architecture Behavioral of REGISTERS is
type REG_FILE_TYPE is array(0 to 15) of STD_LOGIC_VECTOR(15 downto 0);
signal REG_ARRAY : REG_FILE_TYPE:= (others => X"0000");
begin
process(WRITE_ENABLE,REG_CLOCK) is
begin
case (WRITE_ENABLE) is
when '1' => REG_ARRAY(to_integer(unsigned(WRITE_ADDRESS))) <= REG_INPUT;
when '0' => NULL;
when others => NULL;
end case;
end process;
You forgot to add the clock to the process. If there's no clock, and the synthesizer infers that you have a need some memory for the implementation, then it has no other option than to add latches. This is almost always a mistake.
Besides that, I don't like your case structure here, where a simple if-then would do. This is how I'd implement it:
process(REG_CLOCK) is
begin
if rising_edge(REG_CLOCK) then
if WRITE_ENABLE = '0' then
REG_ARRAY(to_integer(unsigned(WRITE_ADDRESS))) <= REG_INPUT;
end if;
end if:
end process;
Related
I have written the following VHDL code with the assumption that it will generate a counter with a synchronous reset! however, when I looked at the elaborated design in Vivado 2020.2, the counter has an ASYNCHRONOUS reset! The process should not get evaluated without seeing the rising/falling edges of the clock! How did the tool infer an asynchronous reset?!
PS. count is defined as an unsigned signal (not std_logic_vector)
Any explanation is greatly appreciated!
process(clk)
begin
if rst='1' then
count <= (others => '0');
elsif rising_edge(clk) then
count <= count + 1;
end if;
end process;
Synthesis tools generally ignore the sensitivity list, and create the logic from design patterns in the code. In your code, the rst branch overrides the clock branch, hence it creates an asynchronous reset. In addition, the reset is not reliant on clk.
To create a synchronous reset, the rst branch should be inside the clock branch as the reset should only occur on a clock edge.
process(clk)
begin
if rising_edge(clk) then
count <= count + 1;
if rst = '1' then
count <= (others => '0');
end if;
end if;
end process;
Here is a design for 4-bit asynchronous ripple counter (using T flip flop however I didn't define a component for Tff and just coded the behavior of circuit regarding T signals).
Following are the questions:
1.) inout ports, I first defined Q as inout (since it's obviously my output and the bits are also used as clk inputs to their following flip flops). Still, when I wanted to simulate my code, the Q output was UUUU which makes sense cause I had to initialize it with the number I wanted my count to begin with. Though I didn't know how to set an inout initial value (I tried Process ... Q <= "0000"; wait; end process but it didn't work)!
2.) In order to solve the above-mentioned problem I changed my inout port to out (Q_out) and defined Q as a signal, this worked BUT...my counter only changed the Q(0) bit and not the others...thus it counts like: 0,1,0,1,0,1,...
3.) I want to debug this code. I tried another style, instead of a 4-bit output I defined 4 1-bit output signals (Q_out1 to Q_out2) in addition to 4 internal signals Q0 to Q1 and this perfectly works
I just want to know why the first style (Q as a 4_bit vector) didn't work out.
thanks in advance for your help.
Here is my code and its test bench:
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
entity four_bit_Asynch_Counter is
Port ( T0,T1,T2,T3 : in STD_LOGIC;
clk : in STD_LOGIC;
Q_out: out STD_LOGIC_VECTOR (3 downto 0));
end four_bit_Asynch_Counter;
architecture Behavioral of four_bit_Asynch_Counter is
signal Q : STD_LOGIC_VECTOR (3 downto 0) := "0000";
begin
Process (clk,Q(0),Q(1),Q(2))
begin
if (falling_edge(clk)) then
if (T0 = '1') then
Q(0) <= not Q(0);
else
Q(0) <= Q(0);
end if;
end if;
if (falling_edge(Q(0))) then
if (T1 = '1') then
Q(1) <= not Q(1);
else
Q(1) <= Q(1);
end if;
end if;
if (falling_edge(Q(1))) then
if (T2 = '1') then
Q(2) <= not Q(2);
else
Q(2) <= Q(2);
end if;
end if;
if (falling_edge(Q(2))) then
if (T3 = '1') then
Q(3) <= not Q(3);
else
Q(3) <= Q(3);
end if;
end if;
Q_out <= Q;
end Process;
end Behavioral;
--------------- Test Bench------------
LIBRARY ieee;
USE ieee.std_logic_1164.ALL;
ENTITY tb_counter IS
END tb_counter;
ARCHITECTURE behavior OF tb_counter IS
-- Component Declaration for the Unit Under Test (UUT)
COMPONENT four_bit_Asynch_Counter
PORT(
T0 : IN std_logic;
T1 : IN std_logic;
T2 : IN std_logic;
T3 : IN std_logic;
clk : IN std_logic;
Q_out : OUT std_logic_vector(3 downto 0)
);
END COMPONENT;
--Inputs
signal T0 : std_logic := '1';
signal T1 : std_logic := '1';
signal T2 : std_logic := '1';
signal T3 : std_logic := '1';
signal clk : std_logic := '0';
--Outputs
signal Q_out : std_logic_vector(3 downto 0);
-- Clock period definitions
constant clk_period : time := 10 ns;
BEGIN
-- Instantiate the Unit Under Test (UUT)
uut: four_bit_Asynch_Counter PORT MAP (
T0 => T0,
T1 => T1,
T2 => T2,
T3 => T3,
clk => clk,
Q_out => Q_out
);
-- Clock process definitions
clk_process :process
begin
clk <= '0';
wait for clk_period/2;
clk <= '1';
wait for clk_period/2;
end process;
-- Stimulus process
stim_proc: process
begin
-- hold reset state for 100 ns.
