Simulation

AMS Support

OrigenSim supports analog/mixed-signal (AMS) simulations via real-number modeling (RNM), whereby top-level pins can be defined as real wire types (WREALs) and then Origen APIs can be used to drive and measure real number values from them.

By default, OrigenSim’s built in simulation setups will run a digital simulation, allowing such real number inputs to be used by behavioral models within that DUT which can themselves output real numbers to be observed on the DUT’s pins by Origen APIs.

The DUT could also contain full electical models which consume the real number inputs, in that case a custom simulation configuration will be required to define the run command to start the simulation.

AMS support must be added in when building the testbench, this is done by adding the --wreal switch to the sim:build command:

origen sim:build path/to/my_dut.v --wreal

By adding this switch, any pins which are defined as wreal types within the given top level will be assigned an analog pin driver by the testbench rather than a digital driver which is the default.

Pins can be defined as a wreal type either by adding the real type to their definition:

input real vddc,

or by adding a wreal wire within the module body:

wreal vddc;

Here is an example which uses both approaches to declare the vdd and ana pins as analog pins whenever the USE_WREAL define is enabled:

module my_dut(
  input tck, tdi, tms, trstn,
  input rstn,
  input [31:0] din,
  input p1,
  input p2,
  `ifdef USE_WREAL
  inout real vdd,
  `else
  inout vdd,
  `endif

  output tdo,
  output done,
  output [15:0] test_bus,
  output [31:0] dout,
  output ana
);

`ifdef USE_WREAL
  wreal ana;
`endif
endmodule

A digital testbench for this would be built via this command:

origen sim:build path/to/my_dut.v

while AMS support would be added by running:

origen sim:build path/to/my_dut.v --wreal --define USE_WREAL

Within the corresponding Origen DUT model of the design, the wreal pins should be declared as either an analog, power or ground pins.

From the above example, the wreal pins could be modeled like this:

add_power_pin :vdd    # Could also be a regular analog pin too, if you prefer

add_pin :ana, type: :analog

See the Pins Guide for more information on modeling pins in Origen.

With all of the AMS configuration done, real values can now be driven and read from Origen application code during a simulation like this:

dut.power_pin(:vdd).drive(1.25)

dut.pin(:ana).read     # => 0.7

# .measure is available as an alias of read for analog pins
dut.pin(:ana).measure  # => 0.7

The peek, poke and force methods from the Direct DUT Manipulation APIs are also available to manipulate real valued nets during simulation.

The analog pin APIs will not work correctly when generating a pattern for an ATE and the application code is responsible for handling them safely, typically like this:

# Example of a simulation-only assertion
if tester.sim?
  measured = dut.pin(:ana).measure
  if measured != 0.3125
    OrigenSim.error "Expected to measure 0.3125V from the ana pin, got #{measured}V!"
  end
end

It is easy to build more complex functionality in your application code from these simple APIs, for example to ramp a vdd pin:

# Ramp up the power on VDD
v = 0
dut.power_pin(:vdd).drive!(v)
until v >= 1.25
  v += 0.05
  dut.power_pin(:vdd).drive!(v)   # Note the use of drive! here which will generate a cycle
end

It is hoped that the community will contribute plugins that contain higher-level functionality like this to make such functions available off-the-shelf in the future.