signal myreg: std_logic := '0';what is initializer?
Yes, you can write an initialization also in a port definition. But, it will be only effective as long as the port isn't driven by another signal.is it possible to initialize signals that are defined in port of an entity
Async resets have always had the issue of asynchronous deassertion. What this means is that, when skew is taken into account, some registers can come out of reset on different cycles. For high speed designs, the systems might come out of reset several cycles apart in which case it could fail every time. For low speed designs, the circuit might just fail sometimes. The slower the design, the lower the probability of failure.
Thank you...
but i am a beginner.. this is my first time into synthesis....
till now i have done only simulation, using functions, procedures, packages & components...
could u suggest some books... (synthesizable statements, creating modules and including them into top-level code, dealing with clocks/delays)
thanks in advance...
process(clk, reset) --leave out reset if you want
begin
if reset = '1' then
--reset your signals here
elsif rising_edge(clk) then
--do stuff here
end if;
end process;
The below example follows your suggestion, as far as I understand it. It sets output q to '1' in an asynchronous reset path. The reset signal is tied to '0' in the top entity. What do you expect as actual initial state of output q?At least with Quartus, it determines the power-up value from the aync reset path in the code. Then, assuming you have no real need for an async reset, just tie it to '0' at the top level.
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity reset_test is
port
(
clk : in std_logic;
d : in std_logic;
q : out std_logic
);
end entity;
architecture rtl of reset_test is
signal reset: std_logic := '0';
begin
process (clk, reset)
begin
if reset = '1' then
q <= '1';
elsif rising_edge(clk) then
q <= d;
end if;
end process;
end rtl;
Perhaps for that most of engineer prefers Async reset assertion and Sync desertion. And always clock and reset will be routed on global network so there is minimum skew on these nets compare to others.Async resets have always had the issue of asynchronous desertion. What this means is that, when skew is taken into account, some registers can come out of reset on different cycles. For high speed designs, the systems might come out of reset several cycles apart in which case it could fail every time. For low speed designs, the circuit might just fail sometimes. The slower the design, the lower the probability of failure.
The below example follows your suggestion, as far as I understand it. It sets output q to '1' in an asynchronous reset path. The reset signal is tied to '0' in the top entity. What do you expect as actual initial state of output q?
Actually it's '0', as predictable in my opinion. You have to add an initializer q: std_logic := '1' to change it. Do you understand why?
Yes, if the reset signal is actually connected to a reset source and asserted at power on. The example however ties reset to '0' to implement a suggestion from a previous post. In this case, the reset path is completely removed from the synthesized logic and the default '0' power on default gets effective.so when board powers up there will be reset pulse due to capacitor charging and discharging so initial state of q will be 1
UUT: entity work.my_entity
port map (
clk => clk,
reset => '0'
...
);
It does not work in my test. There's no different result between different methods that disable the reset path before synthesis. How do you check the initial state of the register?I was thinking of just shorting it in the port map (and this works):
library ieee;
use ieee.std_logic_1164.all;
entity reset_test is
port
(
clk : in std_logic;
d : in std_logic;
reset : in std_logic;
q : out std_logic
);
end entity;
architecture rtl of reset_test is
begin
process (clk, reset)
begin
if reset = '1' then
q <= '1';
elsif rising_edge(clk) then
q <= d;
end if;
end process;
end rtl;
library ieee;
use ieee.std_logic_1164.all;
entity reset_test_top is
port
(
clk : in std_logic;
d : in std_logic;
q : out std_logic
);
end entity;
architecture rtl of reset_test_top is
begin
reset_inst: entity work.reset_test
port map (
clk => clk,
d => d,
reset => '0',
q =>q
);
end rtl;
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