always @* block in sequential circuit

Question

as far as I know, always @* block is used in combinational circuit, and

the logic inside the always @* are sequentially executed regardless of the clock (asynchronous).

However, it seems that the logic inside always @* block in the below code

works synchronously according to the rising edge of the clock.

module univ_bin_counter
#(
 parameter N = 8
)
(
 input wire clk, reset,
 input wire syn_clr, load, en, up,
 input wire [N-1:0] d,
 output wire max_tick, min_tick,
 output wire [N-1:0] q
);
 //signal declaration
 reg [N-1:0] r_reg, r_next;
 //body
 always @(posedge clk, posedge reset)
 if (reset)
 r_reg <= 0;
 else
 r_reg <= r_next;
 //next statement
 always @*
 if (syn_clr)
 r_next = 0;
 else if (load)
 r_next = d;
 else if (en && up)
 r_next = r_reg + 1;
 else if (en && ~up)
 r_next = r_reg - 1;
 else
 r_next = r_reg;
 //output statement
 assign q = r_reg;
 assign max_tick = (r_reg == {N{1'b1}});
 assign min_tick = (r_reg == 0);
endmodule

Before I run the simulation, I expected that the syn_clr works asynchronously but it works synchronously.

Although the syn_clr is set at 90ns, the data q[2:0] becomes zero at the next rising edge of the clk. Although the syn_clr is set at 90ns, the data q[2:0] becomes zero at the next rising edge of the clk.

If my understanding is right, it should be located in the above

always @(posedge clk, posedge reset) 

block instaed of always @* block.

Could you explain what makes the always@* block executed synchronously?

Answer 1

The always @* statement executes asynchrounously. It is equivalent to writing the code below:

always begin
 // wait for one of these signals to change
 @(syn_clr or load or up or en or r_reg)
 if (syn_clr)
 r_next = 0;
 else if (load)
 r_next = d;
 else if (en && up)
 r_next = r_reg + 1;
 else if (en && ~up)
 r_next = r_reg - 1;
 else
 r_next = r_reg;
end

If you look at the value of r_next, you will see that it gets evaluated after any change in the listed signals. It may be that all those signals are changing on a clock edge as well, so you don't see the difference.