection of interconnected logic gates that define the logic. The most common format is verilog. I had seen some VHDL and EDIF designs when I started my career, but I have only really worked with Verilog files.
2. Standard Cell Library
   In digital design, you have a ready made standard cell library which will be used for synthesis and subsequent layouts. Your netlist will have instantiation of these cells. For digital layout, you need layout and timing abstracts for these cells.
   • Layout Model – An abstract model of the standard cell layout is used instead of the complete layout. This will have PINs defined, so as to facilitate automatic routing by the tool as per your netlist. Synopsys tool ICCompiler use “FRAM” views as a PnR abstract. FRAM view is a cell view that has only the PINs and metal and via blockages defined. This makes sure that the interconnection between the PINs can be routed automatically and that the routing tool will not route over existing metal/via areas thus ruling out any shorts. Cadence EDI tools use LEF views, which again has only the PINs and Obstructions (blockages) defined. LEF is an ascii file, so go ahead and have a read.
   • Timing Model – Tools also need a timing model in the form of a .lib file. ICC takes a .db file, which is generated from a .lib. This liberty format file will have timing numbers for the various arcs in a cell, generally in a look up model. Please note that .libs may also have cell power information.
3. Technology File
   The rules pertaining to the process you have selected should also be given to the PnR tool. This includes metal widths, spacing, via definitions etc. ICC takes a milkyway techfile format, while EDI tools take a technology LEF file.
4. Timing Constraints
   SDC files define the timing constraints of your design. You will have the clock definitions, false paths, any input and output delay constraints etc.

These inputs once read in, will get you started with your database.

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perl tutor : How to modify your @INC include path - What to do when Perl modules aren't in their normal locations

The final thing to remember is that if this message comes back with an error, it doesn't exactly mean that this module isn't installed on the current system, it just means that the module isn't in your @INC include path.

perl tutor : How to modify your @INC include path - What to do when Perl modules aren't in their normal locations

When you have root access to a Unix server, it's pretty easy to install Perl modules in their proper locations, and forget about them. But if you don't have root access and you need to install your Perl modules in non-standard directories, how will you get your programs to find your modules?

In this article we'll demonstrate how to use the use lib statement in your Perl programs to include the non-standard location of your Perl modules into Perl's @INC search list.

Perl modules - use and require examples (with root access)

If you're lucky enough to have root access to your Unix server, it's easy to install Perl modules into their default locations. Once they're installed in their default locations, you just include the modules into your Perl/CGI programs with a "use" or "require" statement like this:

use "LWP.pm";  

or this:

require "LWP.pm";  

Include Perl modules when you don't have root access ...

If you don't have access to the root password on your Unix server, or you're not allowed to add Perl modules to the Perl installation directories, what can you do? (Note: This problem usually arises when you're renting web space on somebody else's web server, and they don't have the module installed that you need -- a fairly common occurrence.)

In cases like this, the thing to do is to install the Perl module yourself into a directory where you do have write permission. For instance, if my name is George, and my HOME directory is /home/george, I might install my Perl modules into a directory named /home/george/modules. If you follow the installation instructions for Perl modules, this is very easy to do.

Assuming that goes okay and you now have the module installed in /home/george/modules, how do you get your Perl/CGI programs to find the module? Fortunately, that too is easy. All you have to do is modify your Perl/CGI program slightly to tell the program where else it should look for modules.

For instance, if the people that host my web site didn't have the CGI.pm module available, I'd install it in /home/george/modules. Then I'd modify my Perl/CGI program to find the module by adding this line near the top of my Perl programs:

use lib '/home/george/modules';  

This simple line of code tells your Perl program to add this directory to it's @INC search directory. @INC contains the list of directories Perl searches when looking for modules. The use lib command is the easiest way I know to modify @INC.

read more at http://alvinalexander.com/perl/edu/articles/pl010015