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Design Languages >> Verilog-AMS >> Using $table_model with time as one axis
https://designers-guide.org/forum/YaBB.pl?num=1353589209

Message started by RolfK on Nov 22^nd, 2012, 5:00am

Title: Using $table_model with time as one axis
Post by RolfK on Nov 22^nd, 2012, 5:00am

Dear Experts,

I would like to trigger a curent wave by some event.
The event could by a kind of clock signal or an abstract verilog named event.

My questions is a conceptual one.
In case I would use a table model like shown below, I could imagine to assign it to a signal.
The constant inputs are MIN|TYP|MAX (as enumerator 0,1,2) and the current simulation dtime (time - the start time of this process ).
This way I hope the simulator interpolates the current given by the table dependent on simulation time.
This is quite similar to a PWL. BUT, I do not know how to start a PWL on some event and additinally it looks very benefitial to me to have the option of more consatnt parameters in one table.

Here is some PSEUDO CODE snipet:
analog
@(starting event) begin
start_time = 0 ;
always(@ $current_time ) begin
dtime = $current_time - start_time ;
I(p,n) n <+ $table_model (TYP, dtime , "iwave.tbl", "?,?,?") ;
end
end
end

######### table file #########################
// Look up table for i(t) 2d with time
// Index mtm time current
1 0 0 0
2 0 1.00e-12 0.1
3 0 2.00e-12 0.3
4 0 10.00e-12 0.05
5 1 0 0
6 1 1.00e-12 0.15
7 1 2.00e-12 0.35
8 1 10.00e-12 0.06
9 2 0 0
10 2 1.00e-12 0.2
11 2 2.00e-12 0.4
12 2 10.00e-12 0.07

If there is some alternative or better idea I wopuld like to learn about it.
Additionally I need to learmn more about the control string.
I had a look into the Cadence Verilog-A Reference model, but did not find an exhaustive explantion there.

Thanks a lot

RolfK

Title: Re: Using $table_model with time as one axis
Post by Geoffrey_Coram on Nov 26^th, 2012, 6:30am

Your pseudo-code isn't right; you shouldn't have everything inside the @(starting event), and you shouldn't have contributions inside an event. I've re-written it below, assuming you want 0 current up until the trigger event.

analog begin
@(initial_step) begin
start_time = 0;
enable = 0;
end
@(starting event) begin
start_time = $abstime;
enable = 1;
end
if (enable) begin
dtime = $abstime - start_time ;
ival = $table_model (TYP, dtime , "iwave.tbl", "?,?,?") ;
end else begin
ival = 0 ;
end

I(p,n) n <+ ival;

end // analog