wait for 100 ns;
wait for clk_period*10;
-- insert stimulus here
wait;
end process;
END;
The TL;DR answer is that q(3) doesn't show up in your process sensitivity list.
architecture behavioral of four_bit_asynch_counter is
signal q: std_logic_vector (3 downto 0) := "0000";
begin
process (clk, q(0), q(1), q(2))
begin
if falling_edge(clk) then
if t0 = '1' then
q(0) <= not q(0);
-- else
-- q(0) <= q(0);
end if;
end if;
if falling_edge(q(0)) then
if t1 = '1' then
q(1) <= not q(1);
-- else
-- q(1) <= q(1);
end if;
end if;
if falling_edge(q(1)) then
if t2 = '1' then
q(2) <= not q(2);
-- else
-- q(2) <= q(2);
end if;
end if;
if falling_edge(q(2)) then
if t3 = '1' then
q(3) <= not q(3);
-- else
-- q(3) <= q(3);
end if;
end if;
q_out <= q;
end process;
end architecture behavioral;
For your process sensitivity list you've discovered a feature in how the sensitivity list is constructed from the expression consisting of primaries - clk, q(0), q(1), q(2).
From IEEE Std 1076 -1993, 8.1 Wait statement:
...
The sensitivity set is initially empty. For each primary in the condition of the condition clause, if the primary is
-- A simple name that denotes a signal, add the longest static prefix of the name to the sensitivity set
-- A selected name whose prefix denotes a signal, add the longest static prefix of the name to the sensitivity set
-- An expanded name whose prefix denotes a signal, add the longest static prefix of the name to the sensitivity set
-- An indexed name whose prefix denotes a signal, add the longest static prefix of the name to the sensitivity set and apply this rule to all expressions in the indexed name
...
...
This rule is also used to construct the sensitivity sets of the wait statements in the equivalent process statements for concurrent procedure call statements( 9.3 ), concurrent assertion statements ( 9.4 ), and concurrent signal assignment statements ( 9.5 ).
If a signal name that denotes a signal of a composite type appears in a sensitivity list, the effect is as if the name of each scalar subelement of that signal appears in the list.
...
I only included elements of the rule that are of interest here, the first covers the clock the last element shown covers the std_logic_vector elements specified by selected names.
It helps to understand what is meant by the longest static prefix. This explained in -1993 6.1 Names.
The primaries (indexed names) are static names (q(0), q(1), q(2)), every expression that's part of each indexed name is static.
This means the longest static prefix is the indexed name comprising each primary.
And this leaves q(3) dangling in the breeze for the process signal assignment statement:
q_out <= q;
Without sensitivity to q(3) the value of q_out is not updated until the next event in the sensitivity list, which happens to be on clk:
There are two ways to cure this, you could move the q_out assignment outside the process statement, where it becomes a concurrent signal assignment (with an elaborated equivalent process with a sensitivity list set to q), or you can change the sensitivity list in the present process:
process (clk, q)
So that q_out is updated for an event on q(3) (noting the last quoted paragraph in 8.1 above).
This behavior hold true for later revisions of the standard as well.
With the process sensitivity list is fixed:
Your counter behaves properly.
Also note I commented out the redundant else assignments to the q(0), q(1), q(2) and q(3) a signal will hold it's value until assigned and these are sequential (clocked) statements. Also eliminated the redundant parentheses pairs.
When implementing counters in realisable hardware (either ASIC or FPGA) you should never use a ripple counter. By using the flip-flop output as a clock to the next you will have sub-optimal timing, the tools will not be able to accurately validate the setup and hold times and you are not able to take advantage of dedicated clock routing. In general asynchronous design is a bad idea for real implementations.
