Why verilog "always_comb block contains only one event control" error flagged on always procedural block with multiple "@"

Question

the following code below generates this error message:

"verilog always_comb imposes the restriction that it contains one and only one event control and no blocking timing controls"

always_comb begin
if (sig_a)begin
 @(posedge sig_b); // wait for a sig_b posedge event
 @(negedge sig_b); // then wait for a sig_b negedge event
 event_true=1;  
end

if (event_true)begin
  @((sig_c==1)&&(sig_a==0)); //wait for sig_a to deassert and sig_c assert event to be true
  yes =1;
 end
 else yes =0;

end

Why does the combi logic procedural block generate this error ?

And would an always block with @ event wait be synthesizable ?

Serge Serge · Accepted Answer · 2018-11-16T12:18:38

always_comb is for combinational logic only. the @ statements you use have nothing to do with combinational.

from lrm 9.2.2.2.2

Statements in an always_comb shall not include those that block, have blocking timing or event controls, or fork-join statements.

In your case you need to use a general purpose always. always @* will probably do.

Why verilog "always_comb block contains only one event control" error flagged on always procedural block with multiple "@"

2 Answers