Minutes of: VAMS Compact Modeling conf call May 20

V-AMS Compact Modeling Extensions subcommittee
Minutes of May 20, 2004

Attendees:
Geoffrey Coram, Analog Devices
Ilya Yusim, Cadence
Colin McAndrew, Freescale
Srikanth Chandrasekaran, Motorola
David Zweidinger, Texas Instruments
Jim Barby, U Waterloo
Ninglong Lu, Intel
Jung-Hoon Rhew, Intel

1. Approval of previous minutes (Apr 20).

2. LRM 2.2 draft b

The following items are not in the draft, as mentioned in the e-mail:
a) paramsets
    I just didn't get to it in time
b) mfactor
    Waiting for a proposal from Cadence to the main AMS committee
c) dc sweep for mixed-signal
    Sri will send me an updated proposal in the next day or so
d) simparam descriptions
    Colin e-mailed me these; they will be in the next draft
e) module descriptions
    an oversight on my part

3. Noise extension

Ilya wanted to know if an extra node was required; I used one
in my implementation of the Philips MOS11 (level 1101) model.

Colin said one simulator does not get correlation, only magnitude,
because it doesn't add an extra node

Ilya said another simulator does get correlation without the
extra node, but it requires a change to the noise analysis
routine (adjoint solution method) to support loading two values
into the RHS vector.

Colin noted that we require a smart compiler to optimize away
unneeded nodes in other contexts; so probably it is reasonable
to expect the compiler to optimize away my noise node if its
noise analysis routine supports correlated noise natively.

4. String parameters

Sri asked if the range (from {"a", "b"}) is mandatory.
This tells the simulator how much space to allocate for the string,
(similar to how digital reg variables have a size), so I believe
it should be mandatory.

5. Limit call

Sri asked if, when using a UDF, the arguments be inouts or outputs?
We believe they should not be; the Spice functions have only one
return value. Thus we will sematically restrict inputs to a UDF
use for limiting to be inputs only.

Sri also asked that a table of Spice limiting functions be added
to Annex E, in addition to the text descriptions in Section 4.4.15.

6. ddx(ddt(expr))

This syntax is allowed by the LRM and should not be restricted.
We should be sure that the examples for ddx show the proper use.

In general, any analog_operator can be part of the first argument
(expression) to ddx. However, the ddx() operator takes partial
derivatives with respect to the independent variable given as
the second argument; if the simulator creates internal nodes to
implement the analog_operator, then one cannot reference that
node to take a partial derivative, and that node will be held
constant when taking the partial wrt other variables.

7. Paramset resolution

Addressing Kevin's concerns from e-mails to the reflector, we
agreed that we do need some resolution rules to determine
which paramset is chosen for an instance.

Setting up the system to find the "best" paramset could
require extra memory, but it is done once at elaboration time,
so the memory can be "recycled" for simulation.

... except for dc sweeps, in which one might be sweeping the
transistor length and hence the "bin" or paramset selected.

We could specify that the paramset selected is fixed at
elaboration time. That's a bad idea; the user should not
have to figure out how to split up a sweep into pieces
that are contained in one "bin."

It seems that each point of a dc sweep will require a new
elaboration / selection of paramsets. This is consistent
with the idea that a dc sweep is a collection of operating
points. It may cause trouble if the topology changes
(eg, the new paramset adds gate resistance and thus a new
node to the system); the simulator may have to re-start
the simulation from scratch rather than using values from
the previous point of the sweep.

8. Upcoming meetings:
  May 24, 7:30PM ET, 4:30PM PT - main AMS to discuss LRM 2.2 draft
  June 1, 9AM ET, 6AM PT - CM subcommittee meeting
  June 1, 7:30PM ET, 4:30PM PT - main AMS to discuss mfactor and mix-sig OP

-- 
Geoffrey J. Coram, Ph.D.    Senior CAD Engineer     
Analog Devices, Inc.        Geoffrey.Coram@analog.com 
804 Woburn St., MS-422,     Tel (781) 937-1924
Wilmington, MA 01887        Fax (781) 937-1014