VHDL 综合警告 FF/Latch 的常数值为 0
[英]VHDL synthesis warning FF/Latch has a constant value of 0
问题描述
我正在尝试一些主要涉及使用 FPGA 并从温度传感器读取值的代码。
代码如下：
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;
---- Uncomment the following library declaration if instantiating
---- any Xilinx primitives in this code.
--library UNISIM;
--use UNISIM.VComponents.all;
entity ds18b20 is
Port ( clk : in STD_LOGIC; --50Mhz oscillator onboard
dq : inout STD_LOGIC;
temp_h : out STD_LOGIC_VECTOR (7 downto 0);
temp_l : out STD_LOGIC_VECTOR (7 downto 0);
temperature : out STD_LOGIC_VECTOR (11 downto 0));
end ds18b20;
architecture Behavioral of ds18b20 is
--RESET : RESET AND PRESENCE PULSE
--CMD_CC : SKIP ROM [CCh]
--WRITE_BYTE : WRITE SCRATCHPAD COMMAND
--WRITE_LOW
--WRITE_HIGH
--READ_BIT :
TYPE STATE_TYPE is (RESET,CMD_CC,WRITE_BYTE,WRITE_LOW,WRITE_HIGH,READ_BIT,CMD_44,WAIT800MS,CMD_BE,GET_TMP,WAIT4MS);
signal STATE: STATE_TYPE:=RESET;
signal clk_temp : std_logic:='0';
signal clk1m : std_logic;
signal write_temp : std_logic_vector(7 downto 0) := (others => '0');
signal TMP : std_logic_vector(11 downto 0);
signal tmp_bit : std_logic;
signal WRITE_BYTE_CNT : integer range 0 to 8:=0;
signal WRITE_LOW_CNT : integer range 0 to 2:=0;
signal WRITE_HIGH_CNT : integer range 0 to 2:=0;
signal READ_BIT_CNT : integer range 0 to 3:=0;
signal GET_TMP_CNT : integer range 0 to 12:=0;
signal cnt : integer range 0 to 100001:=0;
signal count : integer range 0 to 25:=0;
signal WRITE_BYTE_FLAG : integer range 0 to 4:=0;
begin
ClkDivider: process (clk)
begin
if rising_edge(clk) then
if (count = 24) then
count <= 0;
clk_temp<= not clk_temp;
else
count <= count +1;
end if;
end if;
clk1m<=clk_temp;
end Process;
STATE_TRANSITION: process(STATE,clk)
begin
if rising_edge(clk) then
case STATE is
--Master issues RESET pulse
when RESET=>
if (cnt>=0 and cnt<500) then
dq<='0';
cnt<=cnt+1;
STATE<=RESET;
--Master waits for PRESENCE pulse
elsif (cnt>=500 and cnt<1000) then
dq<='Z';
cnt<=cnt+1;
STATE<=RESET;
elsif (cnt>=1000) then
cnt<=0;
STATE<=CMD_CC; -- SKIP ROM COMMAND STATE
end if;
when CMD_CC=> -- SKIP ROM COMMAND
write_temp<="11001100"; -- SKIP ROM BINARY COMMAND
STATE<=WRITE_BYTE; -- modified here
--Master issues write scratchpad command
when WRITE_BYTE=>
case WRITE_BYTE_CNT is
when 0 to 7=>
if (write_temp(WRITE_BYTE_CNT)='0') then
STATE<=WRITE_LOW;
else
STATE<=WRITE_HIGH;
end if;
WRITE_BYTE_CNT<=WRITE_BYTE_CNT+1;
when 8=>
if (WRITE_BYTE_FLAG=0) then -- ????0XCC??
STATE<=CMD_44; --CONVERT TEMPERATURE
WRITE_BYTE_FLAG<=1;
elsif (WRITE_BYTE_FLAG=1) then --?0X44??
STATE<=RESET;
WRITE_BYTE_FLAG<=2;
elsif (WRITE_BYTE_FLAG=2) then --????0XCC??
STATE<=CMD_BE; -- READ SCRATCHPAD
WRITE_BYTE_FLAG<=3;
elsif (WRITE_BYTE_FLAG=3) then --?0XBE??
