Delay Counter

[nigel502]

Please forgive me - as I am very very new to VHDL - but I'm trying to
get a LCD to work strictly in VHDL that uses a HLL (C/Java) to dump
down a command - and then enters a delay loop. We decided that the loop
was too costly and are trying to move it into vhdl. So I entered this
loop into the section of the FSM where waiting needs to happen:

x <= '1';
for i in 5000 downto 0 loop
x <= not x;
end loop;

However this loop isn't synthesised, in fact when we flash the chip it
skips over it stating that the sof files are the same. I'm assuming
that you can't syntesize a delay loop?

My next question is how do I create and subsequently call a counter
that will satisfy my requirments. Say I need a 500ns delay - how do I
call this from the procedure that houses the FSM?

 

[D Stanford]

My next question is how do I create and subsequently call a counter

that will satisfy my requirments. Say I need a 500ns delay - how do I
call this from the procedure that houses the FSM?

What you need is a synchronous counter, and it's easy to do:

process(clk, reset)
if reset = '1' then
counter <= (others => '0')
elsif clk'event and clk = '1' then -- or rising_edge(clk)
if counter_en = '1'
counter <= counter + 1;
end if;
end if;
end process;

What you need to define is what is the clock rate? What input do I have
that I can use to enable the counter? What output from the counter do I
want to use? What do I want the counter do do when I'm not using it?

 

[dipesh.trivedi]

hey buddy,
u just have to do is. reset the counter with the required sequence. n
then
assign it within the clock. i mean to say u can make the ring counter
which will provide u with the sequence generator.

signal seq: std_logic_vector (3 downto 0);

process( clk)

begin

if ( reset = '1')
seq <= "0101";
elsif (clk'event and clk = '1') then
seq <= seq (0) & seq( 3 downto 1);
end if ;

end process;

i hope its write. neways am writting code in VHDL aftr 2 years.
if am wrong pls correct me.

 

[dipesh.trivedi]

hi there,
i think David is right, the thing you have to do is, u have to count
the clock cycle to make the delay in ckt. for that u must know the clk
frquency n then decide the min. no. of flip-flop u wil require to
count. for eg. if ur clock period is 50ns then u have to count 100 for
5000ns delay. aftr counting 100 set the flag which directs u to the nxt
state.
i hope it wil work n it is synthesisable.

what do u say David?
m i right? if not then pls correct me.
wil be waiting for ur reply.

 

[Jonathan Bromley]

i hope it wil work n it is synthesisable.

what do u say David?
m i right? if not then pls correct me.
wil be waiting for ur reply.

Can someone kindly explain to me why it is appropriate to
write "synthesisable" in full, and then use the ugly text-speak
abbreviations "u", "ur", "m" and so on? Do our readers
not deserve the courtesy of a decently written response?
--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
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Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
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The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

 

[dipesh.trivedi]

Can someone kindly explain to me why it is appropriate to
write "synthesisable" in full, and then use the ugly text-speak
abbreviations "u", "ur", "m" and so on? Do our readers
not deserve the courtesy of a decently written response?

dnt think so. this group is for sharing knowledge n not to show the
spelling mistakes or grammer mistakes.
i think u shud b a english language consultant rather then techno
consultant.

so pls if u r nt solving or helping to solve problems here in
technology.
then jus dnt reply MY DEAR FRIEND.

 

[D Stanford]

dnt think so. this group is for sharing knowledge n not to show the
spelling mistakes or grammer mistakes.
i think u shud b a english language consultant rather then techno
consultant.

And in sharing knowledge, ones speech and writing implies things about
them as well. It can be much easier to take someone seriously when
they're grammar and spelling attempt to be correct instead of using
'text' or 'l33t' speek, since these are typically not used in a
professional setting.

 

[Rtafas]

You shoud read about digital systems and take some look on synchrous
digital logic. Another point is search about sequential/concurrent
statements. You cannot understand this problem of yours without this
background.

After those reading, lets say that the compiler check this code this
way, if it is not within a process: the same time x gets 1, it gets
5001 times not x. and x can't have 5002 sources at the SAME time.

Regards

Rtafas

 

[Mike Treseler]

so pls if u r nt solving or helping to solve problems here in
technology.
then jus dnt reply

Post whatever you like,
but consider that you may be reducing
the pool of potential question-answering readers
to the very few who write with your particular style.

-- Mike