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I am a beginner in verilogA .
I wrote a code but when I ran PSS it showed hidden state in the code and didn't run.
My behavioural model is as below:-
// VerilogA for VERILOG_A_MODEL, HARD_LIMIT_GM, veriloga`include "constants.vams"`include "disciplines.vams"module HARD_LIMIT_GM(in,out); inout in,out; parameter real vtrans = 0; parameter real tdelay = 0 from [0:inf); parameter real trise = 1p from (0:inf); parameter real tfall = 1p from (0:inf); parameter real Gm=-5m; electrical in,out; real vout_val; analog begin @ (cross(V(in) - vtrans, 1)) vout_val = 1; @ (cross(V(in) - vtrans, -1)) vout_val = 0; I(out) <+ Gm * transition( vout_val, tdelay, trise, tfall); end endmodule
Could anybody please tell how I can avoid the hidden state (vout_val ) ?
The reason you have a hidden state is because vout_val is not updated on every iteration. It's only set within the @cross blocks - and is held (and hence retains state) between cross events.
If you replace the two @cross statements with:
@ (cross(V(in) - vtrans)) ; vout_val = V(in)>vtrans;
Then it will do what you want. The first @cross will force there to be a timestep close to either the positive or negative transition - but doesn't have any action within the @cross itself. The second line is simply computing the vout_val based on whether V(in) is greater than vtrans - but does this at every timestep and hence is not a hidden state.
In reply to Andrew Beckett:
Thanks a lot for your reply. I have some doubts.
1:- Every iteration means:-
Is it each simulation time step ?
2:- Suppose instead of having vout_val= 1 or 0, If I want let's say vout_val= -5 ( for positive edge cross )or +7 ( for -ve edge cross ),
is there a way to achieve this ?
In reply to RFStuff:
Thanks a lot for clarifying the Hidden State problem.
Hello, I'm also new with Verilog A. I would like to run a pss in a circuit that has a block with only a veriloga view. It's a D flip-flop double edge. I wrote the veriloga code according to the advice in this post, but I still have a hidden state.
Here's the code:
// Double edge Flip Flop
module DFF_DEDGE(q, clk, d);
voltage q, clk, d;
parameter real tdelay = 1n from [0:inf),
ttransit = 20p from [0:inf),
vout_high = 1,
vout_low = 0 from (-inf:vout_high),
vth = 0.5;
@(initial_step) x = 0;
@(cross(V(clk) - vth )) x = (V(d) > vth);
V(q) <+ transition( vout_high*x + vout_low*!x, tdelay, ttransit );
If I run a PSS, I got the following error message
ERROR (SPCRTRF-15177): PSS analysis doesn't support behavioral module components with hidden states found in component 'DFF_DEDGE'. Skipped.
... : Hidden state variable: x
How can I modify the code to avoid it?
Thanks a lot!
In reply to Clidre:
See a good coverage of the problem in general in this paper on Hidden State in SpectreRF and also a hidden state free DFF model. In your case, the x variable is only updated on the crossing event, and so is clearly retaining the state of the flip flop from one iteration of the simulator to the next (which is fine normally, but not for SpectreRF as it can't see the hidden state, since it's not on a node).
The model I pointed you too uses a resetable integrator (reset on the clock edge) to achieve the same thing. This doesn't have any hidden states because the variables are updated on every timestep.
Thanks a lot!
Based on the example you pointed me, I adapted my code and this is my newest code:
// Double edge Flip Flop
module DFF_DEDGE_NOHIDDEN(q, clk, d);
parameter integer init_state=0 from [0:1];
integer actNow, out, state;
actNow = 0;
actNow = 1;
state = init_state;
@(cross(V(clk) - vth, +1) or cross(V(clk) - vth, -1)) begin
state = (V(d) > vth);
out = idt(0, state, actNow);
V(q) <+ transition(out ? vout_high : vout_low, tdelay, ttransit);
But I still have hidden state problems:
"ERROR (SPCRTRF-15177): PSS analysis doesn't support behavioral module components with hidden states found in component 'DFF_DEDGE_NOHIDDEN'. Skipped.
... Hidden state variable: state "
I've read the documentation you attached and I think that variable "state" is constantly updated. Am I missing something?
Thank you again for yuor precious help
Hello, it seems I solved it: I added state=0 and now the pss run.
This is my last working code, maybe can be useful to someone. I put in bold the line I added to make it work:
Thank you very much for your help