Multiplexer

[john]

Hi,

I am trying to build a multiplexer. It has two select lines . When its
'11', it selects A when its '00' it selects B . Now, i do not want to
select anything else and maintain A or B at its output even the select
line changes to anything else like "01" or "10"... How can I do that .
I saw may examples and found that { when others => A <= ( others =>
'X' ) }

John.

 

[Rtafas]

Have you ever considered reading some digital systems book and/or heard
about flip-flops or latches? VHDL and it's "a lot more understandable"
with this knowledge. You shoud *really* do this.

You must use a process to save information. Flip-flops and latches are
generetade only this way. Something like:

process(clk)
begin
if clk'event and clk = '1' then
if sel_lines = "11" then
out <= a;
elsif sel_lines = "00" then
out <= b;
end if;
end if;
end process;

What you have been reading was about assyncronous logic (gates). It
won't save any value for you.

Regards,

Ricardo

john escreveu:

 

[john]

Hi Rtafas,
Thanks for ur reply! Have you ever considered to read writers like
shakespear or milton or books like Bible or Quran or intorduction to
ethics. If not, you should read them, before you die. because
engineering or sciences , did take alot of common sense from your
brain.
I had a simple confusion about the mux. I apologize that I put it on
the group. But you do not have to answer it if you think its too simple
for u. why did u bother to answer it. let it answer by other people.
Regards
John

 

[Mike Treseler]

I am trying to build a multiplexer.

http://www.google.com/search?q=vhdl+multiplexer+example+process

 

[Rtafas]

Hey, calm down buddy. You sound a little nervous.

You had a doubt. I've made some suggestion, and it was a very good and
important one. You really should consider reading a good digital
systems book (that is not an offense, its a resonable hint due to your
question: you do not seem experienced in digital design and if so,
sorry then). Then I´ve replied your question with a piece of code and
I gave you a good explanation. No need for flaming.

Maybe you should take things with a more technical view than a personal
one, because no one is here to flame you. I won't give you a hug after
answering any question.

Sorry for the joke, regards

RTafas

john escreveu:

 

[john]

Hi,
I am sorry that I sounded angry or flaming. I do believe that u do need
to study those books and writers atleast once in ur lifetime for ur own
good. The materail, I mentioned is easily available at barnes and
nobles or any book shop in any counrty of the world. i will recommed
Hamlet by shakespear and bible. Please read them. Thanks again for ur
piece of code and reply.
Bye
John

 

[Andy]

You do not have to have an _explicit_ process to save information:

q <= d when rising_edge(clk); -- flip flop

q <= d when en = '1'; -- latch

output <= a when sel = '11' else b when sel = '00'; -- mux + latch

That said, latches are not generally a good idea in ASICs and FPGAs.

Andy

 

[Rtafas]

A very good point. Have you tried using a reset? Something like:

q <= d when clk'event and clk = '1' else '0' when rst = '1';

I imagine that using a clock enable, must be this way.

q <= d when clk'event and clk = '1' and clken = '1' else '0' when rst =
'1';

If synthesizable, it is an interesting aprouch. Have you tried such
situations?


Andy escreveu:

 

[Jonathan Bromley]

Have you ever considered to read writers like
shakespear or milton or books like Bible or Quran or intorduction to
ethics. If not, you should read them, before you die. because
engineering or sciences , did take alot of common sense from your
brain.

It is entirely possible to be a good engineer who also possesses
common-sense and some familiarity with both high culture and
ethics. It is, however, impossible to be a good engineer
without first finding out something about what other
engineers have done in the past. Consequently, the advice
you received to consult a standard digital design text was
well-considered and highly appropriate.

As others have pointed out, it is perfectly possible to mix
Shakespeare and HDLs:

architecture soliloquy of prince_of_denmark is
signal question: bit_vector (7 downto 0);
begin
question <= X"2B" OR (NOT X"2B");
end;

Milton may be a little trickier, and of course Goedel is
in principle impossible to express in any finite
programming language
--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
(e-mail address removed)
http://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

 

[Martin Thompson]

A very good point. Have you tried using a reset? Something like:

q <= d when clk'event and clk = '1' else '0' when rst = '1';

I imagine that using a clock enable, must be this way.

q <= d when clk'event and clk = '1' and clken = '1' else '0' when rst =
'1';

If synthesizable, it is an interesting aprouch. Have you tried such
situations?

I just stuck this through Synplify, and it's fine with Andy's
original. It doesn't like yours, but it's OK with:
q1 <= '0' when rst = '1' else d1 when rising_edge(clk);
q2 <= '0' when rst = '1' else d2 when rising_edge(clk) and clken =
'1';

Anyone else care to try it on other tools? Here's the full code:

library ieee;
use ieee.std_logic_1164.all;

entity ffs is
port (
clk : in std_logic;
clken : in std_logic;
rst: in std_logic;
d,d1,d2 : in std_logic;
q, q1,q2 : out std_logic
);

end entity ffs adder;
architecture a1 of ffs adder is
begin
q <= d when rising_edge(clk);
q1 <= '0' when rst = '1' else d1 when rising_edge(clk);
q2 <= '0' when rst = '1' else d2 when rising_edge(clk) and clken = '1';
end architecture a1;

Cheers,
Martin

 

