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AR# 22608

8.1i XST - What is the recommended way to pass attributes in Verilog code?

Description

Keywords: XST, IOSTANDARD, constraint, synthesis, attribute, 2001

Urgency: Standard

General Description:
In Verilog, there are two methods of passing the attributes; one is to use the meta comments and the other is to use the new Verilog 2001 method.

Which method is preferred?

Solution

Using the IOSTANDARD attribute as an example below:

Meta comments method:
//synthesis attribute IOSTANDARD of "rxd" is LVDCI_33;
//synthesis attribute IOSTANDARD of "data_ready" is LVDCI_33

Verilog-2001 method:
(* IOSTANDARD="LVDCI_33" *) input rxd ;
(* IOSTANDARD="LVDCI_33" *) output data_ready ;

When using XST, the preferred method is Verilog-2001. Xilinx is migrating away from meta comments for two reasons:
- Verilog-2001 method makes the code more portable.
- This is the direction the HDL industry is moving.

The XST User Guide will be updated in the future to use the Verilog-2001 methods. This is planned for ISE 9.1i.
AR# 22608
Date Created 12/22/2005
Last Updated 01/08/2009
Status Archive
Type General Article