# Creating Verilog-A Models in ADS from VHDL-A Code: Part 1 - Simple Resistor

Blog Post created by Colin Warwick on Dec 6, 2018

Verilog-AMS and VHDL-AMS are hardware description languages, capable of describing mixed-signal (i.e. analog and digital) hardware. In this series of postings, we’ll be talking about Verilog-A (i.e. the officially defined subset of Verilog-AMS that supports analog) and “VHDL-A” (not officially defined, but defined here as "the parts of VHDL-AMS that supports analog").

ADS support Verilog-A but not VHDL-A so this series of posting will explain how to create a Verilog-A model if all you have to start from is a VHDL-A model. It isn’t a comprehensive Verilog or VHDL reference manual: you can find plenty of those by searching on the Internet. The idea here is to get you started. You’ll have to refer to those other resources for the in-depth information. Another good resource is Best Practices for Compact Modeling in Verilog-A, J. Electron Devices Soc. (Aug. 2015) by Colin C. McAndrew et al. It's not about how to code models in Verilog-A, it's about how to code them well.

Let jump right in and compare how the two languages express the simplest possible analog component, the resistor:

Code fragment 1a: Resistor in VHDL-A

`use electrical_system.all;entity resistor is   generic (r: real := 0.0);   port(terminal p, n: electrical);end entity resistor;architecture signal_flow of resistor is   quantity vr across ir through p to n;begin   vr == ir * r;end architecture signal_flow;`

Code fragment 1b: Resistor in Verilog-A

``include "disciplines.vams"module resistor(p, n);     parameter real r=0.0;     inout p, n;     electrical p, n;     analog begin           V(p,n) <+ I(p,n)  * r;     endendmodule`

Keywords are in bold. The other identifiers are defined in the code itself, or in the included libraries. For example in both langauges, the identifier electrical is not a keyword: it is defined in a standard library. VHDL-A has:

`use electrical_system.all;`

…and Verilog-A has:

``include "disciplines.vams"`

We won’t go into the contents of these yet. Suffice it to say that they serve a similar purpose, namely to save you from writing the basic “plumbing” required to set up your model.

The two languages diverge in the next lines.

VHDL-A divides the code up into entity and architecture sections. The entity keyword is used to introduce the description of the interface of the component. Here we define ports p and n and a parameter, r. Then, the architecture keyword is used to introduce the description of its internal details. An entity can have more than one architecture ("polymorphism"), so we have to give each one a name, signal_flow in this case, so we can specify which morphology we want to use later on. To implement an architecture with ohmic behavior, we first state explicitly that the voltage V and current I are the across and through variables, respectively, of our ports with:

quantity vr across ir through p to n

(Side note: The terminology “through and across variables” comes from the mathematics of the class of problem that all SPICE-like solvers solve, namely differential algebraic equations or DAEs. In Verilog-A, through and across variables like voltage and currents are called "natures" and a pair of related through and across variables is called a "discipline." Voltage and current natures, plus some other information like tolerances, form the electrical discipline. Confusingly, VHDL-A uses the term "nature" for the pair of natures, whereas Verilog-A uses "discipline.")

Then we specify Ohm’s law, V=IR, for the branch constitutive equation:

vr == ir * r

The "double equal signs" operator implies the simulator should force the equation to be true for every time step in the simulation: it isn’t a one-time assignment like the a = b in C and Java nor the a := b in Pascal and ADA. (And it isn't at all like the equality test A == B in C and Java.)

In contrast, Verilog-A combines the entity/architecture ideas. The module keyword is used to introduce both interface (the parameter, inout, and electrical lines) and the internals (the block that is bounded by analog begin end ).

The line:

V(p,n) <+ I(p,n) * r;

...is called a contribution statement. It is similar to the one in VHDL with double equals == except that the operator in Verilog-A is <+ . Why <+ ? It's because Verilog-A allows multiple contributions to add to the voltage V(p,n). For example,

V(p,n) <+ V(p2,n2);

V(p,n) <+ V(p3,n3);

…is equivalent to:

V(p,n) <+ V(p2,n2)+ V(p3,n3);

Similar to == in VHDL, <+ in Verilog-A implies the simulator should do whatever it takes (iteration!) to force the equation to be true for every time step in the simulation.

In Verilog-A, there is no need for a line like quantity vr across ir through p to n that we had in VHDL because the so-called access functions, V() and I(), are defined in the `included disciplines.vams header file and thereby associated with any instances of the electrical discipline.

So, that's a component. Part 2 shows you how include the Verilog-A version into ADS create a testbench that instantiates the component, adds some stimulus, and shows you the response.