在原程序上修改一下即可！

 

if(reset='0') then

 cnt<=0;

 bus<=0;

elsif clk'event and clk='1' then

start_reg<=start;

stop_reg<=stop;

if (start='1') and (start_reg='0') then

           start_flag<='1';

           cnt<=0;

elsif (stop='1') and (stop_reg='0') then

           start_flag<='0';

           bus<=cnt;

elsif (start_flag='1') then

          cnt<=cnt+1;

end if;

end if;