关于串口rs232程序设计

ti

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
entity transfer is
  generic(framlent:integer:=8);
  port(bclkt,resett,xmit_cmd_p:in std_logic; --????/????
  txdbuf:in std_logic_vector(7 downto 0):="11001010";
  txd:out std_logic;
  txd_done:out std_logic);
end transfer;
architecture behavioral of transfer is
type states is (x_idle,x_start,x_wait,x_shift,x_stop);    --??????
signal state:states:=x_idle;
signal tcnt:integer:=0;
begin
  process(bclkt,resett,xmit_cmd_p,txdbuf)       --?????????
variable xcnt16:std_logic_vector(4 downto 0):="00000";  --??????
variable xbitcnt:integer:=0;
variable txds:std_logic;
begin
   if resett='1'then
      state<=x_idle;txd_done<='0';txds:='1';
   elsif rising_edge(bclkt)then
   case state  is
      when x_idle=>if xmit_cmd_p='1'then state<=x_start;txd_done<='0';
   else state<=x_idle;       --状态1，等待数据帧发送命令
end if;
  when x_start=>if xcnt16>="01111"  then state<=x_wait;xcnt16:="00000";
  else xcnt16:=xcnt16+1;txds:='0';state<=x_start;
  end if;                         --状态2，发送信号至起始位
when x_wait=>if xcnt16>="01110"then
  if xbitcnt=framlent then state<=x_stop;xbitcnt:=0;
  else state<=x_shift;
  end if;
  xcnt16:="00000";
else xcnt16:=xcnt16+1;state<=x_wait;
end if;                         --状态3，等待状态
when x_shift=>txds:=txdbuf(xbitcnt);xbitcnt:=xbitcnt+1;state<=x_wait;
                        --状态4，将待发数据并串转换
when x_stop=>if xcnt16>="01111"then
  if xmit_cmd_p='0'then state<=x_idle;xcnt16:="00000";
  else xcnt16:=xcnt16;state<=x_stop;
  end if; txd_done<='1';
else xcnt16:=xcnt16+1;txds:='1';state<=x_stop;
end if;       --状态5，停止位发送状态
when others=>state<=x_idle;
end case;
end if;
txd<=txds;
end process;
end behavioral;

我的疑惑是红色的部分为什么是01110，而不是01111。串口是以波特率的16倍频率对数据进行采样的，求各位大神帮忙分析一下，小弟不胜感激！