STATE<=GET_TMP;
WRITE_BYTE_FLAG<=0;
end if;
WRITE_BYTE_CNT<=0;
end case;
when WRITE_LOW=>
case WRITE_LOW_CNT is
when 0=>
dq<='0';
if (cnt=78) then
cnt<=0;
WRITE_LOW_CNT<=1;
else
cnt<=cnt+1;
end if;
when 1=>
dq<='Z';
if (cnt=2) then
cnt<=0;
WRITE_LOW_CNT<=2;
else
cnt<=cnt+1;
end if;
when 2=>
STATE<=WRITE_BYTE;
WRITE_LOW_CNT<=0;
when others=>WRITE_LOW_CNT<=0;
end case;
when WRITE_HIGH=>
case WRITE_HIGH_CNT is
when 0=>
dq<='0';
if (cnt=8) then
cnt<=0;
WRITE_HIGH_CNT<=1;
else
cnt<=cnt+1;
end if;
when 1=>
dq<='Z';
if (cnt=72) then
cnt<=0;
WRITE_HIGH_CNT<=2;
else
cnt<=cnt+1;
end if;
when 2=>
STATE<=WRITE_BYTE;
WRITE_HIGH_CNT<=0;
when others=>WRITE_HIGH_CNT<=0;
end case;
when READ_BIT=>
case READ_BIT_CNT is
when 0=>
dq<='0';
if (cnt=4) then
READ_BIT_CNT<=1;
cnt<=0;
else
cnt<=cnt+1;
end if;
when 1=>
dq<='Z';
if (cnt=4) then
READ_BIT_CNT<=2;
cnt<=0;
else
cnt<=cnt+1;
end if;
when 2=>
TMP_BIT<=dq;
if (cnt=1) then
READ_BIT_CNT<=3;
cnt<=0;
else
cnt<=cnt+1;
end if;
when 3=>
if (cnt=45) then
cnt<=0;
READ_BIT_CNT<=0;
STATE<=GET_TMP;
else
cnt<=cnt+1;
end if;
when others=>READ_BIT_CNT<=0;
end case;
when CMD_44=> -- CONVERT TEMPERATURE
write_temp<="01000100"; -- CONVERT TEMPERATURE BINARY COMMAND
STATE<=WRITE_BYTE;
when WAIT800MS=>
if (cnt>=100000) then
STATE<=RESET;
cnt<=0;
else
cnt<=cnt+1;
STATE<=WAIT800MS;
end if;
when CMD_BE=> -- READ SCRATCHPAD
write_temp<="10111110"; -- READ SCRATHPAD BINARY COMMAND
STATE<=WRITE_BYTE;
when GET_TMP=>
case GET_TMP_CNT is
when 0 to 11=>
STATE<=READ_BIT;
TMP(GET_TMP_CNT)<=TMP_BIT;
GET_TMP_CNT<=GET_TMP_CNT+1;
when 12=>
GET_TMP_CNT<=0;
STATE<=WAIT4MS;
end case;
when WAIT4MS=>
if (cnt>=4000) then
STATE<=RESET;
cnt<=0;
else
cnt<=cnt+1;
STATE<=WAIT4MS;
end if;
when others=>STATE<=RESET;
end case;
end if;
end process;
temp_h<='0'&TMP(11 downto 5);
temp_l<="0000"&TMP(4 downto 1);
temperature <= TMP;
end Behavioral;
我得到的警告是
WARNING:Xst:1293 - FF/Latch <write_temp_0> has a constant value of 0 in block <ds18b20>. This FF/Latch will be trimmed during the optimization process.
WARNING:Xst:1710 - FF/Latch <Mtridata_dq> (without init value) has a constant value of 0 in block <ds18b20>. This FF/Latch will be trimmed during the optimization process.
write_temp 是一个变量，用于保存传感器的二进制命令。 所以基本上，我将通过“dq”双向端口将这些命令发送到传感器。 现在，警告指出 write_temp 始终为 0，这意味着我根本无法指示传感器执行任何操作，因为它始终为 0。
任何人都可以请说明如何克服这个问题吗？ 非常感激。
3 个解决方案
解决方案1
5 已采纳 2012-08-29 07:10:16
这里没有错。 警告说write_temp_0始终为 0 - 也就是说，警告仅适用于write_temp 0 位，而不适用于其他 7 位。
这是意料之中的，因为您从未将write_temp位 0 设置为 0 write_temp任何值。合成器对此进行了处理，并通过简单地将其修剪为常量 0 而不是连接到逻辑来对其进行优化。
因此，请尝试代码并查看它是否有效 - 如果无效，则可能是由于其他原因。
此外，在编写和验证这样的代码时，模拟器是一个很棒的工具 - 它可以让您非常轻松地定位代码中的逻辑错误。 所以没有理由不了解如何使用它。
解决方案2
1 2014-02-06 16:53:10
亲爱的同样的问题，我在 fir 上运行我的代码时遇到了同样的问题，我已经运行了你的代码它综合但由于警告它显示了你的输入和时钟的未定义值，不要分配 0 值，尝试放置不同的值，如 write_temp 7到 1. 我不是 vhdl 的完全专家，但是当我以类似的方式进行此更改时，我克服了这些警告，希望这也适用于您。
解决方案3
-1 2020-04-02 23:04:37
我猜，这个警告（FF/Latch）发生在我们忘记为某些信号分配一些值（甚至可能我们认为没有必要分配一些值）时，尤其是当有一些条件时，比如 if-else。 预计我们会为所有条件分配一个值。 因此，它使我们的代码在每个条件下都可以整齐且长时间地重复一些赋值，但是我们可以在进程开始之后和上升之前为所有这些信号（FF/Latch 警告指的是它们）设置一个初始值-边缘。 例如，在 STATE_Transition procecc 中，在 begin 和 ifrising_edge 之间，你应该写 write_temp <= (others=>'0')。
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