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Hidden states in digital converter control blocks
Mar 08^th, 2021, 1:48am

Hello,
I'm trying to develop a hidden state-free behavioral model for a buck converter digital compensator. I tried to follow the directions from "Hidden State in SpectreRF" by Ken Kundert, therefore I used idt operators to store the values of the variables. However, I noticed that some variables or events will not update or execute, and even so, the hidden state error is still present.

To show an example of the problem, the following code tries to implement a digital averaging block that can be used to sense the output of the converter. What was noticed is that:
- if the 0.25 value is remove from "var_v1 = 0.25*var_adc;"(last timer event) the whole event will no longer be executed.

- the idt storage variables were seen as hidden states

- if I move the timer storage variable asignement "ev_clk_rise_t_idt = idt(0, ev_clk_rise_t, ev_clk_rise_ack);" before the first timer event, the this particular variable will no longer be seen as hidden and the code will still work.

virtuoso version 6.1.8-64b 11/10/2020

`include "constants.vams"
`include "disciplines.vams"

module BNA_210307_Hidden_state_sum_test(adc_in, clk, avrg_out);
input adc_in;
input clk;
output avrg_out;

electrical adc_in;
electrical clk;
electrical avrg_out;

real ev_clk_rise_t = 0;

real var_avrg = 0;
real var_avrg_idt = 0;

real var_v1 = 0;
real var_v2 = 0;
real var_v3 = 0;
real var_v4 = 0;
real var_v5 = 0;
real var_v6 = 0;
real var_v7 = 0;

real var_v1_idt = 0;
real var_v2_idt = 0;
real var_v3_idt = 0;
real var_v4_idt = 0;
real var_v5_idt = 0;
real var_v6_idt = 0;
real var_v7_idt = 0;

real ev_clk_rise_t_idt = 0;

integer var_adc = 0;

integer ev_clk_rise_ack = 0;

integer ev_var_v_ack1 = 0;
integer ev_var_v_ack2 = 0;
integer ev_var_v_ack3 = 0;
integer ev_var_v_ack4 = 0;
integer ev_var_v_ack5 = 0;
integer ev_var_v_ack6 = 0;
integer ev_var_v_ack7 = 0;

analog begin

var_v1 = 0;
var_v2 = 0;
var_v3 = 0;
var_v4 = 0;
var_v5 = 0;
var_v6 = 0;
var_v7 = 0;

ev_clk_rise_ack = 0;
ev_clk_rise_t = 0;
var_avrg = 0;

var_adc = 0;

ev_var_v_ack7 = 0;
ev_var_v_ack6 = 0;
ev_var_v_ack5 = 0;
ev_var_v_ack4 = 0;
ev_var_v_ack3 = 0;
ev_var_v_ack2 = 0;
ev_var_v_ack1 = 0;

//the average value should only be updated on the rising edge of the clock
@(cross(V(clk) - 0.5,+1))
begin
ev_clk_rise_t = $abstime;

var_avrg = (var_v1_idt + var_v2_idt + var_v3_idt + var_v4_idt + var_v5_idt + var_v6_idt + var_v7_idt)/7;

ev_clk_rise_ack = 1;
end

//the adc input should be read after 3us from the clock rising edge
@(timer(ev_clk_rise_t_idt + 2.94u))
begin
var_v7 = var_v6;
ev_var_v_ack7 = 1;
end

@(timer(ev_clk_rise_t_idt + 2.95u))
begin
var_v6 = var_v5;
ev_var_v_ack6 = 1;
end

@(timer(ev_clk_rise_t_idt + 2.96u))
begin
var_v5 = var_v4;
ev_var_v_ack5 = 1;
end

@(timer(ev_clk_rise_t_idt + 2.97u))
begin
var_v4 = var_v3;
ev_var_v_ack4 = 1;
end

@(timer(ev_clk_rise_t_idt + 2.98u))
begin
var_v3 = var_v2;
ev_var_v_ack3 = 1;
end

@(timer(ev_clk_rise_t_idt + 2.99u))
begin
var_v2 = var_v1;
ev_var_v_ack2 = 1;
end

