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Explorer
Explorer
7,727 Views
Registered: ‎11-24-2013

Unused sequential element was removed - no idea why!

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-- UPTATE 2--

This issue is known. See https://forums.xilinx.com/t5/Synthesis/Many-spurious-quot-Synth-8-6014-Unused-sequential-element-was/m-p/769636#M21691

 


 

Hello everyone,

 

in a simple VHDL module, I'm getting the warning "unused sequential element was removed". In order to find the reason, I created a very simple VHDL counter that produces the same warning. The code is:

 

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;

entity counter is
    port ( clk   : in std_logic;
           count : out std_logic_vector (4 downto 0));
end counter;

architecture behavioral of counter is

signal count_i : unsigned(4 downto 0) := (others=>'0');

begin

  process (clk)
  begin
     if rising_edge(clk) then
        count_i <= count_i + 1;
     end if;
  end process;

  count <= std_logic_vector(count_i);

end behavioral;

And the warning is:

[Synth 8-6014] Unused sequential element count_i_reg was removed. 

 

I really cannot understand what's happening, since the count is going directly to an output of the module.

 

I'm using Vivado 2017.2 with the board Xilinx ZCU102 as target.

 

Has anyone an idea, why this is happening?

 

Thanks and regards,

Ignacio

 

--UPDATE--

 

I forgot to add that the register is actually being synthesized:

 

counter.png 

counter.png

1 Solution

Accepted Solutions
Moderator
Moderator
10,817 Views
Registered: ‎09-15-2016

Re: Unused sequential element was removed - no idea why!

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Hi @imgignacio,

 

I have filed a CR-988379 on this issue with the above test case since it is observed in the latest internal build of Vivado as well.

Thanks for reporting this. The factory will work to fix the warning by default.

 

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10 Replies
Adventurer
Adventurer
7,712 Views
Registered: ‎11-09-2016

Re: Unused sequential element was removed - no idea why!

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Hi, @imgignacio

Are you sure count at the output?

I implement your code, count reg is there!

dd1.png

dd2.png

0 Kudos
Explorer
Explorer
7,706 Views
Registered: ‎11-24-2013

Re: Unused sequential element was removed - no idea why!

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Hi @berker_atel,

 

thanks for your answer!

 

Yes, that's exactly the problem. The register is there, but nevertheless the Warning is always present...

 

Regards,

Ignacio

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Adventurer
Adventurer
7,689 Views
Registered: ‎11-09-2016

Re: Unused sequential element was removed - no idea why!

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Hi,

Maybe you can try to delete user created message in the below message panel(with right tick ).

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Explorer
Explorer
7,680 Views
Registered: ‎11-24-2013

Re: Unused sequential element was removed - no idea why!

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Hi @berker_atel

 

I have just found that this issue is known. https://forums.xilinx.com/t5/Synthesis/Many-spurious-quot-Synth-8-6014-Unused-sequential-element-was/m-p/769636#M21691

 

I tested it with 2016.4 and there's no warning.

 

Anyway, thanks four your help!

 

Regards,

Ignacio

0 Kudos
Moderator
Moderator
10,818 Views
Registered: ‎09-15-2016

Re: Unused sequential element was removed - no idea why!

Jump to solution

Hi @imgignacio,

 

I have filed a CR-988379 on this issue with the above test case since it is observed in the latest internal build of Vivado as well.

Thanks for reporting this. The factory will work to fix the warning by default.

 

--------------------------------------------------------------
Please mark the appropriate answer as "Accept as solution" if information provided is helpful.
Give 'Kudos' to a post which you think is useful and reply oriented.
--------------------------------------------------------------

View solution in original post

Moderator
Moderator
6,829 Views
Registered: ‎09-15-2016

Re: Unused sequential element was removed - no idea why!

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Hi @imgignacio,

 

The factory have fixed this issue in Vivado 2018.1 build.

 

Regards,
Prathik
-----------------------------------------------------------------------------------------------
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Explorer
Explorer
6,818 Views
Registered: ‎11-24-2013

Re: Unused sequential element was removed - no idea why!

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Hi @prathikm,

 

thanks a lot for the information!

 

Regards,

Ignacio

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Newbie kburke32
Newbie
6,559 Views
Registered: ‎02-13-2018

Re: Unused sequential element was removed - no idea why!

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Prathikm:

 

While we wait for Vivado 2018.1 to be released (although our team will probably not upgrade yet for fear of additional unknown bugs), can you tell us what coding styles seem to cause this bug of fake warning messages?  We are using System Verilog and we also get these spurious warnings.  If there is anything we can do in our RTL to remove them, we'd be happy to make those changes.  Reviewing our netlist after every synthesis to search for and confirm all cells being present is NOT an option.

 

Thanks.

Visitor cmhicks
Visitor
5,771 Views
Registered: ‎03-10-2011

Re: Unused sequential element was removed - no idea why!

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@kburke32wrote:

Prathikm:

 

While we wait for Vivado 2018.1 to be released (although our team will probably not upgrade yet for fear of additional unknown bugs), can you tell us what coding styles seem to cause this bug of fake warning messages?


 

I get lots of these warnings on a design I am currently working on, and I think they are all triggered when the register in question is absorbed into a DSP48.

 

Excerpts from the synthesis log:

 

WARNING: [Synth 8-6014] Unused sequential element pwrmix_inst/m0r_reg was removed.  [/home/cmh/z/axi4s_dns/axi4s_dns.srcs/sources_1/new/axi4s_pwrmix.vhd:183]

 

...further down...

 

WARNING: [Synth 8-6014] Unused sequential element pwrmix_inst/m0_mpy/xrar_reg was removed.  [/home/cmh/z/axi4s_dns/axi4s_dns.srcs/sources_1/imports/new/cmplx_mpy.vhd:26]

 

...further down ...

 

DSP Report: Generating DSP pwrmix_inst/m0_mpy/xrar_reg, operation Mode is: (A*B2)'.
DSP Report: register pwrmix_inst/m0r_reg is absorbed into DSP pwrmix_inst/m0_mpy/xrar_reg.
DSP Report: register pwrmix_inst/m0_mpy/xrar_reg is absorbed into DSP pwrmix_inst/m0_mpy/xrar_reg.

 

It's as if the fabric register is duplicated by a register in a DSP, the connections are moved to the new register leaving the fabric register with unconnected outputs, fabric register gets removed triggering the warning. It's a pity that the warning about the register removal isn't (a) more explicit about what's happening and/or (b) closer in the report file to the message about the absorption into DSP.

CH
==

 

 

Visitor mfouillat
Visitor
3,624 Views
Registered: ‎05-11-2018

Re: Unused sequential element was removed - no idea why!

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Hello, i use vivado 2017.4

I had the same issue.

I created a counter.vhd and a test bench counter_tb.vhd. 

You need to add in your test_bench file :" use IEEE.NUMERIC_STD.ALL; "

And the warning message regarding the counter.vhd vanishes.

 

You need to use the sames libraries in your main description and in your test bench.

 

Regards,