[Ngspice-devel] ngspice and xspice

[Tim E. <ti...@op...>]

Hello ng-spice developers,

    I'm surprised at the lack of readable documentation on how to use
xspice.  I spent some time about a year ago or maybe more trying to
figure out how to use it, using the existing ngspice and xspice
documents.  I wasn't successful until I got a few tips from someone
using Electric, and suddenly I find I can do mixed-mode simulation
. . . I had no idea it could be that easy.  Previously I must have
been doing something wrong with the A/D and D/A bridges, but anyway,
I can get it all to work now.

    I was thinking about an addition to my digital flow qflow to
convert digital synthesis output into an xspice subcircuit for spice
mixed-mode simulation.  However, for that, I have to map any arbitrary
standard cell set into xspice digital primitives.  The problem is that
the xspice digital primitives aren't a complete set.  I thought about
adding a few cells like and-or-invert, etc., but then I realized what
it really needs is a cell "d_lut" for an N-input, lookup table, that
can implement any arbitrary function of N inputs.  So I wrote one.
It's so obvious and useful I'm not sure why it hasn't been done before.
The attached tarball goes in (should be untarred in) xspice/icm/digital/,
and creates a new subdirectory called "d_lut".  The name "d_lut" needs
to be added to file modpath.lst in xspice/icm/digital/.  The d_lut cell
follows the general form of the other digital cells, but it has an
extra parameter called "table_values", which is a string length 2^N for
an N input instance of the cell.  Values can be "1", "0", or "X".  So
a 2-input AND gate could be implemented using a d_lut with 2 inputs and
a string "0001" for table_values.  The d_lut cell can implement any
combinational logic cell in a standard cell set.  That will allow me to
write the script for qflow to map an entire standard cell set into
xspice primitives.

    Can someone please help me get this code added to the ngspice code
base?
					Thanks,
					Tim

+--------------------------------+-------------------------------------+
| R. Timothy Edwards (Tim)       | email: ti...@op...    |
| Open Circuit Design            | web:   http://opencircuitdesign.com |
| 19412 Cissel Manor Drive       | phone: (301) 528-5030               |
| Poolesville, MD 20837          | cell:  (408) 828-8212               |
+--------------------------------+-------------------------------------+