@(timer(ev_clk_rise_t_idt + 3u))
begin

var_adc = 4095*V(adc_in);

if(var_adc > 4095) var_adc = 4095;
if(var_adc < 0) var_adc = 0;

var_v1 = 0.25*var_adc;

ev_var_v_ack1 = 1;
end

// the idt storage variables
ev_clk_rise_t_idt = idt(0, ev_clk_rise_t, ev_clk_rise_ack);

var_v7_idt = idt(0, var_v7,ev_var_v_ack7);
var_v6_idt = idt(0, var_v6,ev_var_v_ack6);
var_v5_idt = idt(0, var_v5,ev_var_v_ack5);
var_v4_idt = idt(0, var_v4,ev_var_v_ack4);
var_v3_idt = idt(0, var_v3,ev_var_v_ack3);
var_v2_idt = idt(0, var_v2,ev_var_v_ack2);
var_v1_idt = idt(0, var_v1,ev_var_v_ack1);

var_avrg_idt = idt(0, var_avrg, ev_clk_rise_ack);

V(avrg_out) <+ transition(var_avrg_idt,0, 100n);

end
endmodule

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Ken Kundert Global Moderator Offline Posts: 2386 Silicon Valley	Re: Hidden states in digital converter control blocks Reply #1 - Mar 13^th, 2021, 7:24pm Try removing the initialization in the declaration for those variables. -Ken
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Nicolae Adrian

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Posts: 2

Re: Hidden states in digital converter control blocks
Reply #2 - Mar 24^th, 2021, 6:58am

Ken Kundert wrote on Mar 13^th, 2021, 7:24pm:

Try removing the initialization in the declaration for those variables.

-Ken

Thanks Ken for your answer. I removed the initialization for those variables but still, the simulator sees them as hidden. I also made some corrections in the timers code since the update of the variable was not correct so here the new code:

Code:

`include "constants.vams"
`include "disciplines.vams"

module BNA_210307_Hidden_state_sum_test(adc_in, clk, avrg_out);
input adc_in;
input clk;
output avrg_out;

electrical adc_in;
electrical clk;
electrical avrg_out;


real ev_clk_rise_t � � � � � �= � � �0;

real var_avrg � � � � � � � � �= � � �0;
real var_avrg_idt � � � � � �= � � �0;


real var_v1 � � � � � � � � � � � �= � � �0;
real var_v2 � � � � � � � � � � � �= � � �0;
real var_v3 � � � � � � � � � � � �= � � �0;
real var_v4 � � � � � � � � � � � �= � � �0;
real var_v5 � � � � � � � � � � � �= � � �0;
real var_v6 � � � � � � � � � � � �= � � �0;
real var_v7 � � � � � � � � � � � �= � � �0;

real var_v1_idt;
real var_v2_idt;
real var_v3_idt;
real var_v4_idt;
real var_v5_idt;
real var_v6_idt;
real var_v7_idt;

real ev_clk_rise_t_idt;


integer var_adc					= � � �0;



integer � � �ev_clk_rise_ack	= � � �0;

integer � � �ev_var_v_ack1 � � �= � � �0;
integer � � �ev_var_v_ack2 � � �= � � �0;
integer � � �ev_var_v_ack3 � � �= � � �0;
integer � � �ev_var_v_ack4 � � �= � � �0;
integer � � �ev_var_v_ack5 � � �= � � �0;
integer � � �ev_var_v_ack6 � � �= � � �0;
integer � � �ev_var_v_ack7 � � �= � � �0;

analog begin


 � � var_v1						= � � �0;
 � � var_v2						= � � �0;
 � � var_v3						= � � �0;
 � � var_v4						= � � �0;
 � � var_v5						= � � �0;
 � � var_v6						= � � �0;
 � � var_v7						= � � �0;

 � � ev_clk_rise_ack			= � � �0; � � �
 � � ev_clk_rise_t				= � � �0;
 � � var_avrg					= � � �0;

 � � var_adc					= � � �0;

 � � ev_var_v_ack7				= � � �0;
 � � ev_var_v_ack6				= � � �0;
 � � ev_var_v_ack5				= � � �0;
 � � ev_var_v_ack4				= � � �0;
 � � ev_var_v_ack3				= � � �0;
 � � ev_var_v_ack2				= � � �0;
 � � ev_var_v_ack1				= � � �0;

 � � //the average value should only be updated on the rising edge of the clock
 � � @(cross(V(clk) - 0.5,+1))
 � � begin
 � � � � � ev_clk_rise_t � � �= � � �$abstime;