A true synchronous design will be much better for synthesis and is much easier to infer in the VHDL code.
Examples of Counter implementations
See the above link for both verilog and vhdl examples of counter implementation.
So I have a modulo counter going from 1->15, then looping back around constantly in a seperate entity. I would like to, in a case statement depending on some outputs, sample this value on the rising_edge of a clock, but only do it once, otherwise the value will be constantly changing. Is there a way to do this? Assign the signal and have it stay static?
I've posted some code that I hope will demonstrate what I am trying to say a bit better.
process(all)
begin --sensitivity list?
if(reset) then
playerCards <= "0000000000000000";
dealerCards <= "0000000000000000";
elsif rising_edge(clock) then
case? deal & dealTo & dealToCardSlot is
when "1100" =>
playerCards(3 downto 0) <= singleCard;
playerCards(15 downto 4) <= (others => '0');
when "1101" =>
playerCards(7 downto 4) <= singleCard;
playerCards(15 downto 8) <= (others => '0');
when "1110" =>
playerCards(11 downto 8) <= singleCard;
playerCards(15 downto 12) <= (others => '0');
when "1111" =>
playerCards(15 downto 12) <= singleCard;
when "1000" =>
dealerCards(3 downto 0) <= singleCard; --dcard1
dealerCards( 15 downto 4) <= (others => '0');
when "1001" =>
dealerCards(7 downto 4) <= singleCard; --dcard2
dealerCards( 15 downto 8) <= (others => '0');
when "1010" =>
dealerCards(11 downto 8) <= singleCard; --dcard3
dealerCards( 15 downto 12) <= (others => '0');
when "1011" =>
dealerCards(15 downto 12) <= singleCard; --dcard4
when "0--0" => null; --do nothing when in win/end
when others => --this covers the start case --sets cards to 0
playerCards <= "0000000000000000";
dealerCards <= "0000000000000000";
end case?;
end if;
end process;
Here I have singleCard being linked to the output of my counter, which increments every clock edge. So basically I would like my case statement to only happen update the value of playerCards once, and then stop.
Thanks for any help.
Your requirement can be stated as : the process can be in two states - sampling, or not sampling, and to transition to not sampling when you sample.
So you can do this with a state variable, which in this case can be a boolean - this turns your process into a state machine. As the other answer says, you can use a signal here instead of a variable, but that breaks encapsulation, making the processes internal workings visible.
Some people would insist you separate out the state machine into another two (or even 3) processes, but I think the single process form is a lot smaller and easier to understand.
process(reset,clock) -- original sens list is incorrect for a clocked process
variable sampling : boolean;
begin
if(reset) then
sampling <= true;
-- other reset actions
elsif rising_edge(clock) then
if sampling then -- add conditions here, or it'll sample on the first clock.
sampling := false;
case ... -- sampling process and program logic
end case;
else
-- if you need to turn sampling back on, set "sampling" here
end if;
end if;
end process;
Yes, you need an additional internal signal that will control your operation mode: updating and non-updating.
It would be actually a kind of feedback signal. You generate it depending on the input conditions, and then you also use it on input to modify the behaviour.
How to change this code to get it working with several (2, 3 or 4) buttons?
signal lastButtonState : std_logic := '0';
process(clk)
begin
if(rising_edge(clk)) then
if(buttonState = '1' and lastButtonState = '0') then --assuming active-high
--Rising edge - do some work...
end if;
lastButtonState <= buttonState;
end if;
end process;
I would like to get several buttons work and do some work on press 'state'. This part of code works for 1 button only.
Thanks
The std_logic_vector type can be used for multiple buttons. Code may look
like:
constant BUTTONS : positive := 4; -- Number of buttons
subtype buttons_t is std_logic_vector(1 to BUTTONS); -- Type for buttons
signal buttonsState : buttons_t; -- Input from buttons (remember to debounce)
signal lastButtonsState : buttons_t := (others => '0'); -- Last state
... -- End of declarations, and start of concurrent code (processes etc.)
process (clk)
variable buttonsRise_v : buttons_t; -- Support variable to make writing easier
begin
if rising_edge(clk) then
buttonsRise_v := buttonsState and (not lastButtonsState); -- Make '1' for each button going 0 to 1
if buttonsRise_v(1) = '1' then -- Detect button 1 going 0 to 1
-- Do some work...
end if;
-- More code reacting on buttons...
lastButtonsState <= buttonsState;
end if;
end process;
I'm trying to write RS232 transmitter module in vhdl for Spartan. According to simulation in Xilinx, it seems to be working fine, but when i try to deploy it on device, it simply doesn't work. I have found out that it might be problem with latches, but somehow I'm not able to pinpoint them. I'm using 50 Mhz clock and the bit rate of transmission is 115200 bs.
