latch inferrence in clocked process

[vu]

Hello,

I am a beginner to VHDL and new to this forum, and I hope someone could
give me a hand with this question.

Can a latch be inferred in a process that has clk in the sensitivity
list? If no, then does this mean the if statement does not require and
else statement to match it? Something along the lines of :

.....
process(clk) is
begin
if (rising_edge(clk)) then
if (a = '1') then b<= '0'
end if;
end process;
.....

 

[Kai Harrekilde-Petersen]

Hello,

I am a beginner to VHDL and new to this forum, and I hope someone could
give me a hand with this question.

Can a latch be inferred in a process that has clk in the sensitivity
list?

Yes. The following code will infer a latch for q(1) in Synopsys DC (at
least last time I 'tried'):

architecture rtl of example is
signal q : std_logic_vector(1 downto 0);
begin
process (reset, clk) is
begin
if reset = '0' then
q <= (others => '0');
elsif rising_edge(clk) then
if ena = '1' then
q(0) <= d;
end if;
end if;
end process;
end rtl;

If no, then does this mean the if statement does not require and
else statement to match it? Something along the lines of :

(snip)

If you don't have an else cause, the effectively create a flop with an
enable bit - see the code snippet above which will infer an FF with an
enable bit for q(0).


Kai

 

[Mike Treseler]

Can a latch be inferred in a process that has clk in the sensitivity
list?

no
A synchronous "latch" is a d-flop.

If no, then does this mean the if statement does not require and
else statement to match it?
yes

Something along the lines of :

....
process(clk) is
begin
if (rising_edge(clk)) then
if (a = '1') then b<= '0'
end if;
end process;
....

Once you fix the syntax errors,
your example would latch low forever
when A goes high. But since an fpga
registers start low on configuration,
synthesis would likely just wire
B to ground for you.

A more interesting example would give
the output some way to recover.

For a synchronous design, think about
how you want your registers to
initialize and update Describe that
exactly, and don't worry about what
synthesis will do.


-- Mike Treseler

____________________________
library ieee;
use ieee.std_logic_1164.all;
entity no_else is
port (a, clk : in std_ulogic;
b : out std_ulogic);
end no_else;

architecture sim of no_else is
begin
process(clk) is
begin
ck : if (rising_edge(clk)) then
b <= a;
if (a = '1') then
b <= '1';
end if;
end if ck;
end process;
end architecture;

 

[arant]

> Can a latch be inferred in a process that has clk in the sensitivity
> list?

One situation that I can think of is for a clock gate block like this

entity clk_gate_cell
port (
clk , enable : in std_logic;
clk_out : out std_logic);
end clk_gate_cell;

architecture str of clk_gate_cell is
begin
--- combo logic for clk gating
--- pass clock only when enable = '1'
p_clkgate : process(clk,enable)
begin
if (enable = '1') then
clk_out <= clk;
else
clk_out <= '0';
end if;
end process p_clkgate;
end str;

if the italicised portion is left out then a latch will be inferred even when clock is there in the sentivity list ... ok I agree we never use clock gating like this in real SoCs but hey what the heck