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# Switching from voltage source to high impedance port in VerilogA: Convergence problem

#### yayw35

##### Newbie
I'm trying to write a VerilogA testbench, I need a port that can act as both a voltage source, and as a high impedance.
As far as I know, there are two ways to implement a high impedance, either by assigning a 0 (or very close to 0) current to the port, or using a very high resistor value. (ex: V(port,n)<+I(port,n)*R_big ).

Given that there is no way (as far as I know) to use a transition while switching between current and voltage source, I went for the second way.
The problem is that when switching from a current source to high impedance, I have convergence issues. My code looks something like this:

Code Verilog - [expand]1
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if(write=0) begin
temp=I(port,n)*R_big;
end
else begin
temp=A_Value;
end
V(port,n)<+transition(temp,0,1n);

For more context, my ultimate goal is to be able to both charge a capacitor and sense it's voltage, through a transmission gate.
Trying the same approach with different simpler circuits, and it doesn't seem to give a problem.
I tried adding resistors to help in converging, doesn't seem to help much.

Any help would be appreciated.
Thank you.

[MODERATOR ACTION]
• Added SYNTAX tags enclosing the code

Last edited by a moderator:

Effectively switching topology might muss up its mind some.
Perhaps formulating it as a vccs cum switch as a time varying
resistance (any -real- source of voltage has a series R, albeit
as low as necessary) could be a smoother approach. Numerical
solvers like smooth and hate discontinuous derivatives.

I think your voltage sense wants to go around, not through,
the tgate if you are about the source position. Inboard, if you
are about the hold-value or its drift / droop while holding.