[Ben Jones]

As others have pointed out, it is perfectly possible to mix
Shakespeare and HDLs:

architecture soliloquy of prince_of_denmark is
signal question: bit_vector (7 downto 0);
begin
question <= X"2B" OR (NOT X"2B");
end;

Milton may be a little trickier

The top-level would be something like:

architecture lost of paradise is

type satan_t is (GIANT, CHERUB, CORMORANT, TOAD, SERPENT);
signal satan : satan_t;

begin

process(reset, uriel, man, satan)
begin
if reset = '1' then
satan <= GIANT;
elsif uriel = '1' then
satan <= CHERUB;
elsif satan'descending then
satan <= CORMORANT;
elsif falling_edge(man) then
satan <= TOAD;
satan <= SERPENT after 604800000 ms;
end if;
end process;

end lost;

I think you'd need a 10-man verification team and another 50,000 lines of
HDL to do it properly though.

-Ben-

 

[Andy]

I should have noted in my previous post that these concurrent statement
versions of registers are not the most efficient to simulate. The
sensitivity list of the process implied by the concurrent assignment
statement includes every signal appearing on the RHS of the statement,
therefore including D. This does not affect the simulation results.
For a few occurrences it would not make much difference in performance
either, since the processes will not really do anything but wake up,
check the clock, and go back to sleep on events on D. But this thwarts
an important optimization in a lot of simulators: merging processes
that share the same sensitivity list. In traditional synchronous
processes, all of the lists consist of only the clock and maybe an
asynchronous reset, allowing lots of processes to be merged, saving the
setup/teardown of process invocation on all but one.

So, while these are fine for the occasional use, where you "just need a
flop", especially in structural code that does not have any (explicit)
processes already, I would not make a habit of coding them this way,
particularly on large projects.

Andy

 

[john]

Hi,

Thanks guys.. I was out of town so I did not get a chance to look at
the group. I will get back to you soon!

John

 

[Chris Foster]

Hi Rtafas,
Thanks for ur reply! Have you ever considered to read writers like
shakespear or milton or books like Bible or Quran or intorduction to
ethics. If not, you should read them, before you die. because
engineering or sciences , did take alot of common sense from your
brain.
I had a simple confusion about the mux. I apologize that I put it on
the group. But you do not have to answer it if you think its too simple
for u. why did u bother to answer it. let it answer by other people.
Regards
John

WOW After helping him, he gives you a rash of shit. What an ass

 

[john]

Hello,
It is, however, impossible to be a good engineer
without first finding out something about what other
engineers have done in the past.

I agree with you. The same rules applies to writers too. I suggest you
that when you try to experiemnt with ethics, you should lookup the past
works of shakespear, milton or jesus. And do not mock them by
arrogantly mixing them with your living making tool VHDL. God knows why
DOD invented VHDL? What were they intentions to build it? But history
will tell you that the writers I mentioned above did have true
intentions about the people and their true prosperity.
I think you should think about reading those books before it gets too
late. you can not define those works with VHDL or believe me you or
yourself do not want to be defined by VHDL too
Please consider my highly appropiate advice!
Regards,
John

 

[Jonathan Bromley]

[...]

I think you should think about reading those books before it gets too
late.

I think you should reconsider your arrogant and unjustified
assumptions about my reading habits.
--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
(e-mail address removed)
http://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

 

[Jonathan Bromley]

I am trying to build a multiplexer. It has two select lines . When its
'11', it selects A when its '00' it selects B . Now, i do not want to
select anything else and maintain A or B at its output even the select
line changes to anything else like "01" or "10"

This isn't really a multiplexer; it's similar to a "Muller C-element"
used in self-timed logic design. It's quite a challenge to build
self-timed logic primitives in an FPGA, because the
optimisations generally are designed to work well on
synchronous rather than asynchronous circuits. A simulation
model is easy enough to create:

signal A, B, Y: ...;
signal S: std_logic_Vector (1 downto 0);
....
process (A, B, S)
begin
case S is
when "00" =>
Y <= B;
when "11" =>
Y <= A;
when "10" | "01" =>
null; -- no change
when others =>
report "Selector contains meta-values"
severity error;
end case;
end process;

Synthesis will build from this a latch that is frozen when the XOR of
S(0) and S(1) is '1', and a multiplexer selected by either bit of
S (obviously it doesn't matter which). It is not at all obvious that
synthesis and place-and-route will preserve a hazard-free
implementation all the way through to the FPGA implementation.
You may need to develop a hazard-free netlist of this element
and then set it up as a relatively-placed macro, making sure
that it has "don't optimize" attributes on it whenever you
use it. This is definitely not my area of expertise, but
I hope some of the comments above may point you in
the right direction.

For a readable introduction, with a few useful references, try
http://www.sun.com/processors/throughput/SciAm_Reprint.pdf
--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
(e-mail address removed)
http://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

 

[Mike Treseler]

Please consider my highly appropiate advice!

It is neither appropriate nor on-topic in this newsgroup.

-- Mike Treseler

 

[john]

Hi,
Thanks Mike.

Regards
John

 

[john]

Hi,
I think you should reconsider your arrogant and unjustified

assumptions about my reading habits.

I am not concern about your reading habits my friend. I am just
concerned about what you do after reading them.

Regards
John