時脈

module system_clock(clk);
parameter PERIOD=100;
output clk;reg clk; //reg可以用在像C語言的用途
initialclk=0; //clk初始值設為0
always
begin
#(PERIOD/2) clk=~clk;
#(PERIOD/2) clk=~clk;
end
always@(posedge clk)
if($time>1000)#(PERIOD-1)$stop;
endmodule