Re: (* synthesis, keep [=<optional_value>] *)

Subject: Re: (* synthesis, keep [=] *)
From: VhdlCohen@aol.com
Date: Fri Feb 01 2002 - 12:15:31 PST

In a message dated 2/1/02 11:44:04 AM Pacific Standard Time,
jbhasker@cadence.com writes:

> Based on the trail of emails on this topic, here is what I propose to change
> in draft 1.9:
>
> 1. Delete the ff_preserve attribute and use the "keep" attribute for
> flip-flops also (Good idea, Ben).
>
> 2. Add comment to the effect that "... shall be preserved (not deleted nor
> replicated) ...".
>
>
Thus, we could write something like:
keep
This attribute preserves nets, sequential elements, or instantiated instances
from optimization. The objects associated with the "keep" directive shall be
preserved , and not deleted nor replicated. In addition, the "keep"
directive shall make the port connections
of the instance and all associated drivers kept throughout the hierarchy.

Example:
wire [7:0] out_a (* synthesis, keep *); // default keep is keep = 1 when
keep is used
reg [7:0] b (* synthesis, keep *);
my_design my_design1 (out1, in1, clk_in)(* synthesis, keep= 1*); // keep
my_design my_design2 (out, in, clk_in)(* synthesis, keep= 0*); // may
optimize out

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Ben Cohen Publisher, Trainer, Consultant (310) 721-4830
<A HREF="http://www.vhdlcohen.com/">http://www.vhdlcohen.com/> vhdlcohen@aol.com
Author of following textbooks:
* Real Chip Design and Verification Using Verilog and VHDL, 2002 isbn
0-9705394-2-8
* Component Design by Example ", 2001 isbn 0-9705394-0-1
* VHDL Coding Styles and Methodologies, 2nd Edition, 1999 isbn 0-7923-8474-1
* VHDL Answers to Frequently Asked Questions, 2nd Edition, isbn 0-7923-8115
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