This is my vhdl code:
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
USE ieee.std_logic_arith.all; -- Uncomment the following library declaration if using
-- arithmetic functions with Signed or Unsigned values
use IEEE.NUMERIC_STD.ALL;
-- Uncomment the following library declaration if instantiating
-- any Xilinx primitives in this code.
--library UNISIM;
--use UNISIM.VComponents.all;
entity nadajnikRS is
Port ( start : in STD_LOGIC;
reset : in STD_LOGIC;
clk : in STD_LOGIC;
DI : in STD_LOGIC_VECTOR(7 downto 0);
RS_TX : out STD_LOGIC;
busy : out STD_LOGIC);
end nadajnikRS;
architecture Behavioral of transRS is
signal register : STD_LOGIC_VECTOR(8 downto 0) := (others => '1' );
signal counter : INTEGER range 0 to 9 := 0;
signal baud_clk : STD_LOGIC := '0';
signal ready : STD_LOGIC := '0';
type states is (working, free);
signal state: states := free;
signal baud_counter : INTEGER range 0 to 220 := 215;
begin
baud_clock: process (clk)
begin
if rising_edge(clk) then
if (ready = '1') then
if (baud_counter < 218) then
if (baud_counter = 217) then
baud_clk <= '1';
end if;
baud_counter <= baud_counter+1;
else
baud_counter <= 0;
baud_clk <= '0';
end if;
else
baud_counter <= 0;
end if;
end if;
end process baud_clock;
shiftregister : process (baud_clk)
begin
if rising_edge(baud_clk) then
if (state = free) then
RS_TX <= '0';
register (7 downto 0) <= DI;
else
RS_TX <= register(0);
register <= '1' & register(8 downto 1);
end if;
end if;
end process shiftregister;
bitcounter : process (baud_clk)
begin
if rising_edge(baud_clk) then
counter <= counter + 1;
if (counter = 10) then
counter <= 1;
end if;
end if;
end process bitcounter;
shiftstate: process (reset, counter, start)
begin
if (reset = '1') then
ready <= '0';
end if;
if (start = '1') then
ready <= '1';
state <= free;
end if;
if (counter = 1 ) then
state <= working;
elsif (counter = 10) then
state <= free;
end if;
end process;
statemachine : process (state)
begin
case state is
when working => busy <= '1';
when free => busy <= '0' ;
end case;
end process statemachine;
end Behavioral;
During synthesis I get two latch warnings:
Xst:737 - Found 1-bit latch for signal <ready>. Latches may be generated from incomplete case or if statements. We do not recommend the use of latches in FPGA/CPLD designs, as they may lead to timing problems.
Xst:737 - Found 1-bit latch for signal <state_0>. Latches may be generated from incomplete case or if statements. We do not recommend the use of latches in FPGA/CPLD designs, as they may lead to timing problems.
I tried to eliminate them by adding additional if statements, but nothing seems to work.
I will be grateful for any help,
Ghaad
A process describing a register should have exactly one signal in the sensitivity list, clk (possibly a reset signal as well if you use asynchronous resets), since a register is only sensitive to a single event, namely a clock edge.
Thus your process sensitivity list baud_clock: process (clk,ready) and shiftregister : process (baud_clk, state) already indicate that you have a problem.
When describing a register, always make sure that your if(rising_edge(clk)) surrounds ALL of the described logic. A simple registered process should look like this:
process(clk) begin
-- NO LOGIC HERE
if(rising_edge(clk)) then
if(reset='1') then
-- Synchronous reset logic here.
else
-- All other logic here.
end if;
end if;
-- NO LOGIC HERE
end process;
Look at your 'shiftstate' process, which is responsible for driving 'ready'. How does it drive 'ready' when 'reset' is not 1, and 'start' is not 1? You haven't told it, so it keeps 'ready' unchanged in those cases. That's what 'latch' means: the process needs to remember what 'ready' was before, and keep it the same; your code therefore infers a memory. Make sure that 'ready' is driven in all branches; you can do this easily with a default assignment at the top.
Having said that, your code has multiple other issues. Did someone suggest in another thread that you shouldn't have your rising edge detection inside an if statement? Or was that someone else? Go back and read it again.
Try to fill all the posibilities of if statements so that for every run the program will know which value correspond to a variable. If statement has almost always go with else or elsif options to not produce latches..
A latch can occur when a process is allowed to go from start to finish without the driven outputs being assigned a value. That is if you have any conditional statements in your process and your outputs are driven inside these conditional statements then there a high chance that the outputs may never be driven. To avoid this it is good practice to place a concurrent statement at the beginning of your process to ensure your outputs are being set at least once. This will tell your synthesiser not to create a latch.