verilog output is delay by 1 clock cycle

Question

i have this sample module

module control(input clk,input [5:0] x, input [6:0] y, 
input [7:0] done_gamestate,
output [7:0] gamestateout
    );

    wire [7:0] connector;
    state_ctrl a(clk,x,y,done_gamestate,connector);
    datapath_ctrl b(clk,connector,gamestateout);

endmodule

and here is the output waveform enter image description here

Basically, how can we make module b respond at the first positive edge of the clock? It seems that the output is delay by 1 positive edge cycle.

here is the state_ctrl module

module state_ctrl(input clk,input [5:0] x, input [6:0] y,
input [7:0] done_gamestate,
output reg [7:0] gamestate
    );
initial begin
gamestate = 0;
end

always@(posedge clk)
begin
case(done_gamestate)
8'b0000_0001 : gamestate <= 8'b0000_0100; // init -> offsets
8'b0000_0100 : gamestate <= 8'b0000_0010; // offsets -> getcells
8'b0000_0010 : gamestate <= 8'b0001_0000; // getcells -> countcells
8'b0001_0000 : gamestate <= 8'b0000_1000; // countcells -> applyrules
8'b0000_1000 : gamestate <= 8'b0010_0000;
8'b0010_0000 : 
    begin
        if(x==8'd62 && y==8'd126)
            gamestate <= 8'b1000_0000;
        else
            gamestate <= 8'b0000_0100;
    end
8'b1000_0000 : gamestate <= 8'b0100_0000; // copy -> delay
8'b0100_0000 : gamestate <= 8'b0000_0100; // delay -> offset
default : begin
    gamestate <= 8'b00000000;
    $display ("error, check done_gamestate %b",done_gamestate); 
    end
endcase
end

endmodule

and here is the datapath_ctrl module

module datapath_ctrl(input clk,input [7:0] gamestate,output reg [7:0] gamestateout
    );

initial begin
gamestateout = 0;
end

always@(posedge clk)
begin
   #1 case(gamestate)
    8'b00000001: gamestateout = 8'b00000001; //init
    8'b00000010: gamestateout = 8'b00000010; //getcells
    8'b00000100: gamestateout = 8'b00000100; //offsets
    8'b00001000: gamestateout = 8'b00001000; //applyrules
    8'b00010000: gamestateout = 8'b00010000; //countcells
    8'b00100000: gamestateout = 8'b00100000; //writecell
    8'b01000000: gamestateout = 8'b01000000; //delay
    8'b10000000: gamestateout = 8'b10000000; //copy
    default : begin
    gamestateout = 8'b00000000;
    $display ("error, check gamestate %b",gamestate); 
    end
endcase
end
endmodule

Morgan Morgan · Accepted Answer · 2013-02-17T10:25:28

There is not enough information to say if there is a mistake in the implementation but i am guessing that there is a 1cycle delay because it is synchronous logic. Data changes and it sampled on the next clock edge therefore states appears to change 1 cycle after its input changed.

If you need outputs to change immediately then you must use combinatorial logic.

verilog output is delay by 1 clock cycle

1 Answers