 � � � � � var_avrg � � � � � �= � � �(var_v1_idt � � �+ � � �var_v2_idt � � �+ � � �var_v3_idt � � �+ � � �var_v4_idt � � �+ � � �var_v5_idt � � �+ � � �var_v6_idt � � �+ � � �var_v7_idt)/7;

 � � � � � ev_clk_rise_ack � � �= � � �1; � � �
 � � end


 � � //the adc input should be read after 3us from the clock rising edge
 � � @(timer(ev_clk_rise_t_idt + 2.94u))
 � � begin
 � � � � � var_v7 � � � � � � � � �= � � �var_v6_idt;
 � � � � � ev_var_v_ack7 � � �= � � �1;
 � � end

 � � @(timer(ev_clk_rise_t_idt + 2.95u))
 � � begin
 � � � � � var_v6 � � � � � � � � �= � � �var_v5_idt;
 � � � � � ev_var_v_ack6 � � �= � � �1;
 � � end

 � � @(timer(ev_clk_rise_t_idt + 2.96u))
 � � begin
 � � � � � var_v5 � � � � � � � � �= � � �var_v4_idt;
 � � � � � ev_var_v_ack5 � � �= � � �1;
 � � end

 � � @(timer(ev_clk_rise_t_idt + 2.97u))
 � � begin
 � � � � � var_v4 � � � � � � � � �= � � �var_v3_idt;
 � � � � � ev_var_v_ack4 � � �= � � �1;
 � � end

 � � @(timer(ev_clk_rise_t_idt + 2.98u))
 � � begin
 � � � � � var_v3 � � � � � � � � �= � � �var_v2_idt;
 � � � � � ev_var_v_ack3 � � �= � � �1;
 � � end

 � � @(timer(ev_clk_rise_t_idt + 2.99u))
 � � begin
 � � � � � var_v2 � � � � � � � � �= � � �var_v1_idt;
 � � � � � ev_var_v_ack2 � � �= � � �1;
 � � end

 � � @(timer(ev_clk_rise_t_idt � � �+ � � �3u))
 � � begin

 � � � � � var_adc � � �= � � �4095*V(adc_in);

 � � � � � if(var_adc � � �> � � �4095) � � �var_adc � � �= � � �4095;
 � � � � � if(var_adc � � �< � � �0) � � � � � �var_adc � � �= � � �0;

 � � � � � var_v1 � � �= � � �0.25*var_adc;

 � � � � � ev_var_v_ack1 � � �= � � �1;
 � � end

 � � // the idt storage variables
 � � ev_clk_rise_t_idt � � �= � � �idt(0, ev_clk_rise_t, ev_clk_rise_ack);

 � � var_v7_idt � � � � � � � � �= � � �idt(0, var_v7,ev_var_v_ack7);
 � � var_v6_idt � � � � � � � � �= � � �idt(0, var_v6,ev_var_v_ack6); � � �
 � � var_v5_idt � � � � � � � � �= � � �idt(0, var_v5,ev_var_v_ack5);
 � � var_v4_idt � � � � � � � � �= � � �idt(0, var_v4,ev_var_v_ack4);
 � � var_v3_idt � � � � � � � � �= � � �idt(0, var_v3,ev_var_v_ack3);
 � � var_v2_idt � � � � � � � � �= � � �idt(0, var_v2,ev_var_v_ack2);
 � � var_v1_idt � � � � � � � � �= � � �idt(0, var_v1,ev_var_v_ack1);


 � � var_avrg_idt � � �= � � �idt(0, var_avrg, ev_clk_rise_ack);


 � � V(avrg_out) � � �<+ � � �transition(var_avrg_idt,0, 100n);

end
endmodule

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Ken Kundert

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Posts: 2386
Silicon Valley

Re: Hidden states in digital converter control blocks
Reply #3 - Mar 24^th, 2021, 7:39pm

When asking for help, you should present the simplest version of the model that exhibits the problem. As such, you should trim it down so that you don't have seven copies of each line. So much clutter makes your model harder to understand.

Having said that, start from the beginning of the analog block and search for var_v1_idt. If you use it before you assign it, which you do, you have hidden state. When you are using it, in @cross block, you are using the value from the last time point. Any use of variable values from a previous time point is hidden state.

-Ken

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