Spice to Spectre conversion

Dear Shriram,

sram17 said:

// Is this a valid way to define the current source using node voltages and user-defined function in Spectre?

Did you happen to check the spectre manual or the On-line support partal? If not, the following document details the syntax and scope of user defined functions:

https://support.cadence.com/apex/techpubDocViewerPage?xmlName=spectreref.xml&title=Spectre%20Circuit%20Simulator%20Reference%20--%20Other%20Simulation%20Topics%20-%20User%20Defined%20Functions%20(functions)&hash=pgfId-1068879&c_version=20.1&path=spectreref/spectreref20.1/chap4.html#pgfId-1068879

Otherwise, from the UNIX prompt, type:

spectre -h expressions

and

spectre -h bsource

The short answer is your syntax does not conform with that required by spectre. spectre uses parenthesis to enclose node names. As an example from the spectre -h output,

Voltage-controlled current source
         vsrc (n1 n2) resistor r=100k
         vccs1 (n3 n4) bsource i=v(n1,n2)/2000.0

I hope this provides enough information and I understood your questions Shriram!

Shawn

Thanks a lot, Shawn! I noticed the paranthesis, however, ADE somehow did not throw up a syntax error when I saved the Spectre file without paranthesis, so I assumed they were optional.

It is still not clear to me based on the examples if I am allowed to send a node voltage as a variable to a user-defined function in bsource and have the sub-circuit behave as it does in SPICE.

Are you suggesting that it is correct?

Thanks again,

Shriram

Dear Shriram,

sram17 said:

It is still not clear to me based on the examples if I am allowed to send a node voltage as a variable to a user-defined function in bsource and have the sub-circuit behave as it does in SPICE.

Perhaps I am not understanding your question correctly and I apologize if I am not!

However, in the example I provided, v(n1,n2) represents the voltages between circuit nodes n1 and n2 and is used in the bsource  function v(n1,n2)/2000.0 as its current. Are you suggesting defining the voltage v(n1,n2) as a separate distinct variable? In lieu of that, perhaps you can just define a voltage net in your circuit, say vdelta, as the difference between node voltages n1 and n2, and then use voltage vdelta in your expression, for example i=v(vdelta)/2000.0.

Did I manage to understand your comment?

Shawn

Hi Shawn - I think we are converging on the same item. I have reduced the complexity of my dependent current source for the purpose of the forum posting. My current source is a somewhat complicated function of not only the voltage difference v(n1,n2), but also a node voltage v(p1). I have tried to express that as f1(v(p1)), with a user-defined function. I was trying to understand if this is a valid way to define a complicated functional relationship in Spectre for a dependent-current source. Hope my question is clear.

Thanks again for your time and comments!

Shriram

Sorry, maybe I messed up and wrote f1(v(p1)) when clearly f1 has 3 variables. So, the correct expression should have been:

G_p1 0 p1 bsource i=-f1(v(p1), 2, alpha)-v(n1,n2)

where x=v(p1) and y=2

I fixed the expressions in the original post. Sorry about the confusion!

Dear Shiram,

sram17 said:

My current source is a somewhat complicated function of not only the voltage difference v(n1,n2), but also a node voltage v(p1). I have tried to express that as f1(v(p1)), with a user-defined function. I was trying to understand if this is a valid way to define a complicated functional relationship in Spectre for a dependent-current source. Hope my question is clear.

Did you examine the examples when you type spectre -h expressions at the UNIX command line? If you examined it, it suggest you type spectre -h function for additional information on calling your own functions in expressions. Typing this at the UNIX prompt yiedls:

"$ spectre -h functions

**********************
User Defined Functions
**********************

Spectre's user-defined function capability allows you to build upon the provided set of built-in mathematical and trigonometric functions. You can write
your own functions, and call these functions from within any expression. The syntax for calling a user-defined function is the same as the syntax for
calling a built-in algebraic or trigonometric function. Note that user-defined functions must be defined before they are referenced (called). Arguments
to user-defined functions will be taken as real values, and the functions will return real values. A user-defined function may contain only a single
statement in braces and this statement must return an expression (which will typically be an expression involving the function arguments). The return
expression may reference the built in parameters `temp' and `tnom'. User-defined functions must be declared at the top level only, and must not be
declared within subcircuits. User-defined functions may be called from anywhere that an expressions can be currently used in Spectre. User-defined
functions may call other functions (both user-defined and built-in), however any user-defined function will need to be declared before it can be called.
User-defined functions can override built-in mathematical and trigonometric functions.
NOTE: Only real values for arguments and return values are supported in this release.
See `spectre -h expressions' for a list of built-in algebraic and trigonometric functions

Synopsis:
real myfunc( [real arg1, ...real argn] ) {
   return <expr of arg1..argn>
}


Examples:


real myfunc( real a, real b ) {
    return a+b*2+sqrt(a*sin(b));
}

An example of a function calling a previously defined function follows

real yourfunc( real a, real b ) {
    return a+b*myfunc(a,b);     // call "myfunc"
}

The final example shows how a user-defined function may be called from an expression in the Spectre netlist:

r1 (1 0) resistor r=myfunc(2.0, 4.5)"

Does this help?

Shawn

Hi Shawn - I read all that already. There are a few suspicious things. First, it says one cannot define functions inside sub-circuits, which kind of seems quite problematic, since we work at different levels of abstraction and a subcircuit allows one that flexibility. None of the examples use node voltages as inputs to user functions. 

My question can be stated as below:

Can one use node-voltages as inputs to a user-defined function for the purposes of defining a controlled current source inside a sub-circuit in Spectre?

Thanks,

Shriram

Dear Shihram,

I think you may have overlooked a statement in the help articles I suggested....you added an additional constraint that I either overlooked or you did not include when you noted concerning a subcircuit...:

sram17 said:

Can one use node-voltages as inputs to a user-defined function for the purposes of defining a controlled current source inside a sub-circuit in Spectre?

From the help article:

"User-defined functions must be  declared within subcircuits."

Shawn

Hi Shawn - not sure where that is. This is what I see:

User-defined functions must be declared at the top level only, and must not be
declared within subcircuits. 

Also, do you have any idea about whether one can pass a node voltage as an argument?