Given a SystemVerilog design modeling an ASIC, how can I enforce the rule that all logic/reg elements that are used in flip-flops should not be initialized to a certain value? Is there a directive that I can use in my code that would prevent this?

Here's an example:

module asic (...);
    logic a;    // this is fine
    // logic a = 1;    // but this is not fine

    reg b = 1;  // this is fine since it is not a flip-flop

    always @(posedge clk, posedge rst)
    if (rst)
        a <= 0;
    else if (b)
        a <= ~a;

I think a check like this would be really helpful in ensuring that all registers in the ASIC design have a reset. The tools I use are Verilator (for linting and simulation) and Yosys (for synthesis), but I haven't been able to find a related option in their documentation.

  • \$\begingroup\$ I don't know System Verilog, but can you use logic a = X; ? \$\endgroup\$ Commented Jul 22, 2020 at 23:41
  • \$\begingroup\$ If by 'X' you mean the 'X'/unknown value, what I'm trying to achieve is the prevention of any use of logic initialization of the form 'logic NAME = VALUE'. \$\endgroup\$ Commented Jul 24, 2020 at 21:36
  • \$\begingroup\$ OK, then just verify that all flip flop outputs are X before you assert reset. Or grep the source for "logic .* = [01]" \$\endgroup\$ Commented Jul 25, 2020 at 0:26
  • \$\begingroup\$ I'm actually doing this for now until support is added for the feature. I'll make sure to update this post once I find a solution. \$\endgroup\$ Commented Jul 28, 2020 at 1:42

1 Answer 1


Inline initialization is not good, even for the b signal. A commercial tool I use for ASIC synthesis ignores those initializations and throws warnings.

If b is a constant, the below would be better (still not ideal).

logic b;
assign b = 1;

I experienced failures in formal equivalence checking (RTL vs. synthesis netlist) as well, so I avoid inline initializations in design.

  • \$\begingroup\$ I'm thinking you can't tell me, but could you tell me which tool you were using so I could do some research into how it checks for those initializations? I can't use it myself since I have to use open-source tools since the software I wrote that uses it is used in academia. \$\endgroup\$ Commented Jul 24, 2020 at 21:34
  • \$\begingroup\$ @norandomtechie It was Synopsys Design Compiler. \$\endgroup\$
    – user154136
    Commented Jul 24, 2020 at 22:29
  • \$\begingroup\$ Is Synopsys Design Compiler accepting assignment of logic inside initial block to synthesis ??? \$\endgroup\$
    – Mitu Raj
    Commented Nov 14, 2020 at 8:31
  • 1
    \$\begingroup\$ @MituRaj No. \$\endgroup\$
    – user154136
    Commented Nov 14, 2020 at 14:44

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