Electronics Forum / CAD / April 2007

Well, no offense intended.  Sorry!

Bully for you!

We are leery of folks posting "look at this great new free
tool" messages here since free tools from commerical vendors usually
have strings attached [1].  The classic examples are ExpressPCB, which
offers free design/layout software, but locks you into a proprietary
output file format usable only at ExpressPCB; and Eagle, whose free
version is wildly popular with students, but is essentially crippled
since it limits you to two layers and a fairly small board area.  Once
you bump up against the limits of the freebie Eagle, you've got to pay
for the full-up version [2].

FWIW, there are lots of freebie and open-source simulators of various
flavors out there, including:  

LTSpice -- closed source SPICE with integrated schematic capture.
          Totally rocks!
          http://www.linear.com/designtools/software/

ngspice -- open source SPICE 3f5.  Still uses a CLI, and not as
          optimized as LTSpice, but it works.
          http://ngspice.sourceforge.net/

GnuCap --  Analog simulator with internal engine a generation or two
          ahead of regular SPICE.  Open source.  
          Can do event-driven simulation as
          well as continuous time.  Still uses CLI, and can read
          SPICE netlists (with some caveats).
          http://www.gnucap.org/

QUCS --    A spiffy new GUI-based simulation environment which claims
          to do all kinds of simulation.  Open source.  
          Incorporates schematic
          capture front end.  I believe they have some work to do
          until it's complete.
          http://qucs.sourceforge.net/

Icarus Verilog --
          Excellent Verilog open-source simulator run from the
          command line.  Used with GTKWave for waveform viewing, it's
          a powerful tool for Verilog design.
          http://icarus.com/eda/verilog/
          http://home.nc.rr.com/gtkwave/

Alliance VHDL --
          French university project providing a chip design tool
          suite.  Includes VHDL simulator.  I've never used it so
          I know very little about it.
          http://www-asim.lip6.fr/recherche/alliance//

TkGate --  GUI based logic simulator.  More of an eductional tool than
          a professional design tool, but it's cool nonetheless.
          Open-source.  
          http://www.tkgate.org/

PSpice --  Years ago PSpice 6.x from MicroSim was downloadable off the
          web for free.  Is it still around?

Tina --    TI's simulation program.  Version 7 is a free download.  Is
          it some kind of "me too" response to LTSpice?  I haven't
          used it.
          http://focus.ti.com/docs/toolsw/folders/print/tina-ti.html

Besides these, there's also MyHDL, PyHDL, FreeHDL, regular Spice3f5,
and plenty of other free simulators out there available on the
web.  And don't get me started on schematic capture, layout tools,
or chip design editors!

You can follow the open-source EDA tool space here:

http://www.opencollector.org/

As you can see, CircuitL*gix is just another entrant into a very
crowded market space.  Maybe it's got some better features than some
other tool?  But from your initial postings it didn't make it seem
so.  Also, commercial giveaways are always to be regarded with
suspicion.  And we're a very cynical group in any event.

Cheers,

Stuart

[1]  LTSpice being a notable exception, probably because they make
their money from chips, not from software.

[2] Nothing wrong with that, since the freebie tool is a loss leader.
But anybody using it should think twice about the long-term dangers of
vendor lock-in before they have too many designs done in such a tool.
Same for CircuitL*gix, I would imagine.

Unless they have very specific needs, I'd suggest it's not really in the best
interest of your students to steer them towards CircuitLogix rather than
LTSpice.  A reasonably sophisticated sernior project could readily exceed the
circuit size limits of the free version of CircuitLogix.  Additionally,
LTSpice is *very* well supported -- for free! -- on the Yahoo! groups; the
program's author still posts regularly.  Just looking at the web site itself,
it's clear that they want to *sell students* the "full educational version."
Of course, there's nothing wrong with trying to make a buck, it's just again
that CircuitLogix doesn't appear to offer $249 worth of "added value" over all
the freeware solutions out there.

Unlike SPICE, which is designed mainly for analog simulation, mixed-mode
simulators such as Multisim and CircuitLogix include both analog and event-
driven digital simulation capabilities in the same executable. This means
that any simulation may contain components that are analog, event driven
(digital or sampled-data), or a combination of both. An entire mixed signal
analysis can be driven from one integrated schematic. All the digital models
in mixed-mode simulators provide accurate specification of propagation time
and rise/fall time delays.

The event driven algorithm provided by mixed-mode simulators is general
purpose and supports non-digital types of data. For example, elements can use
real or integer values to simulate DSP functions or sampled data filters.
Because the event driven algorithm is faster than the standard SPICE matrix
solution simulation time is greatly reduced for circuits that use event
driven models in place of analog models.

Mixed-mode simulation is handled on three levels; (a) with primitive digital
elements that use timing models and the built-in 12 or 16 state digital logic
simulator, (b) with subcircuit models that use the actual transistor topology
of the integrated circuit, and finally, (c) with In-line Boolean logic
expressions.

Exact representations are used mainly in the analysis of transmission line
and signal integrity problems where a close inspection of an IC’s I/O
characteristics is needed. Boolean logic expressions are delay-less functions
that are used to provide efficient logic signal processing in an analog
environment. These two modeling techniques use SPICE to solve a problem while
the third method, digital primitives, use mixed mode capability. Each of
these methods has its merits and target applications. In fact, many
simulations (particularly those which use A/D technology) call for the
combination of all three approaches. No one approach alone is sufficient.

In testing simulators, we ran some SPICE netlist tests on LTSpice,
PSpice and Microcap.  We had heard all the hype about LTSpice but the
simulation speeds in most of the netlists was a good deal slower than
both PSpice and Microcap.  We were not running simulations with their
enhanced Linear models but just a few general circuit files though.
Perhaps if you use the SMPS capability with the Linear models, then it
is a fast simulator, or we just fluked into a few circuits that
LTSpice has problems with.  Was not impressive though.  Great price
however.