AR# 54699

Description

This answer record describes the Vivado Synthesis Attributes, dont_touch, full_case, gated_clock, shreg_extract, and also provides coding examples for them. 

The coding examples are attached to this answer record. The answer record also contains information related to known issues and good coding practices.

Note: Each coding example can be used to directly create a Vivado project. 

Please refer to the header in each source file for the Synthesis attributes covered in each example.

Solution

DONT_TOUCH

The dont_touch attribute works in the same way as the "keep" and "keep hierarchy" attributes. 

Its function is to prevent logic optimization of either signals or hierarchical blocks and forward annotate the netlist to place and route.

Verilog Example
(* dont_touch = "{true|false}" *) <signal_name>;

VHDL Example
attribute dont_touch : string;
attribute dont_touch of <signal_name> : signal is "{true|false}";
 

 FULL_CASE (Verilog only)

The full_case attribute can be used to instruct Vivado Synthesis that all possible values have been specified in the statements. 

If case values are specified, extra logic for case values is not created by Vivado Synthesis. The attribute can only be controlled through RTL.

Verilog Example

        (* full_case *) 
        case (SELECT)
            3'b001 : Q1 <= A[0];
            3'b011 : Q1 <= A[1];
            3'b110 : Q1 <= A[2];
        endcase   

GATED_CLOCK

The gated_clock attribute purpose is to convert clock gating logic to flop enable. 

This makes use of the flip flop clock enables and dedicated clock routing. 

Vivado synthesis allows the conversion of gated clocks. The two items to use to perform this conversion are:

• A switch (-gated_clock_conversion) in the GUI, that instructs the tool to attempt the conversion.
• The RTL attribute that instructs the tool about which signal in the gated logic is the clock.

Verilog Example

(* gated_clock = "{true|false}" *)  input clk_a;

VHDL Example

    Port ( clk_a : in STD_LOGIC;
           en        : in STD_LOGIC;
     ...
end top;

architecture Behavioral of top is

signal gclk_a : STD_LOGIC;

attribute gated_clock : string;
attribute gated_clock of clk_a : signal is "{true|false}"; 

gclk_a <= clk_a AND en;

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