问题
I'm trying out some code that essentially involves using an FPGA and reading values from a temperature sensor.
The code is below:
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;
The warning I get is
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 is a variable that holds the binary commands for the sensor. So essentially, I will be sending these commands to the sensor via the "dq" bidirectional port. Now, the warning states that write_temp is always 0 which means I can't instruct the sensor to do any operation at all since its always 0.
Could anyone please shed some light on how to overcome this? Much appreciated.
回答1:
Nothing is wrong here. The warning says that write_temp_0 is always 0 - that is, the warning only applies to bit 0 of write_temp, not the other 7 bits.
This is to be expected, as you never set bit 0 of write_temp to be anything but 0. The synthesizer picks up on this, and optimizes it by simply trimming it to be a constant 0 instead of being connected to logic.
So try out the code and see if it works - if it doesn't, it's probably due to other reasons.
Also, when writing and verifying code like this, the simulator is a fantastic tool - it'll allow you to locate logic errors in your code very easily. So no reason not to get to know how to use it.
回答2:
dear same problem i am facing same problem running my code on fir, i have run your code it synthesis but due to warning it show's undefined values of your input and clk, don't assign 0 value, try to put different value like write_temp 7 to 1. i am not full expert of vhdl but when i make this change similar way i get over these warning, hope this work for you also.
来源：https://stackoverflow.com/questions/12171815/vhdl-synthesis-warning-ff-latch-has-a-constant-value-of-0