Hello I would love your help
I designed a simple binary counter,
though if I do not put the single Q in the sensitivity list. The counter
updates falling_edge Why is that?
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.all;
use IEEE.STD_LOGIC_UNSIGNED.all;
entity Binary_Counter is
--generic( n : positive:=24);
Port (
Clk : in STD_LOGIC;
-- the LTF(n) signal
Up_Down : in STD_LOGIC;
-- Up_Down ='1' the counter counts up
-- Up_Down ='0' the counter counts down
T_Gate : in STD_LOGIC;
-- Enable count
O : in integer range 0 to 10:=2;
-- The number of times the counter works
ResetS : in STD_LOGIC;
-- Asynchronously reset
Pe : in STD_LOGIC;
-- Asynchronously loads data on the Preset Inputs.
Pi : in STD_LOGIC_VECTOR(23 downto 0);
--Data on these inputs is loaded into the counter when PE is
taken high.
Q_Out : out STD_LOGIC_VECTOR(23 downto 0);
-- Binary data is present outputs
Carry_Out : out STD_LOGIC);
--used to indicate terminal count.
end Binary_Counter;
architecture Behavioral of Binary_Counter is
signal Q : STD_LOGIC_VECTOR(23 downto 0):=(others=>'0');
begin
process(Clk,ResetS,Pe,T_Gate,Up_Down)
variable CountQ : integer range 0 to 10:=O;
begin
if( ResetS = '1') then Q<=(others=>'0');
elsif(CountQ /=0 and T_Gate='1') then
if(rising_edge(clk)) then
if( pe='1' ) then Q<= pi;
elsif(Up_Down='1') then
if(Q=x"111111") then
Carry_Out<='1';
Q<=x"000000";
else
Q<=Q+1;
end if;
else
if(Q=x"000000") then
Carry_Out<='1';
Q<=x"111111";
else
Q<=Q-1;
end if;
end if;
end if;
end if;
Q_Out<=Q;
end process;
end Behavioral;
Attached files:
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120 KB
At first some basics hints: IEEE.STD_LOGIC_ARITH.all thats outdated and should never be used. Next, write a clean clocked process. Only reset should be outside of rising_edge.
Oh well ... where to start? --- 1. --- use IEEE.STD_LOGIC_ARITH.all; use IEEE.STD_LOGIC_UNSIGNED.all; Do not use those! But use instead use ieee.numeric_std.all; --- 2. --- O : in integer range 0 to 10:=2; Try to use only std_logic and std_logic_vectors for ports. --- 3. --- Try to avoid variables if you do not know how they work. And the code suggests you do not know that. --- 4. --- CountQ is only assigned at the start of the process but never changed. So there is no need to use it at all. --- 5. --- process(Clk,ResetS,Pe,T_Gate,Up_Down) You can write it that way, but you only need the asynchronous signals + clock. So O, T_Gate, ResetS and Clk would be sufficient. --- 6. --- I do not understand the function of O. --- 7. --- Try to use "speaking Names" Pe, Pi say nothing to me, you could name them Preset_load and Preset_value or the like. --- 8. --- A very simmilar question (same counter) was asked in the german section of the forum. Look here: Beitrag "VHDL Anfänger braucht Unterstützung (keine Lösung)"
--- 9. --- Carry_Out is undefinded till it is set to '1'. This is an error! --- 10. --- Try to format your code for easy reding + understanding. --- 11. --- To your initial question: daniel wrote: > The counter updates falling_edge Why is that? This is because you have written (shorted):
1 | process (Clk,ResetS,Pe,T_Gate,Up_Down) |
2 | begin |
3 | if rising_edge(clk) then |
4 | |
5 | if Up_Down='1' then |
6 | if Q = x"111111" then |
7 | Carry_Out <= '1'; |
8 | Q <= x"000000"; |
9 | else |
10 | Q <= Q + 1; |
11 | end if; |
12 | else |
13 | if Q = x"000000" then |
14 | Carry_Out <= '1'; |
15 | Q <= x"111111"; |
16 | else |
17 | Q <= Q - 1; |
18 | end if; |
19 | end if; |
20 | end if; |
21 | |
22 | Q_Out <= Q; |
23 | |
24 | end process; |
So Q updates with every rising_edge of the clock. But the process itself is restarted with every action on all of these signals (Clk,ResetS,Pe,T_Gate,Up_Down), so it is restarted on the falling_edge of the clock, too. And because Q was updated on the rising_edge, with Q_Out gets the new value with the next process run which is with the next falling_edge. To avoid this you should write Q_Out <= Q; outside the process or write it inside the if rising_edge(clk) then end if; part. But: If you write it in the if rising_edge(clk) then ... part, you get 1 clock cycle latency. So you may want to write it outside the process.
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