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Design Languages >> Verilog-AMS >> timer function problem
https://designers-guide.org/forum/YaBB.pl?num=1301559713

Message started by Zorro on Mar 31^st, 2011, 1:21am

Title: timer function problem
Post by Zorro on Mar 31^st, 2011, 1:21am

Hi all,

I need some help to find out what is wrong with my code.

I want a signal "slope" to oscillate only when "state" is high.
The oscillation must start after a delay "startup_delay".
when state is low, there is no oscillation and slope retains its value.

the simulation in ADE reacts well for the first cross of state in the positive direction. you can see slope changes after the specified delay and continues oscillating with the specified frequency while state is high.

but for the second cross slope changes after the specified delay but goes automatically to its previous value so instead of having a pulse of lenght period/2 I only have a peak.

here is the code for the timer declaration:

...
real state, slope;
integer armed;

analog begin

@(initial_step) begin
slope = 1.0;
state = 0.0;
t1 = 0.0;
end

// Controlling the slope
@(cross( state - 0.5, +1)) begin
t1 = $abstime;
slope = -1.0;
armed=1;
end
@(cross( state - 0.5, -1)) begin
slope = slope;
armed=0;
t1=0;
end
if (armed) begin
@(timer(t1+startup_delay, duty/fc)) begin
slope = -1.0 * slope;
end
end
...
end

Any suggestion?

Thank you.

Title: Re: timer function problem
Post by boe on Mar 31^st, 2011, 2:14am

Zorro,

Timer event statements inside conditional statements are not allowed (at least by the tools I use).

B O E

[Added] PS: As the start time of the timer event may be dynamic, it is not necessary to have it inside an if statement.

Title: Re: timer function problem
Post by Zorro on Mar 31^st, 2011, 3:08am

thanks for the reply boe,

if I simulate in SMASH I get what I expect.

if I simulate in ADE then I have this problem.

in both the timer event statements inside conditional statements are allowed. May I ask what simulator do you use?

then I don't know if the problem is the code itself or the simulator.

If I modify the timer function like this:

@(timer(t1+startup_delay, duty/fc, 1/fc))

then the pulses are recognized correctly in ADE, but not the delay i.e. it starts to oscillate exactly when the state signal goes high.

I don't know exactly how to limit this new parameter <timeTol>. (Designer's Guide to VerilogAMS, page 206)

PS: As the start time of the timer event may be dynamic, it is not necessary to have it inside an if statement.

ummm... does it mean that it is not possible to, let's say "reset" the start of the timer function?

every time state goes high I am sampling a new t1...but if it is so, this t1 is only being considered the first time and the other t1s are ignored.

from "Designer’s Guide to Verilog-AMS", page 78:

"The timer function produces an event at the (absolute) time given by its first argument, and if its second argument is present,
it will produce subsequent events with a period equal to the value of the second argument."

if you have any suggestion please let me know.

thanks and regards!

Zorro

Title: Re: timer function problem
Post by boe on Mar 31^st, 2011, 3:52am

Zorro wrote on Mar 31^st, 2011, 3:08am:

thanks for the reply boe,
in both the timer event statements inside conditional statements are allowed. May I ask what simulator do you use?

Cadence NCSim: Cadence Verilog-AMS Reference (June 2010) states "do not use the timer function inside conditional statements."

Quote:

then I don't know if the problem is the code itself or the simulator.

NCSim does not seem to support your code (see above).

Quote:

I don't know exactly how to limit this new parameter <timeTol>. (Designer's Guide to VerilogAMS, page 206)

<timeTol> is the timing error for an edge you consider negligible.

Quote:

ummm... does it mean that it is not possible to, let's say "reset" the start of the timer function?

It should be possible. I expect NCSim to test if abs($abstime - (startTime+k*period) ) < timeTol, k non-negative integer. Cadence: "The simulator ... generates events at each multiple of period after start_time."

Quote:

if you have any suggestion please let me know.

For Cadence NCSim, place if statement inside timer event block. If this does not help, you could try to use a non-periodical timer event.

B O E

Title: Re: timer function problem
Post by boe on Mar 31^st, 2011, 9:27am

Zorro,

Did you notice the post http://www.designers-guide.org/Forum/YaBB.pl?num=1299882714?
[edit]Uses a non-periodical timer. Might cause fewer portability issues.[/edit]

B O E

Title: Re: timer function problem
Post by Geoffrey_Coram on Apr 7^th, 2011, 7:14am

What if you change
if (armed) begin
@(timer(t1+startup_delay, duty/fc)) begin
slope = -1.0 * slope;
end
end

to
@(timer(t1+startup_delay, duty/fc)) begin
if (armed) begin
slope = -1.0 * slope;
end
end

?