(newbie) 2 read/write register


M

Max

I have two read/write register; each one has its chip-enable, thane
there are data bus, and read/write signals.

I wrote the following code, and seems to work:
----------------8<---------------------
entity reg_rw is
Port ( rd : in std_logic;
wr : in std_logic;
ce0 : in std_logic;
ce1 : in std_logic;
reset : in std_logic;
db : inout std_logic);
end reg_rw;

architecture Behavioral of reg_rw is

signal data0,data1: std_logic;

begin

--- write 0
write0: process (reset, ce0, wr)
begin
if reset = '1' then
data0 <= '0';
elsif ce0 = '1' then
if rising_edge(wr) then
data0 <= db;
end if;
end if;
end process;
-------------

--- write 1
write1: process (reset, ce1, wr)
begin
if reset = '1' then
data1 <= '0';
elsif ce1 = '1' then
if rising_edge(wr) then
data1 <= db;
end if;
end if;
end process;
------------

--- read global
read_global: process (ce0, ce1, rd)
begin
if ce1 = '0' and ce0 = '0' then
db = 'Z';
elsif rd = '1' then
if ce1 = '1' then
db <= data1;
elsif ce0 = '1' then
db <= data0;
end if;
end if;
end process;
--------------

end Behavioral;
--------------8<-----------------------

There is another way to do the same thing? For examble I would like to
put all code regarding a register in a single process, so how can I
write a process to read/write one register (the processes would be
"read_write0" and "read_write1")?
If it is not possible to put both read and write inside a sigle
process, then I would like to split read: how can I write two process
that each read only one register (4 processes: read0, write0, read1
and write1)?


thanks
 
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E

eadgbe

You need to get into the habit, right now, of using just clock and reset in your sensitivity list.

process(reset,clk)
begin
if reset = '1' then
data0 <= '0';
data1 <= '0';
elsif rising_edge(clk) then
if ce0 = '1' then
data0 <= db;
end if;
if ce1 = '1' then
data1 <= db;
end if;
end if;
end process;

That's the proper way to do an enabled register. Don't try to wrap the clock inside
the enable. You can call your clock "wr"....whatever. But think of it as a clock and
keep your design synchronous.

As for the read....thats just a mux. Do it in a concurrent statement.

db <= 'z' when rd = '0' else
data0 when ce0 = '1' else
data1;

And you have to make damned sure that wr and rd aren't high at the same time or you are going
to trap what you are reading out.

Bob
 

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