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Design Languages >> Verilog-AMS >> verilog ams ADC model with PSS
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Message started by conr on Sep 26^th, 2012, 12:56am

Title: verilog ams ADC model with PSS
Post by conr on Sep 26^th, 2012, 12:56am

I have designed a very simple ADC model in verilog AMS. it works with dc and tran analysis but not with PSS. Anyone know why it happens and how to fix it? It is a problem of PSS analysis or something related to verilog AMS events?
analog begin

@(initial_step("dc")) begin
if (dc_value==`LIMIT) begin
dc_calc=V(vin)/vref*`LIMIT;

end
else begin
dc_calc=dc_value;
end
mod[0]=dc_calc%2;
div[0] = dc_calc /2;

for (i = 1; i <= (`NUM_ADC_BITS-1) ; i = i + 1) begin
mod[i] = div[i-1]%2;
div[i] = div[i-1]/ 2;
end

V(vd7) <+ mod[7]*vlogic_high;
V(vd6) <+ mod[6]*vlogic_high;
V(vd5) <+ mod[5]*vlogic_high;
V(vd4) <+ mod[4]*vlogic_high;
V(vd3) <+ mod[3]*vlogic_high;
V(vd2) <+ mod[2]*vlogic_high;
V(vd1) <+ mod[1]*vlogic_high;
V(vd0) <+ mod[0]*vlogic_high;

end

Title: Re: verilog ams ADC model with PSS
Post by Geoffrey_Coram on Sep 27^th, 2012, 12:24pm

When I read this model, it looks like all the contributions (V(vd7) <+) are done inside the @initial_step block.

I can't imagine how this model works at all, because these contributions do not persist after the first step. If you sweep (dc) or ramp (tran) the input, I don't see the module doing anything after the first point.

Title: Re: verilog ams ADC model with PSS
Post by conr on Sep 28^th, 2012, 12:44am

Hi Geoffrey,
many thanks for your reply.
I did a mistake coping the verilog model inside the topic. the real model had also a second section for transitorial analysis:

@ ( initial_step("tran") ) begin
halfref = vref / 2;
end

@ (cross(V(vclk) - vtrans_clk, 1)) begin
unconverted = V(vin);
for (i = (`NUM_ADC_BITS-1); i >= 0 ; i = i - 1) begin
vd[i] = 0;
if (unconverted > halfref) begin
vd[i] = vlogic_high;
unconverted = unconverted - halfref;
end else begin
vd[i] = vlogic_low;
end
unconverted = unconverted * 2;
end
end

//
// assign the outputs
//
V(vd7) <+ transition( vd[7], tdel, trise, tfall );
V(vd6) <+ transition( vd[6], tdel, trise, tfall );
V(vd5) <+ transition( vd[5], tdel, trise, tfall );
V(vd4) <+ transition( vd[4], tdel, trise, tfall );
V(vd3) <+ transition( vd[3], tdel, trise, tfall );
V(vd2) <+ transition( vd[2], tdel, trise, tfall );
V(vd1) <+ transition( vd[1], tdel, trise, tfall );
V(vd0) <+ transition( vd[0], tdel, trise, tfall );

The problem was the initialization of the variable halfref done just for "tran" simulation. I added "pss" in the initial_step function and now it works also for pss analysis.

Title: Re: verilog ams ADC model with PSS
Post by Geoffrey_Coram on Oct 1^st, 2012, 6:30am

I try to avoid initial_step for reasons such as this: you don't want to have to specify every possible analysis.

A simple division is not much work, even if the simulator does it every iteration. Many compilers will optimize this away so that it only gets executed once.

Alternately, you could use
localparam halfref = vref / 2;