counter + somesteps

[Rafa]

Hello to everyone. I´m new in VHDL and i have (maybe) just a simple
question.
There´s a counter who counts steps from a PLL.
I want to add a special function wich makes the counter add some steps
during a circle
like this:




IF (PLL'event AND PLL = '1') THEN

counter <= counter + 1 + somesteps
END IF;

The problem:

If there is no adjustment the somestep-Input is 0.
If there are somesteps to add the counter adds the value of
"somesteps" for every PLL-Edge.
But I want to add is only one time.

Has someone an idea?

 

[Ralf Hildebrandt]

Rafa wrote:

IF (PLL'event AND PLL = '1') THEN

counter <= counter + 1 + somesteps
END IF;

The problem:

If there is no adjustment the somestep-Input is 0.
If there are somesteps to add the counter adds the value of
"somesteps" for every PLL-Edge.
But I want to add is only one time.

I'm not sure, if I understand you right, but what about:


IF (PLL'event AND PLL = '1') THEN
if (adjustment='1') then
counter <= counter + somesteps + 1;
else
counter <= counter + 1;
end if;
END IF;

Signal adjustment has to be updated before next rising_edge(PLL),
whether somesteps has to be added or not.


The above code will synthesize as a half adder (full adder with constant
"somesteps" input) and carry_in.

Ralf

 

[Rafael. N]

Am Wed, 04 Feb 2004 16:26:23 +0100 schrieb Ralf Hildebrandt

Rafa wrote:




I'm not sure, if I understand you right, but what about:


IF (PLL'event AND PLL = '1') THEN
if (adjustment='1') then
counter <= counter + somesteps + 1;
else
counter <= counter + 1;
end if;
END IF;

Signal adjustment has to be updated before next rising_edge(PLL),
whether somesteps has to be added or not.


The above code will synthesize as a half adder (full adder with constant
"somesteps" input) and carry_in.

Ralf

It´s a good idea!
The problem is, that "adjustment" is driven by a microprocessor so that
it has for a lot of PLL-Circles the value '1'.
That would result in adding "somesteps" for more than one PLL-Circle.

 

[Anders Hellerup Madsen]

It´s a good idea!
The problem is, that "adjustment" is driven by a microprocessor so that
it has for a lot of PLL-Circles the value '1'.
That would result in adding "somesteps" for more than one PLL-Circle.

I've done something like this a couple of times, but I am not entirely
sure I am doing it the best way. I meant to look it up, but when I
weren't encountering any serious problems, I just left it as it were.

Please correct me, someone, if this is crap. I am sort of a newbie.

Anyway, what I'm doing is this:


if (PLL'event and PLL = '1') then
if (adjustment = '1' and adj_old = '0') then
counter <= counter + somesteps + 1;
adj_old = '1';
elsif (adjustment = '0')
counter <= counter + 1;
adj_old <= '0';
else
counter <= counter + 1;
adj_old <= adj_old; -- To avoid latch.
end if;
end if;


This would of course only work, if you can be sure that the ajustment
signal doesn't go low and high between PLL events.

Have you considered the posibillity of making this a clocked process?
That would make things a lot easier, then you just implement it as a
statemachine and make the clock run fast enough to be absolutely sure
that adjustment is high more than one cycle.

-- Anders

 

[Sajan]

May be this will help

if (PLL'event and PLL = '1') then
if (adjustment = '1' and adj_old = '0') then
counter <= counter + somesteps + 1;
else
counter <= counter + 1;
end if;
adj_old <= adjustment;
end if;


Cheers,
Sajan.

 

[Anders Hellerup Madsen]

May be this will help

if (PLL'event and PLL = '1') then
if (adjustment = '1' and adj_old = '0') then
counter <= counter + somesteps + 1;
else
counter <= counter + 1;
end if;
adj_old <= adjustment;
end if;

Yeah, that looks a lot smoother than my solution. If I were Rafa, I'd
hang on to that one.

-- Anders

 

[Rafa]

My special problem was that i wanted to add somesteps allways.
- sometimes within data sometimes without (somesteps <= 0)
..... i didn´t want to chance the state-machine that exist in my previous code.
I solved it now this way.

adj: PROCESS (Inp_Time_Flag,PLL)
BEGIN
IF Inp_Time_Flag = '0' Then
T_counter <= 0;
somesteps <= (others =>'0');

ELSE
IF T_counter < 4 THEN
IF PLL'event AND PLL = '1' THEN

CASE T_Counter IS
WHEN 1 => somesteps <= (Inp_Sec);
WHEN others => somesteps <= (others =>'0');
END CASE;

T_counter <= T_counter + 1;

END IF;
END IF;
END IF;
END PROCESS adj;

I don´t know if this is "clean" code - but it works.
If someone could comment this solution....?!

 

[Ralf Hildebrandt]

adj: PROCESS (Inp_Time_Flag,PLL)
BEGIN
IF Inp_Time_Flag = '0' Then
T_counter <= 0;
somesteps <= (others =>'0');

ELSE
IF T_counter < 4 THEN
IF PLL'event AND PLL = '1' THEN

'event detection inside an if-statement is not allowed for synthesis.
Just exchange the order:

IF PLL'event AND PLL = '1' THEN
IF T_counter < 4 THEN

.... and make sure, that the behavior is o.k. for you.

CASE T_Counter IS
WHEN 1 => somesteps <= (Inp_Sec);
WHEN others => somesteps <= (others =>'0');
END CASE;

O.k. here somesteps are flipflops following a mux.

When adding somesteps to an other value this adder will be a full adder.


Ralf

 

[Rafa]

OK - it works!!!!!!!
-- thanks a lot to all of you..

Regards Rafa